| Message ID | 20200115023843.31325-1-willy@infradead.org (mailing list archive) |
|---|---|
| Headers | show |
| Series | Replacing the readpages a_op | expand |
On Tue, Jan 14, 2020 at 06:38:34PM -0800, Matthew Wilcox wrote: > This is an attempt to add a ->readahead op to replace ->readpages. I've > converted two users, iomap/xfs and cifs. The cifs conversion is lacking > fscache support, and that's just because I didn't want to do that work; > I don't believe there's anything fundamental to it. But I wanted to do > iomap because it is The Infrastructure Of The Future and cifs because it > is the sole remaining user of add_to_page_cache_locked(), which enables > the last two patches in the series. By the way, that gives CIFS access > to the workingset shadow infrastructure, which it had to ignore before > because it couldn't put pages onto the lru list at the right time. > > v2: Chris asked me to show what this would look like if we just have > the implementation look up the pages in the page cache, and I managed > to figure out some things I'd done wrong last time. It's even simpler > than v1 (net 104 lines deleted). I want to discuss whether to change the page refcount guarantees while we're changing the API. Currently, page is allocated with a refcount of 1 page is locked, and inserted into page cache and refcount is bumped to 2 ->readahead is called callee is supposed to call put_page() after submitting I/O I/O completion will unlock the page after I/O completes, leaving the refcount at 1. So, what if we leave the refcount at 1 throughout the submission process, saving ourselves two atomic ops per page? We have to ensure that after the page is submitted for I/O, the submission path no longer touches the page. So the process of converting a filesystem to ->readahead becomes slightly more complex, but there's a bugger win as a result. Opinions?
Hello Matthew! On Tue 14-01-20 18:38:34, Matthew Wilcox wrote: > From: "Matthew Wilcox (Oracle)" <willy@infradead.org> > > This is an attempt to add a ->readahead op to replace ->readpages. I've > converted two users, iomap/xfs and cifs. The cifs conversion is lacking > fscache support, and that's just because I didn't want to do that work; > I don't believe there's anything fundamental to it. But I wanted to do > iomap because it is The Infrastructure Of The Future and cifs because it > is the sole remaining user of add_to_page_cache_locked(), which enables > the last two patches in the series. By the way, that gives CIFS access > to the workingset shadow infrastructure, which it had to ignore before > because it couldn't put pages onto the lru list at the right time. > > v2: Chris asked me to show what this would look like if we just have > the implementation look up the pages in the page cache, and I managed > to figure out some things I'd done wrong last time. It's even simpler > than v1 (net 104 lines deleted). I have an unfinished patch series laying around that pulls the ->readpage / ->readpages API in somewhat different direction so I'd like to discuss whether it's possible to solve my problem using your API. The problem I have is that currently some operations such as hole punching can race with ->readpage / ->readpages like: CPU0 CPU1 fallocate(fd, FALLOC_FL_PUNCH_HOLE, off, len) filemap_write_and_wait_range() down_write(inode->i_rwsem); truncate_pagecache_range(); readahead(fd, off, len) creates pages in page cache looks up block mapping removes blocks from inode and frees them issues bio - reads stale data - potential security issue Now how I wanted to address this is that I'd change the API convention for ->readpage() so that we call it with the page unlocked and the function would lock the page, check it's still OK, and do what it needs. And this will allow ->readpage() and also ->readpages() to grab lock (EXT4_I(inode)->i_mmap_sem in case of ext4) to synchronize with hole punching while we are adding pages to page cache and mapping underlying blocks. Now your API makes even ->readpages() (actually ->readahead) called with pages locked so that makes this approach problematic because of lock inversions. So I'd prefer if we could keep the situation that ->readpages / ->readahead gets called without any pages in page cache locked... Honza
On Tue, Jan 21, 2020 at 12:36:27PM +0100, Jan Kara wrote: > > v2: Chris asked me to show what this would look like if we just have > > the implementation look up the pages in the page cache, and I managed > > to figure out some things I'd done wrong last time. It's even simpler > > than v1 (net 104 lines deleted). > > I have an unfinished patch series laying around that pulls the ->readpage > / ->readpages API in somewhat different direction so I'd like to discuss > whether it's possible to solve my problem using your API. The problem I > have is that currently some operations such as hole punching can race with > ->readpage / ->readpages like: > > CPU0 CPU1 > fallocate(fd, FALLOC_FL_PUNCH_HOLE, off, len) > filemap_write_and_wait_range() > down_write(inode->i_rwsem); > truncate_pagecache_range(); > readahead(fd, off, len) > creates pages in page cache > looks up block mapping > removes blocks from inode and frees them > issues bio > - reads stale data - > potential security > issue > > Now how I wanted to address this is that I'd change the API convention for > ->readpage() so that we call it with the page unlocked and the function > would lock the page, check it's still OK, and do what it needs. And this > will allow ->readpage() and also ->readpages() to grab lock > (EXT4_I(inode)->i_mmap_sem in case of ext4) to synchronize with hole punching > while we are adding pages to page cache and mapping underlying blocks. > > Now your API makes even ->readpages() (actually ->readahead) called with > pages locked so that makes this approach problematic because of lock > inversions. So I'd prefer if we could keep the situation that ->readpages / > ->readahead gets called without any pages in page cache locked... I'm not a huge fan of that approach because it increases the number of atomic ops (right now, we __SetPageLocked on the page before adding it to i_pages). Holepunch is a rather rare operation while readpage and readpages/readahead are extremely common, so can we make holepunch take a lock that will prevent new readpage(s) succeeding? I have an idea to move the lock entries from DAX to being a generic page cache concept. That way, holepunch could insert lock entries into the pagecache to cover the range being punched, and readpage(s) would either skip lock entries or block on them. Maybe there's a better approach though.
On Tue 21-01-20 13:48:45, Matthew Wilcox wrote: > On Tue, Jan 21, 2020 at 12:36:27PM +0100, Jan Kara wrote: > > > v2: Chris asked me to show what this would look like if we just have > > > the implementation look up the pages in the page cache, and I managed > > > to figure out some things I'd done wrong last time. It's even simpler > > > than v1 (net 104 lines deleted). > > > > I have an unfinished patch series laying around that pulls the ->readpage > > / ->readpages API in somewhat different direction so I'd like to discuss > > whether it's possible to solve my problem using your API. The problem I > > have is that currently some operations such as hole punching can race with > > ->readpage / ->readpages like: > > > > CPU0 CPU1 > > fallocate(fd, FALLOC_FL_PUNCH_HOLE, off, len) > > filemap_write_and_wait_range() > > down_write(inode->i_rwsem); > > truncate_pagecache_range(); > > readahead(fd, off, len) > > creates pages in page cache > > looks up block mapping > > removes blocks from inode and frees them > > issues bio > > - reads stale data - > > potential security > > issue > > > > Now how I wanted to address this is that I'd change the API convention for > > ->readpage() so that we call it with the page unlocked and the function > > would lock the page, check it's still OK, and do what it needs. And this > > will allow ->readpage() and also ->readpages() to grab lock > > (EXT4_I(inode)->i_mmap_sem in case of ext4) to synchronize with hole punching > > while we are adding pages to page cache and mapping underlying blocks. > > > > Now your API makes even ->readpages() (actually ->readahead) called with > > pages locked so that makes this approach problematic because of lock > > inversions. So I'd prefer if we could keep the situation that ->readpages / > > ->readahead gets called without any pages in page cache locked... > > I'm not a huge fan of that approach because it increases the number of > atomic ops (right now, we __SetPageLocked on the page before adding it > to i_pages). Yeah, good point. The per-page cost of locking may be noticeable. > Holepunch is a rather rare operation while readpage and > readpages/readahead are extremely common, so can we make holepunch take a > lock that will prevent new readpage(s) succeeding? I'm not opposed - in fact my solution would do exactly that (with EXT4_I(inode)->i_mmap_sem), just the lock ordering wrt page lock and mmap_sem is causing troubles and that's why I need the change in the API for readpage and friends. > I have an idea to move the lock entries from DAX to being a generic page > cache concept. That way, holepunch could insert lock entries into the > pagecache to cover the range being punched, and readpage(s) would either > skip lock entries or block on them. Two notes on the entry locks: The additional traffic in the xarray creating locked entries and then removing them is going to cost as well. But if that's only for hole punching, it would be bearable I guess. This does not solve the problem with the lock ordering. There are quite some constraints on this synchronization scheme. Generally we want to prevent creation of pages in the page cache in a certain range. That means we need to block read(2), readahead(2), madvise(2) MADV_WILLNEED, page faults. The page faults constrain us that the lock has to rank below mmap_sem. On the other hand hole punching needs to hold the lock while evicting pages so that mandates that the lock needs to rank above page lock. Also note that read(2) can cause page faults (to copy data to userspace) so to avoid lock inversion against mmap_sem, any protection must not cover that part of the read path which basically leaves us with ->readpage()/->readpages() as the only place where we can grab the lock. Which nicely covers also all the other places creating pages in the page cache we need to block. Except that ->readpage has the unfortunate property of being called under page lock. But maybe we could create a new hook somewhere in the paths creating pages to acquire the lock early. But so far I don't have an idea for something nice. Honza
On Tue, Jan 21, 2020 at 01:48:45PM -0800, Matthew Wilcox wrote: > On Tue, Jan 21, 2020 at 12:36:27PM +0100, Jan Kara wrote: > > > v2: Chris asked me to show what this would look like if we just have > > > the implementation look up the pages in the page cache, and I managed > > > to figure out some things I'd done wrong last time. It's even simpler > > > than v1 (net 104 lines deleted). > > > > I have an unfinished patch series laying around that pulls the ->readpage > > / ->readpages API in somewhat different direction so I'd like to discuss > > whether it's possible to solve my problem using your API. The problem I > > have is that currently some operations such as hole punching can race with > > ->readpage / ->readpages like: > > > > CPU0 CPU1 > > fallocate(fd, FALLOC_FL_PUNCH_HOLE, off, len) > > filemap_write_and_wait_range() > > down_write(inode->i_rwsem); > > truncate_pagecache_range(); shouldn't fallocate be holding EXT4_I(inode)->i_mmap_sem before it truncates the page cache? Otherwise it's not serialised against page faults. Looks at code ... oh, it does hold the i_mmap_sem in write mode, so.... > > readahead(fd, off, len) > > creates pages in page cache > > looks up block mapping > > removes blocks from inode and frees them > > issues bio > > - reads stale data - > > potential security > > issue .... I'm not sure that this race condition should exist anymore as readahead should not run until the filesystem drops it's inode and mmap locks after the entire extent freeing operation is complete... > > Now how I wanted to address this is that I'd change the API convention for > > ->readpage() so that we call it with the page unlocked and the function > > would lock the page, check it's still OK, and do what it needs. And this > > will allow ->readpage() and also ->readpages() to grab lock > > (EXT4_I(inode)->i_mmap_sem in case of ext4) to synchronize with hole punching > > while we are adding pages to page cache and mapping underlying blocks. > > > > Now your API makes even ->readpages() (actually ->readahead) called with > > pages locked so that makes this approach problematic because of lock > > inversions. So I'd prefer if we could keep the situation that ->readpages / > > ->readahead gets called without any pages in page cache locked... > > I'm not a huge fan of that approach because it increases the number of > atomic ops (right now, we __SetPageLocked on the page before adding it > to i_pages). Holepunch is a rather rare operation while readpage and > readpages/readahead are extremely common, so can we make holepunch take > a lock that will prevent new readpage(s) succeeding? > > I have an idea to move the lock entries from DAX to being a generic page > cache concept. That way, holepunch could insert lock entries into the > pagecache to cover the range being punched, and readpage(s) would either > skip lock entries or block on them. > > Maybe there's a better approach though. Can we step back for a moment and look at how we already serialise readahead against truncate/hole punch? While the readahead code itself doesn't serialise against truncate, in all cases we should be running through the filesystem at a higher layer and provides the truncate/holepunch serialisation before we get to the readahead code. The read() syscall IO path: read() ->read_iter() filesystem takes truncate serialisation lock generic_file_read_iter() generic_file_buffered_read() page_cache_sync_readahead() .... page_cache_async_readahead() .... ..... filesystem drops truncate serialisation lock The page fault IO path: page fault ->fault xfs_vm_filemap_fault filesystem takes mmap truncate serialisation lock filemap_fault do_async_mmap_readahead page_cache_async_readahead .... do_sync_mmap_readahead page_cache_sync_readahead ..... filesystem drops mmap truncate serialisation lock Then there is fadvise(WILLNEED) which calls force_page_cache_readahead() directly. We now have ->fadvise for filesystems to add locking around this: fadvise64 vfs_fadvise ->fadvise xfs_file_fadvise filesystem takes truncate serialisation lock generic_fadvise() filesystem drops truncate serialisation lock XFS and overlay both have ->fadvise methods to provide this serialisation of readahead, but ext4 does not so users could trigger the stated race condition through fadvise(WILLNEED) or readahead() syscalls. Hence, AFAICT, ext4 only needs to implement ->fadvise and the stated readahead vs truncate_pagecache_range() race condition goes away. Unless, of course, I've missed some other entry point into the readahead code that does not vector through the filesystem first. What have I missed? Cheers, Dave.
On Thu 23-01-20 10:47:40, Dave Chinner wrote: > On Tue, Jan 21, 2020 at 01:48:45PM -0800, Matthew Wilcox wrote: > > On Tue, Jan 21, 2020 at 12:36:27PM +0100, Jan Kara wrote: > > > > v2: Chris asked me to show what this would look like if we just have > > > > the implementation look up the pages in the page cache, and I managed > > > > to figure out some things I'd done wrong last time. It's even simpler > > > > than v1 (net 104 lines deleted). > > > > > > I have an unfinished patch series laying around that pulls the ->readpage > > > / ->readpages API in somewhat different direction so I'd like to discuss > > > whether it's possible to solve my problem using your API. The problem I > > > have is that currently some operations such as hole punching can race with > > > ->readpage / ->readpages like: > > > > > > CPU0 CPU1 > > > fallocate(fd, FALLOC_FL_PUNCH_HOLE, off, len) > > > filemap_write_and_wait_range() > > > down_write(inode->i_rwsem); > > > truncate_pagecache_range(); > > shouldn't fallocate be holding EXT4_I(inode)->i_mmap_sem before it > truncates the page cache? Otherwise it's not serialised against > page faults. Looks at code ... oh, it does hold the i_mmap_sem in > write mode, so.... Yes. > > > readahead(fd, off, len) > > > creates pages in page cache > > > looks up block mapping > > > removes blocks from inode and frees them > > > issues bio > > > - reads stale data - > > > potential security > > > issue > > .... I'm not sure that this race condition should exist anymore > as readahead should not run until the filesystem drops it's inode > and mmap locks after the entire extent freeing operation is > complete... Not for XFS but for all the other filesystems see below.. > > > Now how I wanted to address this is that I'd change the API convention for > > > ->readpage() so that we call it with the page unlocked and the function > > > would lock the page, check it's still OK, and do what it needs. And this > > > will allow ->readpage() and also ->readpages() to grab lock > > > (EXT4_I(inode)->i_mmap_sem in case of ext4) to synchronize with hole punching > > > while we are adding pages to page cache and mapping underlying blocks. > > > > > > Now your API makes even ->readpages() (actually ->readahead) called with > > > pages locked so that makes this approach problematic because of lock > > > inversions. So I'd prefer if we could keep the situation that ->readpages / > > > ->readahead gets called without any pages in page cache locked... > > > > I'm not a huge fan of that approach because it increases the number of > > atomic ops (right now, we __SetPageLocked on the page before adding it > > to i_pages). Holepunch is a rather rare operation while readpage and > > readpages/readahead are extremely common, so can we make holepunch take > > a lock that will prevent new readpage(s) succeeding? > > > > I have an idea to move the lock entries from DAX to being a generic page > > cache concept. That way, holepunch could insert lock entries into the > > pagecache to cover the range being punched, and readpage(s) would either > > skip lock entries or block on them. > > > > Maybe there's a better approach though. > > Can we step back for a moment and look at how we already serialise > readahead against truncate/hole punch? While the readahead code > itself doesn't serialise against truncate, in all cases we should be > running through the filesystem at a higher layer and provides the > truncate/holepunch serialisation before we get to the readahead > code. > > The read() syscall IO path: > > read() > ->read_iter() > filesystem takes truncate serialisation lock > generic_file_read_iter() > generic_file_buffered_read() > page_cache_sync_readahead() > .... > page_cache_async_readahead() > .... > ..... > filesystem drops truncate serialisation lock Yes, this is the scheme XFS uses. But ext4 and other filesystems use a scheme where read is serialized against truncate only by page locks and i_size checks. Which works for truncate but is not enough for hole punching. And locking read(2) and readahead(2) in all these filesystem with i_rwsem is going to cause heavy regressions with mixed read-write workloads and unnecessarily so because we don't need to lock reads against writes, just against truncate or hole punching. So I wanted to use i_mmap_sem for the serialization of the read path against truncate. But due to lock ordering with mmap_sem and because reads do take page faults to copy data it is not straightforward - hence my messing with ->readpage(). Now that I'm thinking about it, there's also a possibility of introducing yet another rwsem into the inode that would rank above mmap_sem and be used to serialize ->read_iter and ->fadvise against truncate. But having three rwsems in the inode for serialization seems a bit too convoluted for my taste. Honza
On Wed 22-01-20 10:44:14, Jan Kara wrote: > On Tue 21-01-20 13:48:45, Matthew Wilcox wrote: > > On Tue, Jan 21, 2020 at 12:36:27PM +0100, Jan Kara wrote: > > > > v2: Chris asked me to show what this would look like if we just have > > > > the implementation look up the pages in the page cache, and I managed > > > > to figure out some things I'd done wrong last time. It's even simpler > > > > than v1 (net 104 lines deleted). > > > > > > I have an unfinished patch series laying around that pulls the ->readpage > > > / ->readpages API in somewhat different direction so I'd like to discuss > > > whether it's possible to solve my problem using your API. The problem I > > > have is that currently some operations such as hole punching can race with > > > ->readpage / ->readpages like: > > > > > > CPU0 CPU1 > > > fallocate(fd, FALLOC_FL_PUNCH_HOLE, off, len) > > > filemap_write_and_wait_range() > > > down_write(inode->i_rwsem); > > > truncate_pagecache_range(); > > > readahead(fd, off, len) > > > creates pages in page cache > > > looks up block mapping > > > removes blocks from inode and frees them > > > issues bio > > > - reads stale data - > > > potential security > > > issue > > > > > > Now how I wanted to address this is that I'd change the API convention for > > > ->readpage() so that we call it with the page unlocked and the function > > > would lock the page, check it's still OK, and do what it needs. And this > > > will allow ->readpage() and also ->readpages() to grab lock > > > (EXT4_I(inode)->i_mmap_sem in case of ext4) to synchronize with hole punching > > > while we are adding pages to page cache and mapping underlying blocks. > > > > > > Now your API makes even ->readpages() (actually ->readahead) called with > > > pages locked so that makes this approach problematic because of lock > > > inversions. So I'd prefer if we could keep the situation that ->readpages / > > > ->readahead gets called without any pages in page cache locked... > > > > I'm not a huge fan of that approach because it increases the number of > > atomic ops (right now, we __SetPageLocked on the page before adding it > > to i_pages). > > Yeah, good point. The per-page cost of locking may be noticeable. Thinking about this a bit more, we should be using ->readpages() to fill most of the data. And for ->readpages() there would be no additional overhead. Just for ->readpage() which should be rarely needed. We just need to come up with a good solution for filesystems that have ->readpage but not ->readpages. Honza
On Thu, Jan 23, 2020 at 11:21:01AM +0100, Jan Kara wrote: > On Thu 23-01-20 10:47:40, Dave Chinner wrote: > > On Tue, Jan 21, 2020 at 01:48:45PM -0800, Matthew Wilcox wrote: > > > On Tue, Jan 21, 2020 at 12:36:27PM +0100, Jan Kara wrote: > > > > > v2: Chris asked me to show what this would look like if we just have > > > > > the implementation look up the pages in the page cache, and I managed > > > > > to figure out some things I'd done wrong last time. It's even simpler > > > > > than v1 (net 104 lines deleted). > > > > > > > > I have an unfinished patch series laying around that pulls the ->readpage > > > > / ->readpages API in somewhat different direction so I'd like to discuss > > > > whether it's possible to solve my problem using your API. The problem I > > > > have is that currently some operations such as hole punching can race with > > > > ->readpage / ->readpages like: > > > > > > > > CPU0 CPU1 > > > > fallocate(fd, FALLOC_FL_PUNCH_HOLE, off, len) > > > > filemap_write_and_wait_range() > > > > down_write(inode->i_rwsem); > > > > truncate_pagecache_range(); > > > > shouldn't fallocate be holding EXT4_I(inode)->i_mmap_sem before it > > truncates the page cache? Otherwise it's not serialised against > > page faults. Looks at code ... oh, it does hold the i_mmap_sem in > > write mode, so.... > > Yes. > > > > > readahead(fd, off, len) > > > > creates pages in page cache > > > > looks up block mapping > > > > removes blocks from inode and frees them > > > > issues bio > > > > - reads stale data - > > > > potential security > > > > issue > > > > .... I'm not sure that this race condition should exist anymore > > as readahead should not run until the filesystem drops it's inode > > and mmap locks after the entire extent freeing operation is > > complete... > > Not for XFS but for all the other filesystems see below.. > > > > > Now how I wanted to address this is that I'd change the API convention for > > > > ->readpage() so that we call it with the page unlocked and the function > > > > would lock the page, check it's still OK, and do what it needs. And this > > > > will allow ->readpage() and also ->readpages() to grab lock > > > > (EXT4_I(inode)->i_mmap_sem in case of ext4) to synchronize with hole punching > > > > while we are adding pages to page cache and mapping underlying blocks. > > > > > > > > Now your API makes even ->readpages() (actually ->readahead) called with > > > > pages locked so that makes this approach problematic because of lock > > > > inversions. So I'd prefer if we could keep the situation that ->readpages / > > > > ->readahead gets called without any pages in page cache locked... > > > > > > I'm not a huge fan of that approach because it increases the number of > > > atomic ops (right now, we __SetPageLocked on the page before adding it > > > to i_pages). Holepunch is a rather rare operation while readpage and > > > readpages/readahead are extremely common, so can we make holepunch take > > > a lock that will prevent new readpage(s) succeeding? > > > > > > I have an idea to move the lock entries from DAX to being a generic page > > > cache concept. That way, holepunch could insert lock entries into the > > > pagecache to cover the range being punched, and readpage(s) would either > > > skip lock entries or block on them. > > > > > > Maybe there's a better approach though. > > > > Can we step back for a moment and look at how we already serialise > > readahead against truncate/hole punch? While the readahead code > > itself doesn't serialise against truncate, in all cases we should be > > running through the filesystem at a higher layer and provides the > > truncate/holepunch serialisation before we get to the readahead > > code. > > > > The read() syscall IO path: > > > > read() > > ->read_iter() > > filesystem takes truncate serialisation lock > > generic_file_read_iter() > > generic_file_buffered_read() > > page_cache_sync_readahead() > > .... > > page_cache_async_readahead() > > .... > > ..... > > filesystem drops truncate serialisation lock > > Yes, this is the scheme XFS uses. But ext4 and other filesystems use a > scheme where read is serialized against truncate only by page locks and > i_size checks. Right, which I've characterised in the past as a "performance over correctness" hack that has resulted in infrastructure that can only protect against truncate and not general page invalidation operations. It's also been the source of many, many truncate bugs over the past 20 years and has never worked for hole punch, yet.... > Which works for truncate but is not enough for hole > punching. And locking read(2) and readahead(2) in all these filesystem with > i_rwsem is going to cause heavy regressions with mixed read-write workloads > and unnecessarily so because we don't need to lock reads against writes, > just against truncate or hole punching. .... performance over correctness is still used as the justification for keeping this ancient, troublesome architecture in place even though it cannot support the requirements of modern filesystems. Indeed, this problem had to be fixed with DAX, and the ext4 DAX read path uses shared inode locks. And now the ext4 DIO path uses shared inode locking, too. Only the ext4 buffered read path does not use shared read locks to avoid this race condition. I don't have a "maintain the existing mixed rw performance" solution for you here, except to say that I'm working (slowly) towards fixing this problem on XFS with range locking for IO instead of using the i_rwsem. That fixes so many other concurrency issues, too, such as allowing fallocate() and ftruncate() to run concurrently with non-overlapping IO to the same file. IOWs, moving IO locking away from the page cache will allow much greater concurrency and performance for a much wider range of filesystem operations than trying to screw around with individual page locks. > So I wanted to use i_mmap_sem for the serialization of the read path against > truncate. But due to lock ordering with mmap_sem and because reads do take > page faults to copy data it is not straightforward - hence my messing with > ->readpage(). Another manifestation of the age old "can't take the same lock in the syscall IO path and the page fault IO path" problem. We've previously looked hacking about readpages in XFS, too, but it was too complex, didn't improve performance, and in general such changes were heading in the opposite direction we have been moving the IO path infrastructure towards with iomap. i.e. improving buffered IO efficiency requires use to reduce the amount of work we do per page, not increase it.... > Now that I'm thinking about it, there's also a possibility of > introducing yet another rwsem into the inode that would rank above > mmap_sem and be used to serialize ->read_iter and ->fadvise against > truncate. But having three rwsems in the inode for serialization seems a > bit too convoluted for my taste. Yup, that's called "falling off the locking cliff". :/ Cheers, Dave.
From: "Matthew Wilcox (Oracle)" <willy@infradead.org> This is an attempt to add a ->readahead op to replace ->readpages. I've converted two users, iomap/xfs and cifs. The cifs conversion is lacking fscache support, and that's just because I didn't want to do that work; I don't believe there's anything fundamental to it. But I wanted to do iomap because it is The Infrastructure Of The Future and cifs because it is the sole remaining user of add_to_page_cache_locked(), which enables the last two patches in the series. By the way, that gives CIFS access to the workingset shadow infrastructure, which it had to ignore before because it couldn't put pages onto the lru list at the right time. v2: Chris asked me to show what this would look like if we just have the implementation look up the pages in the page cache, and I managed to figure out some things I'd done wrong last time. It's even simpler than v1 (net 104 lines deleted). Matthew Wilcox (Oracle) (9): mm: Fix the return type of __do_page_cache_readahead readahead: Ignore return value of ->readpages XArray: Add xarray_for_each_range readahead: Put pages in cache earlier mm: Add readahead address space operation iomap,xfs: Convert from readpages to readahead cifs: Convert from readpages to readahead mm: Remove add_to_page_cache_locked mm: Unify all add_to_page_cache variants Documentation/core-api/xarray.rst | 10 +- Documentation/filesystems/locking.rst | 7 +- Documentation/filesystems/vfs.rst | 11 ++ fs/cifs/file.c | 143 +++++--------------------- fs/iomap/buffered-io.c | 72 +++---------- fs/iomap/trace.h | 2 +- fs/xfs/xfs_aops.c | 10 +- include/linux/fs.h | 2 + include/linux/iomap.h | 2 +- include/linux/pagemap.h | 25 ++--- include/linux/xarray.h | 30 ++++++ mm/filemap.c | 72 ++++--------- mm/internal.h | 2 +- mm/readahead.c | 76 +++++++++----- 14 files changed, 180 insertions(+), 284 deletions(-)