| Message ID | 20200319150805.uaggnfue5xgaougx@fiona (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [v2] iomap: return partial I/O count on error in iomap_dio_bio_actor | expand |
I spent a fair amount of time looking over this change, and I am starting to feel very bad about it. iomap_apply() has pretty clear semantics of either return an error, or return the bytes processed, and in general these semantics work just fine. The thing that breaks this concept is the btrfs submit_bio hook, which allows the file system to keep state for each bio actually submitted. But I think you can simply keep the length internally in btrfs - use the space in iomap->private as a counter of how much was allocated, pass the iomap to the submit_io hook, and update it there, and then deal with the rest in ->iomap_end. That assumes ->iomap_end actually is the right place - can someone explain what the expected call site for __endio_write_update_ordered is? It kinda sorta looks to me like something that would want to be called after I/O completion, not after I/O submission, but maybe I misunderstand the code.
On 3/20/20 10:05 AM, Christoph Hellwig wrote: > I spent a fair amount of time looking over this change, and I am > starting to feel very bad about it. iomap_apply() has pretty clear > semantics of either return an error, or return the bytes processed, > and in general these semantics work just fine. > > The thing that breaks this concept is the btrfs submit_bio hook, > which allows the file system to keep state for each bio actually > submitted. But I think you can simply keep the length internally > in btrfs - use the space in iomap->private as a counter of how > much was allocated, pass the iomap to the submit_io hook, and > update it there, and then deal with the rest in ->iomap_end. > > That assumes ->iomap_end actually is the right place - can someone > explain what the expected call site for __endio_write_update_ordered > is? It kinda sorta looks to me like something that would want to > be called after I/O completion, not after I/O submission, but maybe > I misunderstand the code. > I'm not sure what you're looking at specifically wrt error handling, but I can explain __endio_write_update_ordered. Btrfs has ordered extents to keep track of an extent that currently has IO being done on it. Generally that IO takes multiple bio's, so we keep track of the outstanding size of the IO being done, and each bio completes and thus removes its size from the pending size. If any one of those bios has an error we need to make sure we discard the whole ordered extent, as part of it won't be valid. Just a cursory look at the current code I assume that's what's confusing you, we call this when we have an error in the O_DIRECT code. This is just so we get the proper cleanup for the ordered extent. People will wait on the ordered extent to be completed, so if we've started an ordered extent and aren't able to complete the range we need to do __endio_write_update_ordered() so that the ordered extent is finished and we wakeup any waiters. Does this help? If I need to I can context switch into whatever you're looking at, but I'm going to avoid looking and hope I can just shout useful information in your direction ;). Thanks, Josef
On Fri, Mar 20, 2020 at 10:23:43AM -0400, Josef Bacik wrote: > I'm not sure what you're looking at specifically wrt error handling, but I > can explain __endio_write_update_ordered. > > Btrfs has ordered extents to keep track of an extent that currently has IO > being done on it. Generally that IO takes multiple bio's, so we keep track > of the outstanding size of the IO being done, and each bio completes and > thus removes its size from the pending size. If any one of those bios has > an error we need to make sure we discard the whole ordered extent, as part > of it won't be valid. Just a cursory look at the current code I assume > that's what's confusing you, we call this when we have an error in the > O_DIRECT code. This is just so we get the proper cleanup for the ordered > extent. People will wait on the ordered extent to be completed, so if we've > started an ordered extent and aren't able to complete the range we need to > do __endio_write_update_ordered() so that the ordered extent is finished and > we wakeup any waiters. > > Does this help? If I need to I can context switch into whatever you're > looking at, but I'm going to avoid looking and hope I can just shout useful > information in your direction ;). Thanks, Yes, this helps a lot. This is about the patches from Goldwyn to convert btrfs to use the iomap direct I/O code. And in that series he currently calls __endio_write_update_ordered from the ->iomap_end method, which for direct I/O is called after all bios are submitted to complete ordered extents for a range after an I/O error, that is one that no I/O has been submitted to, and the accounting for that is a little complicated..
On 7:35 20/03, Christoph Hellwig wrote: > On Fri, Mar 20, 2020 at 10:23:43AM -0400, Josef Bacik wrote: > > I'm not sure what you're looking at specifically wrt error handling, but I > > can explain __endio_write_update_ordered. > > > > Btrfs has ordered extents to keep track of an extent that currently has IO > > being done on it. Generally that IO takes multiple bio's, so we keep track > > of the outstanding size of the IO being done, and each bio completes and > > thus removes its size from the pending size. If any one of those bios has > > an error we need to make sure we discard the whole ordered extent, as part > > of it won't be valid. Just a cursory look at the current code I assume > > that's what's confusing you, we call this when we have an error in the > > O_DIRECT code. This is just so we get the proper cleanup for the ordered > > extent. People will wait on the ordered extent to be completed, so if we've > > started an ordered extent and aren't able to complete the range we need to > > do __endio_write_update_ordered() so that the ordered extent is finished and > > we wakeup any waiters. > > > > Does this help? If I need to I can context switch into whatever you're > > looking at, but I'm going to avoid looking and hope I can just shout useful > > information in your direction ;). Thanks, > > Yes, this helps a lot. This is about the patches from Goldwyn to > convert btrfs to use the iomap direct I/O code. And in that series > he currently calls __endio_write_update_ordered from the ->iomap_end > method, which for direct I/O is called after all bios are submitted > to complete ordered extents for a range after an I/O error, that > is one that no I/O has been submitted to, and the accounting for that > is a little complicated.. I think you meant "some" instead of "no". Yes, keeping the information in iomap->private and setting in btrfs_submit_direct() would be better. I will modify the code and re-test. Thanks!
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c index fa0ff78..d52c70f 100644 --- a/fs/ext4/inode.c +++ b/fs/ext4/inode.c @@ -3475,7 +3475,7 @@ static int ext4_iomap_end(struct inode *inode, loff_t offset, loff_t length, * the I/O. Any blocks that may have been allocated in preparation for * the direct I/O will be reused during buffered I/O. */ - if (flags & (IOMAP_WRITE | IOMAP_DIRECT) && written == 0) + if (flags & (IOMAP_WRITE | IOMAP_DIRECT) && written < length) return -ENOTBLK; return 0; diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c index 41c1e7c..b5f4d4a 100644 --- a/fs/iomap/direct-io.c +++ b/fs/iomap/direct-io.c @@ -264,7 +264,7 @@ iomap_dio_bio_actor(struct inode *inode, loff_t pos, loff_t length, size_t n; if (dio->error) { iov_iter_revert(dio->submit.iter, copied); - copied = ret = 0; + ret = dio->error; goto out; } @@ -325,8 +325,17 @@ iomap_dio_bio_actor(struct inode *inode, loff_t pos, loff_t length, iomap_dio_zero(dio, iomap, pos, fs_block_size - pad); } out: - /* Undo iter limitation to current extent */ - iov_iter_reexpand(dio->submit.iter, orig_count - copied); + /* + * Undo iter limitation to current extent + * If there is an error, undo the entire extent. However, return the + * bytes copied so far for filesystems such as btrfs to account for + * submitted I/O. + */ + if (ret < 0) + iov_iter_reexpand(dio->submit.iter, orig_count); + else + iov_iter_reexpand(dio->submit.iter, orig_count - copied); + if (copied) return copied; return ret; @@ -499,6 +508,10 @@ iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter, do { ret = iomap_apply(inode, pos, count, flags, ops, dio, iomap_dio_actor); + + if (ret >= 0 && dio->error) + ret = dio->error; + if (ret <= 0) { /* magic error code to fall back to buffered I/O */ if (ret == -ENOTBLK) {
Currently, I/Os that complete with an error indicate this by passing written == 0 to the iomap_end function. However, btrfs needs to know how many bytes were written for its own accounting. Change the convention to pass the number of bytes which were actually written, and change the only user (ext4) to check for a short write instead of a zero length write. For filesystems that do not define ->iomap_end(), check for dio->error again after the iomap_apply() call to diagnose the error. Changes since v1: - Considerate of iov_iter rollback functions - Double check errors for filesystems not implementing iomap_end() Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>