| Message ID | 20181130195812.19536-3-josef@toxicpanda.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | drop the mmap_sem when doing IO in the fault path | expand |
On Fri, Nov 30, 2018 at 02:58:10PM -0500, Josef Bacik wrote: > If we do not have a page at filemap_fault time we'll do this weird > forced page_cache_read thing to populate the page, and then drop it > again and loop around and find it. This makes for 2 ways we can read a > page in filemap_fault, and it's not really needed. Instead add a > FGP_FOR_MMAP flag so that pagecache_get_page() will return a unlocked > page that's in pagecache. Then use the normal page locking and readpage > logic already in filemap_fault. This simplifies the no page in page > cache case significantly. > > Signed-off-by: Josef Bacik <josef@toxicpanda.com> That's a great simplification. Looks correct to me. Acked-by: Johannes Weiner <hannes@cmpxchg.org>
On Fri 30-11-18 14:58:10, Josef Bacik wrote: > If we do not have a page at filemap_fault time we'll do this weird > forced page_cache_read thing to populate the page, and then drop it > again and loop around and find it. This makes for 2 ways we can read a > page in filemap_fault, and it's not really needed. Instead add a > FGP_FOR_MMAP flag so that pagecache_get_page() will return a unlocked > page that's in pagecache. Then use the normal page locking and readpage > logic already in filemap_fault. This simplifies the no page in page > cache case significantly. > > Signed-off-by: Josef Bacik <josef@toxicpanda.com> Thanks for the patch. I like the simplification but I think it could be even improved... see below. > @@ -2449,9 +2426,11 @@ vm_fault_t filemap_fault(struct vm_fault *vmf) > count_memcg_event_mm(vmf->vma->vm_mm, PGMAJFAULT); > ret = VM_FAULT_MAJOR; > retry_find: > - page = find_get_page(mapping, offset); > + page = pagecache_get_page(mapping, offset, > + FGP_CREAT|FGP_FOR_MMAP, > + vmf->gfp_mask); > if (!page) > - goto no_cached_page; > + return vmf_error(-ENOMEM); So why don't you just do: page = pagecache_get_page(mapping, offset, FGP_CREAT | FGP_LOCK, vmf->gfp_mask); if (!page) return vmf_error(-ENOMEM); goto check_uptodate; where check_uptodate would be a label before 'PageUptodate' check? Then you don't have to introduce new flag for pagecache_get_page() and you also don't have to unlock and then lock again the page... And you can still delete all the code you've deleted. Honza
On Fri 07-12-18 10:57:50, Jan Kara wrote: > On Fri 30-11-18 14:58:10, Josef Bacik wrote: > > If we do not have a page at filemap_fault time we'll do this weird > > forced page_cache_read thing to populate the page, and then drop it > > again and loop around and find it. This makes for 2 ways we can read a > > page in filemap_fault, and it's not really needed. Instead add a > > FGP_FOR_MMAP flag so that pagecache_get_page() will return a unlocked > > page that's in pagecache. Then use the normal page locking and readpage > > logic already in filemap_fault. This simplifies the no page in page > > cache case significantly. > > > > Signed-off-by: Josef Bacik <josef@toxicpanda.com> > > Thanks for the patch. I like the simplification but I think it could be > even improved... see below. > > > @@ -2449,9 +2426,11 @@ vm_fault_t filemap_fault(struct vm_fault *vmf) > > count_memcg_event_mm(vmf->vma->vm_mm, PGMAJFAULT); > > ret = VM_FAULT_MAJOR; > > retry_find: > > - page = find_get_page(mapping, offset); > > + page = pagecache_get_page(mapping, offset, > > + FGP_CREAT|FGP_FOR_MMAP, > > + vmf->gfp_mask); > > if (!page) > > - goto no_cached_page; > > + return vmf_error(-ENOMEM); > > So why don't you just do: > > page = pagecache_get_page(mapping, offset, > FGP_CREAT | FGP_LOCK, vmf->gfp_mask); > if (!page) > return vmf_error(-ENOMEM); > goto check_uptodate; > > where check_uptodate would be a label before 'PageUptodate' check? > > Then you don't have to introduce new flag for pagecache_get_page() and you > also don't have to unlock and then lock again the page... And you can still > delete all the code you've deleted. Ah, you don't want lock_page() to block in case someone raced with you and instantiated the page so that you can drop mmap_sem. OK, the patch looks good to me then. You can add: Reviewed-by: Jan Kara <jack@suse.cz> Honza
diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h index 226f96f0dee0..b13c2442281f 100644 --- a/include/linux/pagemap.h +++ b/include/linux/pagemap.h @@ -252,6 +252,7 @@ pgoff_t page_cache_prev_miss(struct address_space *mapping, #define FGP_WRITE 0x00000008 #define FGP_NOFS 0x00000010 #define FGP_NOWAIT 0x00000020 +#define FGP_FOR_MMAP 0x00000040 struct page *pagecache_get_page(struct address_space *mapping, pgoff_t offset, int fgp_flags, gfp_t cache_gfp_mask); diff --git a/mm/filemap.c b/mm/filemap.c index 81adec8ee02c..f068712c2525 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -1503,6 +1503,9 @@ EXPORT_SYMBOL(find_lock_entry); * @gfp_mask and added to the page cache and the VM's LRU * list. The page is returned locked and with an increased * refcount. Otherwise, NULL is returned. + * - FGP_FOR_MMAP: Similar to FGP_CREAT, only it unlocks the page after it has + * added it to pagecache, as the mmap code expects to do it's own special + * locking dance. * * If FGP_LOCK or FGP_CREAT are specified then the function may sleep even * if the GFP flags specified for FGP_CREAT are atomic. @@ -1555,7 +1558,7 @@ struct page *pagecache_get_page(struct address_space *mapping, pgoff_t offset, if (!page) return NULL; - if (WARN_ON_ONCE(!(fgp_flags & FGP_LOCK))) + if (WARN_ON_ONCE(!(fgp_flags & (FGP_LOCK | FGP_FOR_MMAP)))) fgp_flags |= FGP_LOCK; /* Init accessed so avoid atomic mark_page_accessed later */ @@ -1569,6 +1572,13 @@ struct page *pagecache_get_page(struct address_space *mapping, pgoff_t offset, if (err == -EEXIST) goto repeat; } + + /* + * add_to_page_cache_lru lock's the page, and for mmap we expect + * a unlocked page. + */ + if (fgp_flags & FGP_FOR_MMAP) + unlock_page(page); } return page; @@ -2293,39 +2303,6 @@ generic_file_read_iter(struct kiocb *iocb, struct iov_iter *iter) EXPORT_SYMBOL(generic_file_read_iter); #ifdef CONFIG_MMU -/** - * page_cache_read - adds requested page to the page cache if not already there - * @file: file to read - * @offset: page index - * @gfp_mask: memory allocation flags - * - * This adds the requested page to the page cache if it isn't already there, - * and schedules an I/O to read in its contents from disk. - */ -static int page_cache_read(struct file *file, pgoff_t offset, gfp_t gfp_mask) -{ - struct address_space *mapping = file->f_mapping; - struct page *page; - int ret; - - do { - page = __page_cache_alloc(gfp_mask); - if (!page) - return -ENOMEM; - - ret = add_to_page_cache_lru(page, mapping, offset, gfp_mask); - if (ret == 0) - ret = mapping->a_ops->readpage(file, page); - else if (ret == -EEXIST) - ret = 0; /* losing race to add is OK */ - - put_page(page); - - } while (ret == AOP_TRUNCATED_PAGE); - - return ret; -} - #define MMAP_LOTSAMISS (100) /* @@ -2449,9 +2426,11 @@ vm_fault_t filemap_fault(struct vm_fault *vmf) count_memcg_event_mm(vmf->vma->vm_mm, PGMAJFAULT); ret = VM_FAULT_MAJOR; retry_find: - page = find_get_page(mapping, offset); + page = pagecache_get_page(mapping, offset, + FGP_CREAT|FGP_FOR_MMAP, + vmf->gfp_mask); if (!page) - goto no_cached_page; + return vmf_error(-ENOMEM); } if (!lock_page_or_retry(page, vmf->vma->vm_mm, vmf->flags)) { @@ -2488,28 +2467,6 @@ vm_fault_t filemap_fault(struct vm_fault *vmf) vmf->page = page; return ret | VM_FAULT_LOCKED; -no_cached_page: - /* - * We're only likely to ever get here if MADV_RANDOM is in - * effect. - */ - error = page_cache_read(file, offset, vmf->gfp_mask); - - /* - * The page we want has now been added to the page cache. - * In the unlikely event that someone removed it in the - * meantime, we'll just come back here and read it again. - */ - if (error >= 0) - goto retry_find; - - /* - * An error return from page_cache_read can result if the - * system is low on memory, or a problem occurs while trying - * to schedule I/O. - */ - return vmf_error(error); - page_not_uptodate: /* * Umm, take care of errors if the page isn't up-to-date.
If we do not have a page at filemap_fault time we'll do this weird forced page_cache_read thing to populate the page, and then drop it again and loop around and find it. This makes for 2 ways we can read a page in filemap_fault, and it's not really needed. Instead add a FGP_FOR_MMAP flag so that pagecache_get_page() will return a unlocked page that's in pagecache. Then use the normal page locking and readpage logic already in filemap_fault. This simplifies the no page in page cache case significantly. Signed-off-by: Josef Bacik <josef@toxicpanda.com> --- include/linux/pagemap.h | 1 + mm/filemap.c | 73 ++++++++++--------------------------------------- 2 files changed, 16 insertions(+), 58 deletions(-)