Dave Chinner
2020-Feb-18 06:31 UTC
[Ocfs2-devel] [PATCH v6 09/19] mm: Add page_cache_readahead_limit
On Mon, Feb 17, 2020 at 10:45:56AM -0800, Matthew Wilcox wrote:> From: "Matthew Wilcox (Oracle)" <willy at infradead.org> > > ext4 and f2fs have duplicated the guts of the readahead code so > they can read past i_size. Instead, separate out the guts of the > readahead code so they can call it directly.Gross and nasty (hosting non-stale data beyond EOF in the page cache, that is). Code is pretty simple, but...> } > > -/* > - * __do_page_cache_readahead() actually reads a chunk of disk. It allocates > - * the pages first, then submits them for I/O. This avoids the very bad > - * behaviour which would occur if page allocations are causing VM writeback. > - * We really don't want to intermingle reads and writes like that. > +/** > + * page_cache_readahead_limit - Start readahead beyond a file's i_size. > + * @mapping: File address space. > + * @file: This instance of the open file; used for authentication. > + * @offset: First page index to read. > + * @end_index: The maximum page index to read. > + * @nr_to_read: The number of pages to read. > + * @lookahead_size: Where to start the next readahead. > + * > + * This function is for filesystems to call when they want to start > + * readahead potentially beyond a file's stated i_size. If you want > + * to start readahead on a normal file, you probably want to call > + * page_cache_async_readahead() or page_cache_sync_readahead() instead. > + * > + * Context: File is referenced by caller. Mutexes may be held by caller. > + * May sleep, but will not reenter filesystem to reclaim memory. > */ > -void __do_page_cache_readahead(struct address_space *mapping, > - struct file *filp, pgoff_t offset, unsigned long nr_to_read, > - unsigned long lookahead_size) > +void page_cache_readahead_limit(struct address_space *mapping,... I don't think the function name conveys it's purpose. It's really a ranged readahead that ignores where i_size lies. i.e page_cache_readahead_range(mapping, start, end, nr_to_read) seems like a better API to me, and then you can drop the "start readahead beyond i_size" comments and replace it with "Range is not limited by the inode's i_size and hence can be used to read data stored beyond EOF into the page cache." Also: "This is almost certainly not the function you want to call. Use page_cache_async_readahead or page_cache_sync_readahead() instead." Cheers, Dave. -- Dave Chinner david at fromorbit.com
Matthew Wilcox
2020-Feb-18 19:54 UTC
[Ocfs2-devel] [PATCH v6 09/19] mm: Add page_cache_readahead_limit
On Tue, Feb 18, 2020 at 05:31:10PM +1100, Dave Chinner wrote:> On Mon, Feb 17, 2020 at 10:45:56AM -0800, Matthew Wilcox wrote: > > From: "Matthew Wilcox (Oracle)" <willy at infradead.org> > > > > ext4 and f2fs have duplicated the guts of the readahead code so > > they can read past i_size. Instead, separate out the guts of the > > readahead code so they can call it directly. > > Gross and nasty (hosting non-stale data beyond EOF in the page > cache, that is).I thought you meant sneaking changes into the VFS (that were rejected) by copying VFS code and modifying it ...> > +/** > > + * page_cache_readahead_limit - Start readahead beyond a file's i_size. > > + * @mapping: File address space. > > + * @file: This instance of the open file; used for authentication. > > + * @offset: First page index to read. > > + * @end_index: The maximum page index to read. > > + * @nr_to_read: The number of pages to read. > > + * @lookahead_size: Where to start the next readahead. > > + * > > + * This function is for filesystems to call when they want to start > > + * readahead potentially beyond a file's stated i_size. If you want > > + * to start readahead on a normal file, you probably want to call > > + * page_cache_async_readahead() or page_cache_sync_readahead() instead. > > + * > > + * Context: File is referenced by caller. Mutexes may be held by caller. > > + * May sleep, but will not reenter filesystem to reclaim memory. > > */ > > -void __do_page_cache_readahead(struct address_space *mapping, > > - struct file *filp, pgoff_t offset, unsigned long nr_to_read, > > - unsigned long lookahead_size) > > +void page_cache_readahead_limit(struct address_space *mapping, > > ... I don't think the function name conveys it's purpose. It's > really a ranged readahead that ignores where i_size lies. i.e > > page_cache_readahead_range(mapping, start, end, nr_to_read) > > seems like a better API to me, and then you can drop the "start > readahead beyond i_size" comments and replace it with "Range is not > limited by the inode's i_size and hence can be used to read data > stored beyond EOF into the page cache."I'm concerned that calling it 'range' implies "I want to read between start and end" rather than "I want to read nr_to_read at start, oh but don't go past end". Maybe the right way to do this is have the three callers cap nr_to_read. Well, the one caller ... after all, f2fs and ext4 have no desire to cap the length. Then we can call it page_cache_readahead_exceed() or page_cache_readahead_dangerous() or something else like that to make it clear that you shouldn't be calling it.> Also: "This is almost certainly not the function you want to call. > Use page_cache_async_readahead or page_cache_sync_readahead() > instead."+1 to that ;-) Here's what I currently have:>From d202dda7a92566496fe9e233ee7855fb560324ce Mon Sep 17 00:00:00 2001From: "Matthew Wilcox (Oracle)" <willy at infradead.org> Date: Mon, 10 Feb 2020 18:31:15 -0500 Subject: [PATCH] mm: Add page_cache_readahead_exceed ext4 and f2fs have duplicated the guts of the readahead code so they can read past i_size. Instead, separate out the guts of the readahead code so they can call it directly. Signed-off-by: Matthew Wilcox (Oracle) <willy at infradead.org> --- fs/ext4/verity.c | 35 ++-------------------- fs/f2fs/verity.c | 35 ++-------------------- include/linux/pagemap.h | 3 ++ mm/readahead.c | 66 ++++++++++++++++++++++++++++------------- 4 files changed, 52 insertions(+), 87 deletions(-) diff --git a/fs/ext4/verity.c b/fs/ext4/verity.c index dc5ec724d889..172ebf860014 100644 --- a/fs/ext4/verity.c +++ b/fs/ext4/verity.c @@ -342,37 +342,6 @@ static int ext4_get_verity_descriptor(struct inode *inode, void *buf, return desc_size; } -/* - * Prefetch some pages from the file's Merkle tree. - * - * This is basically a stripped-down version of __do_page_cache_readahead() - * which works on pages past i_size. - */ -static void ext4_merkle_tree_readahead(struct address_space *mapping, - pgoff_t start_index, unsigned long count) -{ - LIST_HEAD(pages); - unsigned int nr_pages = 0; - struct page *page; - pgoff_t index; - struct blk_plug plug; - - for (index = start_index; index < start_index + count; index++) { - page = xa_load(&mapping->i_pages, index); - if (!page || xa_is_value(page)) { - page = __page_cache_alloc(readahead_gfp_mask(mapping)); - if (!page) - break; - page->index = index; - list_add(&page->lru, &pages); - nr_pages++; - } - } - blk_start_plug(&plug); - ext4_mpage_readpages(mapping, &pages, NULL, nr_pages, true); - blk_finish_plug(&plug); -} - static struct page *ext4_read_merkle_tree_page(struct inode *inode, pgoff_t index, unsigned long num_ra_pages) @@ -386,8 +355,8 @@ static struct page *ext4_read_merkle_tree_page(struct inode *inode, if (page) put_page(page); else if (num_ra_pages > 1) - ext4_merkle_tree_readahead(inode->i_mapping, index, - num_ra_pages); + page_cache_readahead_exceed(inode->i_mapping, NULL, + index, num_ra_pages, 0); page = read_mapping_page(inode->i_mapping, index, NULL); } return page; diff --git a/fs/f2fs/verity.c b/fs/f2fs/verity.c index d7d430a6f130..f240ad087162 100644 --- a/fs/f2fs/verity.c +++ b/fs/f2fs/verity.c @@ -222,37 +222,6 @@ static int f2fs_get_verity_descriptor(struct inode *inode, void *buf, return size; } -/* - * Prefetch some pages from the file's Merkle tree. - * - * This is basically a stripped-down version of __do_page_cache_readahead() - * which works on pages past i_size. - */ -static void f2fs_merkle_tree_readahead(struct address_space *mapping, - pgoff_t start_index, unsigned long count) -{ - LIST_HEAD(pages); - unsigned int nr_pages = 0; - struct page *page; - pgoff_t index; - struct blk_plug plug; - - for (index = start_index; index < start_index + count; index++) { - page = xa_load(&mapping->i_pages, index); - if (!page || xa_is_value(page)) { - page = __page_cache_alloc(readahead_gfp_mask(mapping)); - if (!page) - break; - page->index = index; - list_add(&page->lru, &pages); - nr_pages++; - } - } - blk_start_plug(&plug); - f2fs_mpage_readpages(mapping, &pages, NULL, nr_pages, true); - blk_finish_plug(&plug); -} - static struct page *f2fs_read_merkle_tree_page(struct inode *inode, pgoff_t index, unsigned long num_ra_pages) @@ -266,8 +235,8 @@ static struct page *f2fs_read_merkle_tree_page(struct inode *inode, if (page) put_page(page); else if (num_ra_pages > 1) - f2fs_merkle_tree_readahead(inode->i_mapping, index, - num_ra_pages); + page_cache_readahead_exceed(inode->i_mapping, NULL, + index, num_ra_pages, 0); page = read_mapping_page(inode->i_mapping, index, NULL); } return page; diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h index 48c3bca57df6..1f7964d2b8ca 100644 --- a/include/linux/pagemap.h +++ b/include/linux/pagemap.h @@ -623,6 +623,9 @@ void page_cache_sync_readahead(struct address_space *, struct file_ra_state *, void page_cache_async_readahead(struct address_space *, struct file_ra_state *, struct file *, struct page *, pgoff_t index, unsigned long req_count); +void page_cache_readahead_exceed(struct address_space *, struct file *, + pgoff_t index, unsigned long nr_to_read, + unsigned long lookahead_count); /* * Like add_to_page_cache_locked, but used to add newly allocated pages: diff --git a/mm/readahead.c b/mm/readahead.c index 9dd431fa16c9..cad26287ad8b 100644 --- a/mm/readahead.c +++ b/mm/readahead.c @@ -142,45 +142,43 @@ static void read_pages(struct readahead_control *rac, struct list_head *pages) blk_finish_plug(&plug); } -/* - * __do_page_cache_readahead() actually reads a chunk of disk. It allocates - * the pages first, then submits them for I/O. This avoids the very bad - * behaviour which would occur if page allocations are causing VM writeback. - * We really don't want to intermingle reads and writes like that. +/** + * page_cache_readahead_exceed - Start unchecked readahead. + * @mapping: File address space. + * @file: This instance of the open file; used for authentication. + * @index: First page index to read. + * @nr_to_read: The number of pages to read. + * @lookahead_size: Where to start the next readahead. + * + * This function is for filesystems to call when they want to start + * readahead beyond a file's stated i_size. This is almost certainly + * not the function you want to call. Use page_cache_async_readahead() + * or page_cache_sync_readahead() instead. + * + * Context: File is referenced by caller. Mutexes may be held by caller. + * May sleep, but will not reenter filesystem to reclaim memory. */ -void __do_page_cache_readahead(struct address_space *mapping, - struct file *filp, pgoff_t index, unsigned long nr_to_read, +void page_cache_readahead_exceed(struct address_space *mapping, + struct file *file, pgoff_t index, unsigned long nr_to_read, unsigned long lookahead_size) { - struct inode *inode = mapping->host; - unsigned long end_index; /* The last page we want to read */ LIST_HEAD(page_pool); unsigned long i; - loff_t isize = i_size_read(inode); gfp_t gfp_mask = readahead_gfp_mask(mapping); bool use_list = mapping->a_ops->readpages; struct readahead_control rac = { .mapping = mapping, - .file = filp, + .file = file, ._start = index, ._nr_pages = 0, }; - if (isize == 0) - return; - - end_index = ((isize - 1) >> PAGE_SHIFT); - /* * Preallocate as many pages as we will need. */ for (i = 0; i < nr_to_read; i++) { - struct page *page; - - if (index > end_index) - break; + struct page *page = xa_load(&mapping->i_pages, index); - page = xa_load(&mapping->i_pages, index); if (page && !xa_is_value(page)) { /* * Page already present? Kick off the current batch @@ -225,6 +223,32 @@ void __do_page_cache_readahead(struct address_space *mapping, read_pages(&rac, &page_pool); BUG_ON(!list_empty(&page_pool)); } +EXPORT_SYMBOL_GPL(page_cache_readahead_exceed); + +/* + * __do_page_cache_readahead() actually reads a chunk of disk. It allocates + * the pages first, then submits them for I/O. This avoids the very bad + * behaviour which would occur if page allocations are causing VM writeback. + * We really don't want to intermingle reads and writes like that. + */ +void __do_page_cache_readahead(struct address_space *mapping, + struct file *file, pgoff_t index, unsigned long nr_to_read, + unsigned long lookahead_size) +{ + struct inode *inode = mapping->host; + loff_t isize = i_size_read(inode); + pgoff_t end_index; + + if (isize == 0) + return; + + end_index = (isize - 1) >> PAGE_SHIFT; + if (end_index < index + nr_to_read) + nr_to_read = end_index - index; + + page_cache_readahead_exceed(mapping, file, index, nr_to_read, + lookahead_size); +} /* * Chunk the readahead into 2 megabyte units, so that we don't pin too much -- 2.25.0