[RFC PATCH 0/7] mm: Make more use of readahead_control

[RFC PATCH 0/7] mm: Make more use of readahead_control

[David Howells]

Hi Willy,

Here's a set of patches to expand the use of the readahead_control struct,
essentially from do_sync_mmap_readahead() down.  Note that I've been
passing the number of pages to read in rac->_nr_pages, and then saving it
and changing it certain points, e.g. page_cache_readahead_unbounded().

Also pass file_ra_state into force_page_cache_readahead().

Also there's an apparent minor bug in khugepaged.c that I've included a
patch for: page_cache_sync_readahead() looks to be given the wrong size in
collapse_file().

David
---
David Howells (7):
      Fix khugepaged's request size in collapse_file()
      mm: Make ondemand_readahead() take a readahead_control struct
      mm: Push readahead_control down into force_page_cache_readahead()
      mm: Pass readahead_control into page_cache_{sync,async}_readahead()
      mm: Make __do_page_cache_readahead() use rac->_nr_pages
      mm: Fold ra_submit() into do_sync_mmap_readahead()
      mm: Pass a file_ra_state struct into force_page_cache_readahead()


 fs/btrfs/free-space-cache.c |  7 +--
 fs/btrfs/ioctl.c            | 10 +++--
 fs/btrfs/relocation.c       | 14 +++---
 fs/btrfs/send.c             | 15 ++++---
 fs/ext4/dir.c               | 12 ++---
 fs/ext4/verity.c            |  8 ++--
 fs/f2fs/dir.c               | 10 +++--
 fs/f2fs/verity.c            |  8 ++--
 include/linux/pagemap.h     | 11 ++---
 mm/fadvise.c                |  6 ++-
 mm/filemap.c                | 33 +++++++-------
 mm/internal.h               | 16 +------
 mm/khugepaged.c             |  6 +--
 mm/readahead.c              | 89 ++++++++++++++++++-------------------
 14 files changed, 127 insertions(+), 118 deletions(-)

[RFC PATCH 1/7] Fix khugepaged's request size in collapse_file()

[David Howells]

collapse_file() in khugepaged passes PAGE_SIZE as the number of pages to be
read ahead to page_cache_sync_readahead().  It seems this was expressed as a
number of bytes rather than a number of pages.

Fix it to use the number of pages to the end of the window instead.

Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem) FS")
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Matthew Wilcox <willy@infradead.org>
cc: Song Liu <songliubraving@fb.com>
---

 mm/khugepaged.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 6d199c353281..f2d243077b74 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1706,7 +1706,7 @@ static void collapse_file(struct mm_struct *mm,
 				xas_unlock_irq(&xas);
 				page_cache_sync_readahead(mapping, &file->f_ra,
 							  file, index,
-							  PAGE_SIZE);
+							  end - index);
 				/* drain pagevecs to help isolate_lru_page() */
 				lru_add_drain();
 				page = find_lock_page(mapping, index);

Re: [RFC PATCH 1/7] Fix khugepaged's request size in collapse_file()

[Song Liu]

On Tue, Sep 1, 2020 at 9:28 AM David Howells <dhowells@redhat.com> wrote:
>
> collapse_file() in khugepaged passes PAGE_SIZE as the number of pages to be
> read ahead to page_cache_sync_readahead().  It seems this was expressed as a
> number of bytes rather than a number of pages.
>
> Fix it to use the number of pages to the end of the window instead.
>
> Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem) FS")
> Signed-off-by: David Howells <dhowells@redhat.com>
> cc: Matthew Wilcox <willy@infradead.org>
> cc: Song Liu <songliubraving@fb.com>

Thanks for the fix!

Acked-by: Song Liu <songliubraving@fb.com>

> ---
>
>  mm/khugepaged.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 6d199c353281..f2d243077b74 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1706,7 +1706,7 @@ static void collapse_file(struct mm_struct *mm,
>                                 xas_unlock_irq(&xas);
>                                 page_cache_sync_readahead(mapping, &file->f_ra,
>                                                           file, index,
> -                                                         PAGE_SIZE);
> +                                                         end - index);
>                                 /* drain pagevecs to help isolate_lru_page() */
>                                 lru_add_drain();
>                                 page = find_lock_page(mapping, index);
>
>
>

[RFC PATCH 2/7] mm: Make ondemand_readahead() take a readahead_control struct

[David Howells]

Make ondemand_readahead() take a readahead_control struct in preparation
for making do_sync_mmap_readahead() pass down an RAC struct.

Signed-off-by: David Howells <dhowells@redhat.com>
---

 mm/readahead.c |   35 +++++++++++++++++++++--------------
 1 file changed, 21 insertions(+), 14 deletions(-)

diff --git a/mm/readahead.c b/mm/readahead.c
index 91859e6e2b7d..0e16fb4809f5 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -511,14 +511,15 @@ static bool page_cache_readahead_order(struct readahead_control *rac,
 /*
  * A minimal readahead algorithm for trivial sequential/random reads.
  */
-static void ondemand_readahead(struct address_space *mapping,
-		struct file_ra_state *ra, struct file *file,
-		struct page *page, pgoff_t index, unsigned long req_size)
+static void ondemand_readahead(struct readahead_control *rac,
+			       struct file_ra_state *ra,
+			       struct page *page)
 {
-	DEFINE_READAHEAD(rac, file, mapping, index);
-	struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
+	struct backing_dev_info *bdi = inode_to_bdi(rac->mapping->host);
 	unsigned long max_pages = ra->ra_pages;
 	unsigned long add_pages;
+	unsigned long req_size = rac->_nr_pages;
+	unsigned long index = rac->_index;
 	pgoff_t prev_index;
 
 	/*
@@ -556,7 +557,7 @@ static void ondemand_readahead(struct address_space *mapping,
 		pgoff_t start;
 
 		rcu_read_lock();
-		start = page_cache_next_miss(mapping, index + 1, max_pages);
+		start = page_cache_next_miss(rac->mapping, index + 1, max_pages);
 		rcu_read_unlock();
 
 		if (!start || start - index > max_pages)
@@ -589,14 +590,14 @@ static void ondemand_readahead(struct address_space *mapping,
 	 * Query the page cache and look for the traces(cached history pages)
 	 * that a sequential stream would leave behind.
 	 */
-	if (try_context_readahead(mapping, ra, index, req_size, max_pages))
+	if (try_context_readahead(rac->mapping, ra, index, req_size, max_pages))
 		goto readit;
 
 	/*
 	 * standalone, small random read
 	 * Read as is, and do not pollute the readahead state.
 	 */
-	__do_page_cache_readahead(&rac, req_size, 0);
+	__do_page_cache_readahead(rac, req_size, 0);
 	return;
 
 initial_readahead:
@@ -622,10 +623,10 @@ static void ondemand_readahead(struct address_space *mapping,
 		}
 	}
 
-	rac._index = ra->start;
-	if (page && page_cache_readahead_order(&rac, ra, thp_order(page)))
+	rac->_index = ra->start;
+	if (page && page_cache_readahead_order(rac, ra, thp_order(page)))
 		return;
-	__do_page_cache_readahead(&rac, ra->size, ra->async_size);
+	__do_page_cache_readahead(rac, ra->size, ra->async_size);
 }
 
 /**
@@ -645,6 +646,9 @@ void page_cache_sync_readahead(struct address_space *mapping,
 			       struct file_ra_state *ra, struct file *filp,
 			       pgoff_t index, unsigned long req_count)
 {
+	DEFINE_READAHEAD(rac, filp, mapping, index);
+	rac._nr_pages = req_count;
+
 	/* no read-ahead */
 	if (!ra->ra_pages)
 		return;
@@ -659,7 +663,7 @@ void page_cache_sync_readahead(struct address_space *mapping,
 	}
 
 	/* do read-ahead */
-	ondemand_readahead(mapping, ra, filp, NULL, index, req_count);
+	ondemand_readahead(&rac, ra, NULL);
 }
 EXPORT_SYMBOL_GPL(page_cache_sync_readahead);
 
@@ -683,7 +687,10 @@ page_cache_async_readahead(struct address_space *mapping,
 			   struct page *page, pgoff_t index,
 			   unsigned long req_count)
 {
-	/* no read-ahead */
+	DEFINE_READAHEAD(rac, filp, mapping, index);
+	rac._nr_pages = req_count;
+
+	/* No Read-ahead */
 	if (!ra->ra_pages)
 		return;
 
@@ -705,7 +712,7 @@ page_cache_async_readahead(struct address_space *mapping,
 		return;
 
 	/* do read-ahead */
-	ondemand_readahead(mapping, ra, filp, page, index, req_count);
+	ondemand_readahead(&rac, ra, page);
 }
 EXPORT_SYMBOL_GPL(page_cache_async_readahead);
 

[RFC PATCH 3/7] mm: Push readahead_control down into force_page_cache_readahead()

[David Howells]

Push readahead_control down into force_page_cache_readahead() from its
callers in preparation for making do_sync_mmap_readahead() pass down an RAC
struct.

Signed-off-by: David Howells <dhowells@redhat.com>
---

 mm/fadvise.c   |    6 +++++-
 mm/internal.h  |    3 +--
 mm/readahead.c |   20 ++++++++++++--------
 3 files changed, 18 insertions(+), 11 deletions(-)

diff --git a/mm/fadvise.c b/mm/fadvise.c
index 0e66f2aaeea3..b68d2f2959d5 100644
--- a/mm/fadvise.c
+++ b/mm/fadvise.c
@@ -104,7 +104,11 @@ int generic_fadvise(struct file *file, loff_t offset, loff_t len, int advice)
 		if (!nrpages)
 			nrpages = ~0UL;
 
-		force_page_cache_readahead(mapping, file, start_index, nrpages);
+		{
+			DEFINE_READAHEAD(rac, file, mapping, start_index);
+			rac._nr_pages = nrpages;
+			force_page_cache_readahead(&rac);
+		}
 		break;
 	case POSIX_FADV_NOREUSE:
 		break;
diff --git a/mm/internal.h b/mm/internal.h
index bf2bee6c42a1..2eb9f7f5f134 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -49,8 +49,7 @@ void unmap_page_range(struct mmu_gather *tlb,
 			     unsigned long addr, unsigned long end,
 			     struct zap_details *details);
 
-void force_page_cache_readahead(struct address_space *, struct file *,
-		pgoff_t index, unsigned long nr_to_read);
+void force_page_cache_readahead(struct readahead_control *);
 void __do_page_cache_readahead(struct readahead_control *,
 		unsigned long nr_to_read, unsigned long lookahead_size);
 
diff --git a/mm/readahead.c b/mm/readahead.c
index 0e16fb4809f5..e557c6d5a183 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -271,13 +271,12 @@ void __do_page_cache_readahead(struct readahead_control *rac,
  * Chunk the readahead into 2 megabyte units, so that we don't pin too much
  * memory at once.
  */
-void force_page_cache_readahead(struct address_space *mapping,
-		struct file *file, pgoff_t index, unsigned long nr_to_read)
+void force_page_cache_readahead(struct readahead_control *rac)
 {
-	DEFINE_READAHEAD(rac, file, mapping, index);
+	struct address_space *mapping = rac->mapping;
 	struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
-	struct file_ra_state *ra = &file->f_ra;
-	unsigned long max_pages;
+	struct file_ra_state *ra = &rac->file->f_ra;
+	unsigned long max_pages, index, nr_to_read;
 
 	if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages &&
 			!mapping->a_ops->readahead))
@@ -287,14 +286,19 @@ void force_page_cache_readahead(struct address_space *mapping,
 	 * If the request exceeds the readahead window, allow the read to
 	 * be up to the optimal hardware IO size
 	 */
+	index = readahead_index(rac);
 	max_pages = max_t(unsigned long, bdi->io_pages, ra->ra_pages);
-	nr_to_read = min(nr_to_read, max_pages);
+	nr_to_read = min_t(unsigned long, readahead_count(rac), max_pages);
 	while (nr_to_read) {
 		unsigned long this_chunk = (2 * 1024 * 1024) / PAGE_SIZE;
 
 		if (this_chunk > nr_to_read)
 			this_chunk = nr_to_read;
-		__do_page_cache_readahead(&rac, this_chunk, 0);
+
+		rac->_index = index;
+		rac->_nr_pages = this_chunk;
+		// Do I need to modify rac->_batch_count?
+		__do_page_cache_readahead(rac, this_chunk, 0);
 
 		index += this_chunk;
 		nr_to_read -= this_chunk;
@@ -658,7 +662,7 @@ void page_cache_sync_readahead(struct address_space *mapping,
 
 	/* be dumb */
 	if (filp && (filp->f_mode & FMODE_RANDOM)) {
-		force_page_cache_readahead(mapping, filp, index, req_count);
+		force_page_cache_readahead(&rac);
 		return;
 	}
 

[RFC PATCH 4/7] mm: Pass readahead_control into page_cache_{sync,async}_readahead()

[David Howells]

Pass struct readahead_control into the page_cache_{sync,async}_readahead()
functions in preparation for making do_sync_mmap_readahead() pass down an
RAC struct.

Signed-off-by: David Howells <dhowells@redhat.com>
---

 fs/btrfs/free-space-cache.c |    7 ++++---
 fs/btrfs/ioctl.c            |   10 +++++++---
 fs/btrfs/relocation.c       |   14 ++++++++------
 fs/btrfs/send.c             |   15 +++++++++------
 fs/ext4/dir.c               |   12 +++++++-----
 fs/f2fs/dir.c               |   10 +++++++---
 include/linux/pagemap.h     |    8 +++-----
 mm/filemap.c                |   27 +++++++++++++++------------
 mm/khugepaged.c             |    6 +++---
 mm/readahead.c              |   40 ++++++++++++----------------------------
 10 files changed, 75 insertions(+), 74 deletions(-)

diff --git a/fs/btrfs/free-space-cache.c b/fs/btrfs/free-space-cache.c
index dc82fd0c80cb..0ca9361acf30 100644
--- a/fs/btrfs/free-space-cache.c
+++ b/fs/btrfs/free-space-cache.c
@@ -286,16 +286,17 @@ int btrfs_truncate_free_space_cache(struct btrfs_trans_handle *trans,
 static void readahead_cache(struct inode *inode)
 {
 	struct file_ra_state *ra;
-	unsigned long last_index;
+
+	DEFINE_READAHEAD(rac, NULL, inode->i_mapping, 0);
 
 	ra = kzalloc(sizeof(*ra), GFP_NOFS);
 	if (!ra)
 		return;
 
 	file_ra_state_init(ra, inode->i_mapping);
-	last_index = (i_size_read(inode) - 1) >> PAGE_SHIFT;
+	rac._nr_pages = (i_size_read(inode) - 1) >> PAGE_SHIFT;
 
-	page_cache_sync_readahead(inode->i_mapping, ra, NULL, 0, last_index);
+	page_cache_sync_readahead(&rac, ra);
 
 	kfree(ra);
 }
diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index bd3511c5ca81..5025a6a800e9 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -1428,6 +1428,8 @@ int btrfs_defrag_file(struct inode *inode, struct file *file,
 	struct page **pages = NULL;
 	bool do_compress = range->flags & BTRFS_DEFRAG_RANGE_COMPRESS;
 
+	DEFINE_READAHEAD(rac, file, inode->i_mapping, 0);
+
 	if (isize == 0)
 		return 0;
 
@@ -1534,9 +1536,11 @@ int btrfs_defrag_file(struct inode *inode, struct file *file,
 
 		if (i + cluster > ra_index) {
 			ra_index = max(i, ra_index);
-			if (ra)
-				page_cache_sync_readahead(inode->i_mapping, ra,
-						file, ra_index, cluster);
+			if (ra) {
+				rac._index = ra_index;
+				rac._nr_pages = cluster;
+				page_cache_sync_readahead(&rac, ra);
+			}
 			ra_index += cluster;
 		}
 
diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
index 4ba1ab9cc76d..1979803fd475 100644
--- a/fs/btrfs/relocation.c
+++ b/fs/btrfs/relocation.c
@@ -2684,6 +2684,8 @@ static int relocate_file_extent_cluster(struct inode *inode,
 	int nr = 0;
 	int ret = 0;
 
+	DEFINE_READAHEAD(rac, NULL, inode->i_mapping, 0);
+
 	if (!cluster->nr)
 		return 0;
 
@@ -2712,9 +2714,9 @@ static int relocate_file_extent_cluster(struct inode *inode,
 
 		page = find_lock_page(inode->i_mapping, index);
 		if (!page) {
-			page_cache_sync_readahead(inode->i_mapping,
-						  ra, NULL, index,
-						  last_index + 1 - index);
+			rac._index = index;
+			rac._nr_pages = last_index + 1 - index;
+			page_cache_sync_readahead(&rac, ra);
 			page = find_or_create_page(inode->i_mapping, index,
 						   mask);
 			if (!page) {
@@ -2728,9 +2730,9 @@ static int relocate_file_extent_cluster(struct inode *inode,
 		}
 
 		if (PageReadahead(page)) {
-			page_cache_async_readahead(inode->i_mapping,
-						   ra, NULL, page, index,
-						   last_index + 1 - index);
+			rac._index = index;
+			rac._nr_pages = last_index + 1 - index;
+			page_cache_async_readahead(&rac, ra, page);
 		}
 
 		if (!PageUptodate(page)) {
diff --git a/fs/btrfs/send.c b/fs/btrfs/send.c
index d9813a5b075a..ee0a9a2b5d08 100644
--- a/fs/btrfs/send.c
+++ b/fs/btrfs/send.c
@@ -4811,6 +4811,8 @@ static ssize_t fill_read_buf(struct send_ctx *sctx, u64 offset, u32 len)
 	unsigned pg_offset = offset_in_page(offset);
 	ssize_t ret = 0;
 
+	DEFINE_READAHEAD(rac, NULL, NULL, 0);
+
 	inode = btrfs_iget(fs_info->sb, sctx->cur_ino, root);
 	if (IS_ERR(inode))
 		return PTR_ERR(inode);
@@ -4829,15 +4831,18 @@ static ssize_t fill_read_buf(struct send_ctx *sctx, u64 offset, u32 len)
 	/* initial readahead */
 	memset(&sctx->ra, 0, sizeof(struct file_ra_state));
 	file_ra_state_init(&sctx->ra, inode->i_mapping);
+	rac.mapping = inode->i_mapping;
 
 	while (index <= last_index) {
 		unsigned cur_len = min_t(unsigned, len,
 					 PAGE_SIZE - pg_offset);
 
+		rac._index = index;
+		rac._nr_pages = last_index + 1 - index;
+
 		page = find_lock_page(inode->i_mapping, index);
 		if (!page) {
-			page_cache_sync_readahead(inode->i_mapping, &sctx->ra,
-				NULL, index, last_index + 1 - index);
+			page_cache_sync_readahead(&rac, &sctx->ra);
 
 			page = find_or_create_page(inode->i_mapping, index,
 					GFP_KERNEL);
@@ -4847,10 +4852,8 @@ static ssize_t fill_read_buf(struct send_ctx *sctx, u64 offset, u32 len)
 			}
 		}
 
-		if (PageReadahead(page)) {
-			page_cache_async_readahead(inode->i_mapping, &sctx->ra,
-				NULL, page, index, last_index + 1 - index);
-		}
+		if (PageReadahead(page))
+			page_cache_async_readahead(&rac, &sctx->ra, page);
 
 		if (!PageUptodate(page)) {
 			btrfs_readpage(NULL, page);
diff --git a/fs/ext4/dir.c b/fs/ext4/dir.c
index 1d82336b1cd4..6205c6830454 100644
--- a/fs/ext4/dir.c
+++ b/fs/ext4/dir.c
@@ -118,6 +118,8 @@ static int ext4_readdir(struct file *file, struct dir_context *ctx)
 	struct buffer_head *bh = NULL;
 	struct fscrypt_str fstr = FSTR_INIT(NULL, 0);
 
+	DEFINE_READAHEAD(rac, file, sb->s_bdev->bd_inode->i_mapping, 0);
+
 	if (IS_ENCRYPTED(inode)) {
 		err = fscrypt_get_encryption_info(inode);
 		if (err)
@@ -176,11 +178,11 @@ static int ext4_readdir(struct file *file, struct dir_context *ctx)
 		if (err > 0) {
 			pgoff_t index = map.m_pblk >>
 					(PAGE_SHIFT - inode->i_blkbits);
-			if (!ra_has_index(&file->f_ra, index))
-				page_cache_sync_readahead(
-					sb->s_bdev->bd_inode->i_mapping,
-					&file->f_ra, file,
-					index, 1);
+			if (!ra_has_index(&file->f_ra, index)) {
+				rac._index = index;
+				rac._nr_pages = 1;
+				page_cache_sync_readahead(&rac, &file->f_ra);
+			}
 			file->f_ra.prev_pos = (loff_t)index << PAGE_SHIFT;
 			bh = ext4_bread(NULL, inode, map.m_lblk, 0);
 			if (IS_ERR(bh)) {
diff --git a/fs/f2fs/dir.c b/fs/f2fs/dir.c
index 069f498af1e3..982f6d37454a 100644
--- a/fs/f2fs/dir.c
+++ b/fs/f2fs/dir.c
@@ -1027,6 +1027,8 @@ static int f2fs_readdir(struct file *file, struct dir_context *ctx)
 	struct fscrypt_str fstr = FSTR_INIT(NULL, 0);
 	int err = 0;
 
+	DEFINE_READAHEAD(rac, file, inode->i_mapping, 0);
+
 	if (IS_ENCRYPTED(inode)) {
 		err = fscrypt_get_encryption_info(inode);
 		if (err)
@@ -1052,9 +1054,11 @@ static int f2fs_readdir(struct file *file, struct dir_context *ctx)
 		cond_resched();
 
 		/* readahead for multi pages of dir */
-		if (npages - n > 1 && !ra_has_index(ra, n))
-			page_cache_sync_readahead(inode->i_mapping, ra, file, n,
-				min(npages - n, (pgoff_t)MAX_DIR_RA_PAGES));
+		if (npages - n > 1 && !ra_has_index(ra, n)) {
+			rac._index = n;
+			rac._nr_pages = min(npages - n, (pgoff_t)MAX_DIR_RA_PAGES);
+			page_cache_sync_readahead(&rac, ra);
+		}
 
 		dentry_page = f2fs_find_data_page(inode, n);
 		if (IS_ERR(dentry_page)) {
diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 8bf048a76c43..cd7bde29d4cc 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -769,11 +769,9 @@ void delete_from_page_cache_batch(struct address_space *mapping,
 
 #define VM_READAHEAD_PAGES	(SZ_128K / PAGE_SIZE)
 
-void page_cache_sync_readahead(struct address_space *, struct file_ra_state *,
-		struct file *, pgoff_t index, unsigned long req_count);
-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_sync_readahead(struct readahead_control *, struct file_ra_state *);
+void page_cache_async_readahead(struct readahead_control *, struct file_ra_state *,
+				struct page *);
 void page_cache_readahead_unbounded(struct readahead_control *,
 		unsigned long nr_to_read, unsigned long lookahead_count);
 
diff --git a/mm/filemap.c b/mm/filemap.c
index 82b97cf4306c..9f2f99db7318 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2070,6 +2070,8 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb,
 	unsigned int prev_offset;
 	int error = 0;
 
+	DEFINE_READAHEAD(rac, filp, mapping, 0);
+
 	if (unlikely(*ppos >= inode->i_sb->s_maxbytes))
 		return 0;
 	iov_iter_truncate(iter, inode->i_sb->s_maxbytes);
@@ -2097,9 +2099,9 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb,
 		if (!page) {
 			if (iocb->ki_flags & IOCB_NOIO)
 				goto would_block;
-			page_cache_sync_readahead(mapping,
-					ra, filp,
-					index, last_index - index);
+			rac._index = index;
+			rac._nr_pages = last_index - index;
+			page_cache_sync_readahead(&rac, ra);
 			page = find_get_page(mapping, index);
 			if (unlikely(page == NULL))
 				goto no_cached_page;
@@ -2109,9 +2111,9 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb,
 				put_page(page);
 				goto out;
 			}
-			page_cache_async_readahead(mapping,
-					ra, filp, thp_head(page),
-					index, last_index - index);
+			rac._index = index;
+			rac._nr_pages = last_index - index;
+			page_cache_async_readahead(&rac, ra, thp_head(page));
 		}
 		if (!PageUptodate(page)) {
 			/*
@@ -2469,6 +2471,8 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf)
 	pgoff_t offset = vmf->pgoff;
 	unsigned int mmap_miss;
 
+	DEFINE_READAHEAD(rac, file, mapping, offset);
+
 	/* If we don't want any read-ahead, don't bother */
 	if (vmf->vma->vm_flags & VM_RAND_READ)
 		return fpin;
@@ -2477,8 +2481,8 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf)
 
 	if (vmf->vma->vm_flags & VM_SEQ_READ) {
 		fpin = maybe_unlock_mmap_for_io(vmf, fpin);
-		page_cache_sync_readahead(mapping, ra, file, offset,
-					  ra->ra_pages);
+		rac._nr_pages = ra->ra_pages;
+		page_cache_sync_readahead(&rac, ra);
 		return fpin;
 	}
 
@@ -2515,10 +2519,10 @@ static struct file *do_async_mmap_readahead(struct vm_fault *vmf,
 {
 	struct file *file = vmf->vma->vm_file;
 	struct file_ra_state *ra = &file->f_ra;
-	struct address_space *mapping = file->f_mapping;
 	struct file *fpin = NULL;
 	unsigned int mmap_miss;
-	pgoff_t offset = vmf->pgoff;
+
+	DEFINE_READAHEAD(rac, file, file->f_mapping, vmf->pgoff);
 
 	/* If we don't want any read-ahead, don't bother */
 	if (vmf->vma->vm_flags & VM_RAND_READ || !ra->ra_pages)
@@ -2528,8 +2532,7 @@ static struct file *do_async_mmap_readahead(struct vm_fault *vmf,
 		WRITE_ONCE(ra->mmap_miss, --mmap_miss);
 	if (PageReadahead(thp_head(page))) {
 		fpin = maybe_unlock_mmap_for_io(vmf, fpin);
-		page_cache_async_readahead(mapping, ra, file,
-				thp_head(page), offset, ra->ra_pages);
+		page_cache_async_readahead(&rac, ra, thp_head(page));
 	}
 	return fpin;
 }
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index f2d243077b74..0bece7ab0ce7 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1703,10 +1703,10 @@ static void collapse_file(struct mm_struct *mm,
 			}
 		} else {	/* !is_shmem */
 			if (!page || xa_is_value(page)) {
+				DEFINE_READAHEAD(rac, file, mapping, index);
+				rac._nr_pages = end - index;
 				xas_unlock_irq(&xas);
-				page_cache_sync_readahead(mapping, &file->f_ra,
-							  file, index,
-							  end - index);
+				page_cache_sync_readahead(&rac, &file->f_ra);
 				/* drain pagevecs to help isolate_lru_page() */
 				lru_add_drain();
 				page = find_lock_page(mapping, index);
diff --git a/mm/readahead.c b/mm/readahead.c
index e557c6d5a183..7114246b4e41 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -635,24 +635,17 @@ static void ondemand_readahead(struct readahead_control *rac,
 
 /**
  * page_cache_sync_readahead - generic file readahead
- * @mapping: address_space which holds the pagecache and I/O vectors
+ * @rac: Readahead control.
  * @ra: file_ra_state which holds the readahead state
- * @filp: passed on to ->readpage() and ->readpages()
- * @index: Index of first page to be read.
- * @req_count: Total number of pages being read by the caller.
  *
  * page_cache_sync_readahead() should be called when a cache miss happened:
  * it will submit the read.  The readahead logic may decide to piggyback more
  * pages onto the read request if access patterns suggest it will improve
  * performance.
  */
-void page_cache_sync_readahead(struct address_space *mapping,
-			       struct file_ra_state *ra, struct file *filp,
-			       pgoff_t index, unsigned long req_count)
+void page_cache_sync_readahead(struct readahead_control *rac,
+			       struct file_ra_state *ra)
 {
-	DEFINE_READAHEAD(rac, filp, mapping, index);
-	rac._nr_pages = req_count;
-
 	/* no read-ahead */
 	if (!ra->ra_pages)
 		return;
@@ -661,39 +654,30 @@ void page_cache_sync_readahead(struct address_space *mapping,
 		return;
 
 	/* be dumb */
-	if (filp && (filp->f_mode & FMODE_RANDOM)) {
-		force_page_cache_readahead(&rac);
+	if (rac->file && (rac->file->f_mode & FMODE_RANDOM)) {
+		force_page_cache_readahead(rac);
 		return;
 	}
 
 	/* do read-ahead */
-	ondemand_readahead(&rac, ra, NULL);
+	ondemand_readahead(rac, ra, NULL);
 }
 EXPORT_SYMBOL_GPL(page_cache_sync_readahead);
 
 /**
  * page_cache_async_readahead - file readahead for marked pages
- * @mapping: address_space which holds the pagecache and I/O vectors
+ * @rac: Readahead control.
  * @ra: file_ra_state which holds the readahead state
- * @filp: passed on to ->readpage() and ->readpages()
- * @page: The page at @index which triggered the readahead call.
- * @index: Index of first page to be read.
- * @req_count: Total number of pages being read by the caller.
  *
  * page_cache_async_readahead() should be called when a page is used which
  * is marked as PageReadahead; this is a marker to suggest that the application
  * has used up enough of the readahead window that we should start pulling in
  * more pages.
  */
-void
-page_cache_async_readahead(struct address_space *mapping,
-			   struct file_ra_state *ra, struct file *filp,
-			   struct page *page, pgoff_t index,
-			   unsigned long req_count)
+void page_cache_async_readahead(struct readahead_control *rac,
+				struct file_ra_state *ra,
+				struct page *page)
 {
-	DEFINE_READAHEAD(rac, filp, mapping, index);
-	rac._nr_pages = req_count;
-
 	/* No Read-ahead */
 	if (!ra->ra_pages)
 		return;
@@ -709,14 +693,14 @@ page_cache_async_readahead(struct address_space *mapping,
 	/*
 	 * Defer asynchronous read-ahead on IO congestion.
 	 */
-	if (inode_read_congested(mapping->host))
+	if (inode_read_congested(rac->mapping->host))
 		return;
 
 	if (blk_cgroup_congested())
 		return;
 
 	/* do read-ahead */
-	ondemand_readahead(&rac, ra, page);
+	ondemand_readahead(rac, ra, page);
 }
 EXPORT_SYMBOL_GPL(page_cache_async_readahead);
 

[RFC PATCH 5/7] mm: Make __do_page_cache_readahead() use rac->_nr_pages

[David Howells]

Make __do_page_cache_readahead() use rac->_nr_pages rather than passing in
nr_to_read argument.

Signed-off-by: David Howells <dhowells@redhat.com>
---

 fs/ext4/verity.c        |    8 +++++---
 fs/f2fs/verity.c        |    8 +++++---
 include/linux/pagemap.h |    3 +--
 mm/internal.h           |    6 +++---
 mm/readahead.c          |   20 +++++++++++---------
 5 files changed, 25 insertions(+), 20 deletions(-)

diff --git a/fs/ext4/verity.c b/fs/ext4/verity.c
index 6fc2dbc87c0b..3d377110e839 100644
--- a/fs/ext4/verity.c
+++ b/fs/ext4/verity.c
@@ -356,10 +356,12 @@ static struct page *ext4_read_merkle_tree_page(struct inode *inode,
 
 	page = find_get_page_flags(inode->i_mapping, index, FGP_ACCESSED);
 	if (!page || !PageUptodate(page)) {
-		if (page)
+		if (page) {
 			put_page(page);
-		else if (num_ra_pages > 1)
-			page_cache_readahead_unbounded(&rac, num_ra_pages, 0);
+		} else if (num_ra_pages > 1) {
+			rac._nr_pages = num_ra_pages;
+			page_cache_readahead_unbounded(&rac, 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 392dd07f4214..8445eed5a1bc 100644
--- a/fs/f2fs/verity.c
+++ b/fs/f2fs/verity.c
@@ -235,10 +235,12 @@ static struct page *f2fs_read_merkle_tree_page(struct inode *inode,
 
 	page = find_get_page_flags(inode->i_mapping, index, FGP_ACCESSED);
 	if (!page || !PageUptodate(page)) {
-		if (page)
+		if (page) {
 			put_page(page);
-		else if (num_ra_pages > 1)
-			page_cache_readahead_unbounded(&rac, num_ra_pages, 0);
+		} else if (num_ra_pages > 1) {
+			rac._nr_pages = num_ra_pages;
+			page_cache_readahead_unbounded(&rac, 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 cd7bde29d4cc..72e9c44d62bb 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -772,8 +772,7 @@ void delete_from_page_cache_batch(struct address_space *mapping,
 void page_cache_sync_readahead(struct readahead_control *, struct file_ra_state *);
 void page_cache_async_readahead(struct readahead_control *, struct file_ra_state *,
 				struct page *);
-void page_cache_readahead_unbounded(struct readahead_control *,
-		unsigned long nr_to_read, unsigned long lookahead_count);
+void page_cache_readahead_unbounded(struct readahead_control *, unsigned long);
 
 /*
  * Like add_to_page_cache_locked, but used to add newly allocated pages:
diff --git a/mm/internal.h b/mm/internal.h
index 2eb9f7f5f134..e1d296e76fb0 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -50,8 +50,7 @@ void unmap_page_range(struct mmu_gather *tlb,
 			     struct zap_details *details);
 
 void force_page_cache_readahead(struct readahead_control *);
-void __do_page_cache_readahead(struct readahead_control *,
-		unsigned long nr_to_read, unsigned long lookahead_size);
+void __do_page_cache_readahead(struct readahead_control *, unsigned long);
 
 /*
  * Submit IO for the read-ahead request in file_ra_state.
@@ -60,7 +59,8 @@ static inline void ra_submit(struct file_ra_state *ra,
 		struct address_space *mapping, struct file *file)
 {
 	DEFINE_READAHEAD(rac, file, mapping, ra->start);
-	__do_page_cache_readahead(&rac, ra->size, ra->async_size);
+	rac._nr_pages = ra->size;
+	__do_page_cache_readahead(&rac, ra->async_size);
 }
 
 /**
diff --git a/mm/readahead.c b/mm/readahead.c
index 7114246b4e41..28ff80304a21 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -172,10 +172,11 @@ static void read_pages(struct readahead_control *rac, struct list_head *pages,
  * May sleep, but will not reenter filesystem to reclaim memory.
  */
 void page_cache_readahead_unbounded(struct readahead_control *rac,
-		unsigned long nr_to_read, unsigned long lookahead_size)
+				    unsigned long lookahead_size)
 {
 	struct address_space *mapping = rac->mapping;
 	unsigned long index = readahead_index(rac);
+	unsigned long nr_to_read = readahead_count(rac);
 	LIST_HEAD(page_pool);
 	gfp_t gfp_mask = readahead_gfp_mask(mapping);
 	unsigned long i;
@@ -195,6 +196,7 @@ void page_cache_readahead_unbounded(struct readahead_control *rac,
 	/*
 	 * Preallocate as many pages as we will need.
 	 */
+	rac->_nr_pages = 0;
 	for (i = 0; i < nr_to_read; i++) {
 		struct page *page = xa_load(&mapping->i_pages, index + i);
 
@@ -247,7 +249,7 @@ EXPORT_SYMBOL_GPL(page_cache_readahead_unbounded);
  * We really don't want to intermingle reads and writes like that.
  */
 void __do_page_cache_readahead(struct readahead_control *rac,
-		unsigned long nr_to_read, unsigned long lookahead_size)
+			       unsigned long lookahead_size)
 {
 	struct inode *inode = rac->mapping->host;
 	unsigned long index = readahead_index(rac);
@@ -261,10 +263,10 @@ void __do_page_cache_readahead(struct readahead_control *rac,
 	if (index > end_index)
 		return;
 	/* Don't read past the page containing the last byte of the file */
-	if (nr_to_read > end_index - index)
-		nr_to_read = end_index - index + 1;
+	if (readahead_count(rac) > end_index - index)
+		rac->_nr_pages = end_index - index + 1;
 
-	page_cache_readahead_unbounded(rac, nr_to_read, lookahead_size);
+	page_cache_readahead_unbounded(rac, lookahead_size);
 }
 
 /*
@@ -297,8 +299,7 @@ void force_page_cache_readahead(struct readahead_control *rac)
 
 		rac->_index = index;
 		rac->_nr_pages = this_chunk;
-		// Do I need to modify rac->_batch_count?
-		__do_page_cache_readahead(rac, this_chunk, 0);
+		__do_page_cache_readahead(rac, 0);
 
 		index += this_chunk;
 		nr_to_read -= this_chunk;
@@ -601,7 +602,7 @@ static void ondemand_readahead(struct readahead_control *rac,
 	 * standalone, small random read
 	 * Read as is, and do not pollute the readahead state.
 	 */
-	__do_page_cache_readahead(rac, req_size, 0);
+	__do_page_cache_readahead(rac, 0);
 	return;
 
 initial_readahead:
@@ -630,7 +631,8 @@ static void ondemand_readahead(struct readahead_control *rac,
 	rac->_index = ra->start;
 	if (page && page_cache_readahead_order(rac, ra, thp_order(page)))
 		return;
-	__do_page_cache_readahead(rac, ra->size, ra->async_size);
+	rac->_nr_pages = ra->size;
+	__do_page_cache_readahead(rac, ra->async_size);
 }
 
 /**

[RFC PATCH 6/7] mm: Fold ra_submit() into do_sync_mmap_readahead()

[David Howells]

Fold ra_submit() into its last remaining user and pass the previously added
readahead_control struct down into __do_page_cache_readahead().

Signed-off-by: David Howells <dhowells@redhat.com>
---

 mm/filemap.c  |    6 +++---
 mm/internal.h |   11 -----------
 2 files changed, 3 insertions(+), 14 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 9f2f99db7318..c22bb01e8ba6 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2502,10 +2502,10 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf)
 	 * mmap read-around
 	 */
 	fpin = maybe_unlock_mmap_for_io(vmf, fpin);
-	ra->start = max_t(long, 0, offset - ra->ra_pages / 2);
-	ra->size = ra->ra_pages;
+	ra->start = rac._index    = max_t(long, 0, offset - ra->ra_pages / 2);
+	ra->size  = rac._nr_pages = ra->ra_pages;
 	ra->async_size = ra->ra_pages / 4;
-	ra_submit(ra, mapping, file);
+	__do_page_cache_readahead(&rac, ra->async_size);
 	return fpin;
 }
 
diff --git a/mm/internal.h b/mm/internal.h
index e1d296e76fb0..de3b2ce2743a 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -52,17 +52,6 @@ void unmap_page_range(struct mmu_gather *tlb,
 void force_page_cache_readahead(struct readahead_control *);
 void __do_page_cache_readahead(struct readahead_control *, unsigned long);
 
-/*
- * Submit IO for the read-ahead request in file_ra_state.
- */
-static inline void ra_submit(struct file_ra_state *ra,
-		struct address_space *mapping, struct file *file)
-{
-	DEFINE_READAHEAD(rac, file, mapping, ra->start);
-	rac._nr_pages = ra->size;
-	__do_page_cache_readahead(&rac, ra->async_size);
-}
-
 /**
  * page_evictable - test whether a page is evictable
  * @page: the page to test

[RFC PATCH 7/7] mm: Pass a file_ra_state struct into force_page_cache_readahead()

[David Howells]

Pass a file_ra_state struct into force_page_cache_readahead().  One caller
has one that should be passed in and the other doesn't, but the former
needs to pass its in.

Signed-off-by: David Howells <dhowells@redhat.com>
---

 mm/fadvise.c   |    2 +-
 mm/internal.h  |    2 +-
 mm/readahead.c |    6 +++---
 3 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/mm/fadvise.c b/mm/fadvise.c
index b68d2f2959d5..2f1550279757 100644
--- a/mm/fadvise.c
+++ b/mm/fadvise.c
@@ -107,7 +107,7 @@ int generic_fadvise(struct file *file, loff_t offset, loff_t len, int advice)
 		{
 			DEFINE_READAHEAD(rac, file, mapping, start_index);
 			rac._nr_pages = nrpages;
-			force_page_cache_readahead(&rac);
+			force_page_cache_readahead(&rac, &rac.file->f_ra);
 		}
 		break;
 	case POSIX_FADV_NOREUSE:
diff --git a/mm/internal.h b/mm/internal.h
index de3b2ce2743a..977ad7d81b1b 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -49,7 +49,7 @@ void unmap_page_range(struct mmu_gather *tlb,
 			     unsigned long addr, unsigned long end,
 			     struct zap_details *details);
 
-void force_page_cache_readahead(struct readahead_control *);
+void force_page_cache_readahead(struct readahead_control *, struct file_ra_state *);
 void __do_page_cache_readahead(struct readahead_control *, unsigned long);
 
 /**
diff --git a/mm/readahead.c b/mm/readahead.c
index 28ff80304a21..b001720c13aa 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -273,11 +273,11 @@ void __do_page_cache_readahead(struct readahead_control *rac,
  * Chunk the readahead into 2 megabyte units, so that we don't pin too much
  * memory at once.
  */
-void force_page_cache_readahead(struct readahead_control *rac)
+void force_page_cache_readahead(struct readahead_control *rac,
+				struct file_ra_state *ra)
 {
 	struct address_space *mapping = rac->mapping;
 	struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
-	struct file_ra_state *ra = &rac->file->f_ra;
 	unsigned long max_pages, index, nr_to_read;
 
 	if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages &&
@@ -657,7 +657,7 @@ void page_cache_sync_readahead(struct readahead_control *rac,
 
 	/* be dumb */
 	if (rac->file && (rac->file->f_mode & FMODE_RANDOM)) {
-		force_page_cache_readahead(rac);
+		force_page_cache_readahead(rac, ra);
 		return;
 	}
 

Re: [RFC PATCH 0/7] mm: Make more use of readahead_control

[Eric Biggers]

On Tue, Sep 01, 2020 at 05:28:15PM +0100, David Howells wrote:
> 
> Hi Willy,
> 
> Here's a set of patches to expand the use of the readahead_control struct,
> essentially from do_sync_mmap_readahead() down.  Note that I've been
> passing the number of pages to read in rac->_nr_pages, and then saving it
> and changing it certain points, e.g. page_cache_readahead_unbounded().
> 
> Also pass file_ra_state into force_page_cache_readahead().
> 
> Also there's an apparent minor bug in khugepaged.c that I've included a
> patch for: page_cache_sync_readahead() looks to be given the wrong size in
> collapse_file().
> 

What branch does this apply to?

- Eric

Re: [RFC PATCH 0/7] mm: Make more use of readahead_control

[Matthew Wilcox]

On Tue, Sep 01, 2020 at 09:41:32AM -0700, Eric Biggers wrote:
> On Tue, Sep 01, 2020 at 05:28:15PM +0100, David Howells wrote:
> > 
> > Hi Willy,
> > 
> > Here's a set of patches to expand the use of the readahead_control struct,
> > essentially from do_sync_mmap_readahead() down.  Note that I've been
> > passing the number of pages to read in rac->_nr_pages, and then saving it
> > and changing it certain points, e.g. page_cache_readahead_unbounded().
> > 
> > Also pass file_ra_state into force_page_cache_readahead().
> > 
> > Also there's an apparent minor bug in khugepaged.c that I've included a
> > patch for: page_cache_sync_readahead() looks to be given the wrong size in
> > collapse_file().
> > 
> 
> What branch does this apply to?

He's done it on top of http://git.infradead.org/users/willy/pagecache.git

I was hoping he'd include
http://git.infradead.org/users/willy/pagecache.git/commitdiff/c71de787328809026cfabbcc5485cb01caca8646
http://git.infradead.org/users/willy/pagecache.git/commitdiff/f3a1cd6447e29a54b03efc2189d943f12ac1c9b9
http://git.infradead.org/users/willy/pagecache.git/commitdiff/c03d3a5a5716bb0df2fe15ec528bbd895cd18e6e

as the first three patches in the series; then it should apply to Linus'
tree.

Re: [RFC PATCH 0/7] mm: Make more use of readahead_control

[David Howells]

Matthew Wilcox <willy@infradead.org> wrote:

> He's done it on top of http://git.infradead.org/users/willy/pagecache.git

Sorry, yes, I should've mentioned that.

> I was hoping he'd include
> http://git.infradead.org/users/willy/pagecache.git/commitdiff/c71de787328809026cfabbcc5485cb01caca8646
> http://git.infradead.org/users/willy/pagecache.git/commitdiff/f3a1cd6447e29a54b03efc2189d943f12ac1c9b9
> http://git.infradead.org/users/willy/pagecache.git/commitdiff/c03d3a5a5716bb0df2fe15ec528bbd895cd18e6e
> 
> as the first three patches in the series; then it should apply to Linus'
> tree.

Did you want me to carry those patches and pass them to Linus through my
fscache branch?

David

Re: [RFC PATCH 0/7] mm: Make more use of readahead_control

[Matthew Wilcox]

On Tue, Sep 01, 2020 at 05:28:15PM +0100, David Howells wrote:
> Here's a set of patches to expand the use of the readahead_control struct,
> essentially from do_sync_mmap_readahead() down.

I like this.

> Note that I've been
> passing the number of pages to read in rac->_nr_pages, and then saving it
> and changing it certain points, e.g. page_cache_readahead_unbounded().

I do not like this.  You're essentially mutating the meaning of _nr_pages
as the ractl moves down the stack, and that's going to lead to bugs.
I'd keep it as a separate argument.

> Also there's an apparent minor bug in khugepaged.c that I've included a
> patch for: page_cache_sync_readahead() looks to be given the wrong size in
> collapse_file().

This needs to go in as an independent fix.  But you didn't actually cc Song.

Re: [RFC PATCH 0/7] mm: Make more use of readahead_control

[David Howells]

Matthew Wilcox <willy@infradead.org> wrote:

> > Also there's an apparent minor bug in khugepaged.c that I've included a
> > patch for: page_cache_sync_readahead() looks to be given the wrong size in
> > collapse_file().
> 
> This needs to go in as an independent fix.  But you didn't actually cc Song.

Bah.  I forgot to pass --auto to stgit.  No matter, he saw it anyway.

David

Re: [RFC PATCH 0/7] mm: Make more use of readahead_control

[David Howells]

Matthew Wilcox <willy@infradead.org> wrote:

> > Note that I've been
> > passing the number of pages to read in rac->_nr_pages, and then saving it
> > and changing it certain points, e.g. page_cache_readahead_unbounded().
> 
> I do not like this.  You're essentially mutating the meaning of _nr_pages
> as the ractl moves down the stack, and that's going to lead to bugs.
> I'd keep it as a separate argument.

The meaning isn't specified in linux/pagemap.h.  Can you adjust the comments
on the struct and on readahead_count() to make it more clear what the value
represents?

Thanks,
David

Re: [RFC PATCH 0/7] mm: Make more use of readahead_control

[Matthew Wilcox]

On Tue, Sep 01, 2020 at 08:44:14PM +0100, David Howells wrote:
> Matthew Wilcox <willy@infradead.org> wrote:
> 
> > > Note that I've been
> > > passing the number of pages to read in rac->_nr_pages, and then saving it
> > > and changing it certain points, e.g. page_cache_readahead_unbounded().
> > 
> > I do not like this.  You're essentially mutating the meaning of _nr_pages
> > as the ractl moves down the stack, and that's going to lead to bugs.
> > I'd keep it as a separate argument.
> 
> The meaning isn't specified in linux/pagemap.h.  Can you adjust the comments
> on the struct and on readahead_count() to make it more clear what the value
> represents?

It's intentionally not documented.  This documentation is for the
filesystem author, who should not be looking at it.  Neither should
you :-P