[v9,20/25] ext4: Convert from readpages to readahead
diff mbox series

Message ID 20200320142231.2402-21-willy@infradead.org
State New
Headers show
Series
  • Change readahead API
Related show

Commit Message

Matthew Wilcox March 20, 2020, 2:22 p.m. UTC
From: "Matthew Wilcox (Oracle)" <willy@infradead.org>

Use the new readahead operation in ext4

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: William Kucharski <william.kucharski@oracle.com>
---
 fs/ext4/ext4.h     |  3 +--
 fs/ext4/inode.c    | 21 +++++++++------------
 fs/ext4/readpage.c | 22 ++++++++--------------
 3 files changed, 18 insertions(+), 28 deletions(-)

Comments

Eric Biggers March 20, 2020, 5:37 p.m. UTC | #1
On Fri, Mar 20, 2020 at 07:22:26AM -0700, Matthew Wilcox wrote:
> From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
> 
> Use the new readahead operation in ext4
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> Reviewed-by: William Kucharski <william.kucharski@oracle.com>
> ---
>  fs/ext4/ext4.h     |  3 +--
>  fs/ext4/inode.c    | 21 +++++++++------------
>  fs/ext4/readpage.c | 22 ++++++++--------------
>  3 files changed, 18 insertions(+), 28 deletions(-)
> 

Reviewed-by: Eric Biggers <ebiggers@google.com>

> +		if (rac) {
> +			page = readahead_page(rac);
>  			prefetchw(&page->flags);
> -			list_del(&page->lru);
> -			if (add_to_page_cache_lru(page, mapping, page->index,
> -				  readahead_gfp_mask(mapping)))
> -				goto next_page;
>  		}

Maybe the prefetchw(&page->flags) should be included in readahead_page()?
Most of the callers do it.

- Eric
Matthew Wilcox March 20, 2020, 5:48 p.m. UTC | #2
On Fri, Mar 20, 2020 at 10:37:34AM -0700, Eric Biggers wrote:
> On Fri, Mar 20, 2020 at 07:22:26AM -0700, Matthew Wilcox wrote:
> > From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
> > 
> > Use the new readahead operation in ext4
> > 
> > Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> > Reviewed-by: William Kucharski <william.kucharski@oracle.com>
> > ---
> >  fs/ext4/ext4.h     |  3 +--
> >  fs/ext4/inode.c    | 21 +++++++++------------
> >  fs/ext4/readpage.c | 22 ++++++++--------------
> >  3 files changed, 18 insertions(+), 28 deletions(-)
> > 
> 
> Reviewed-by: Eric Biggers <ebiggers@google.com>
> 
> > +		if (rac) {
> > +			page = readahead_page(rac);
> >  			prefetchw(&page->flags);
> > -			list_del(&page->lru);
> > -			if (add_to_page_cache_lru(page, mapping, page->index,
> > -				  readahead_gfp_mask(mapping)))
> > -				goto next_page;
> >  		}
> 
> Maybe the prefetchw(&page->flags) should be included in readahead_page()?
> Most of the callers do it.

I did notice that a lot of callers do that.  I wonder whether it (still)
helps or whether it's just cargo-cult programming.  It can't possibly
have helped before because we did list_del(&page->lru) as the very next
instruction after prefetchw(), and they're in the same cacheline.  It'd
be interesting to take it out and see what happens to performance.
Eric Biggers March 20, 2020, 6:40 p.m. UTC | #3
On Fri, Mar 20, 2020 at 10:48:48AM -0700, Matthew Wilcox wrote:
> On Fri, Mar 20, 2020 at 10:37:34AM -0700, Eric Biggers wrote:
> > On Fri, Mar 20, 2020 at 07:22:26AM -0700, Matthew Wilcox wrote:
> > > From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
> > > 
> > > Use the new readahead operation in ext4
> > > 
> > > Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> > > Reviewed-by: William Kucharski <william.kucharski@oracle.com>
> > > ---
> > >  fs/ext4/ext4.h     |  3 +--
> > >  fs/ext4/inode.c    | 21 +++++++++------------
> > >  fs/ext4/readpage.c | 22 ++++++++--------------
> > >  3 files changed, 18 insertions(+), 28 deletions(-)
> > > 
> > 
> > Reviewed-by: Eric Biggers <ebiggers@google.com>
> > 
> > > +		if (rac) {
> > > +			page = readahead_page(rac);
> > >  			prefetchw(&page->flags);
> > > -			list_del(&page->lru);
> > > -			if (add_to_page_cache_lru(page, mapping, page->index,
> > > -				  readahead_gfp_mask(mapping)))
> > > -				goto next_page;
> > >  		}
> > 
> > Maybe the prefetchw(&page->flags) should be included in readahead_page()?
> > Most of the callers do it.
> 
> I did notice that a lot of callers do that.  I wonder whether it (still)
> helps or whether it's just cargo-cult programming.  It can't possibly
> have helped before because we did list_del(&page->lru) as the very next
> instruction after prefetchw(), and they're in the same cacheline.  It'd
> be interesting to take it out and see what happens to performance.

Yeah, it does look like the list_del() made the prefetchw() useless, so it
should just be removed.  The prefetchw() dates all the way back to
mpage_readpages() being added in 2002, but even then the list_del() was
immediately afterwards, and 'flags' and 'lru' were in the same cache line in
'struct page' even then (assuming at least a 32-byte cache line size), so...

- Eric

Patch
diff mbox series

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 4441331d06cc..1570a0b51b73 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -3279,8 +3279,7 @@  static inline void ext4_set_de_type(struct super_block *sb,
 
 /* readpages.c */
 extern int ext4_mpage_readpages(struct address_space *mapping,
-				struct list_head *pages, struct page *page,
-				unsigned nr_pages, bool is_readahead);
+		struct readahead_control *rac, struct page *page);
 extern int __init ext4_init_post_read_processing(void);
 extern void ext4_exit_post_read_processing(void);
 
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index e60aca791d3f..d674c5f9066c 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -3226,23 +3226,20 @@  static int ext4_readpage(struct file *file, struct page *page)
 		ret = ext4_readpage_inline(inode, page);
 
 	if (ret == -EAGAIN)
-		return ext4_mpage_readpages(page->mapping, NULL, page, 1,
-						false);
+		return ext4_mpage_readpages(page->mapping, NULL, page);
 
 	return ret;
 }
 
-static int
-ext4_readpages(struct file *file, struct address_space *mapping,
-		struct list_head *pages, unsigned nr_pages)
+static void ext4_readahead(struct readahead_control *rac)
 {
-	struct inode *inode = mapping->host;
+	struct inode *inode = rac->mapping->host;
 
-	/* If the file has inline data, no need to do readpages. */
+	/* If the file has inline data, no need to do readahead. */
 	if (ext4_has_inline_data(inode))
-		return 0;
+		return;
 
-	return ext4_mpage_readpages(mapping, pages, NULL, nr_pages, true);
+	ext4_mpage_readpages(rac->mapping, rac, NULL);
 }
 
 static void ext4_invalidatepage(struct page *page, unsigned int offset,
@@ -3587,7 +3584,7 @@  static int ext4_set_page_dirty(struct page *page)
 
 static const struct address_space_operations ext4_aops = {
 	.readpage		= ext4_readpage,
-	.readpages		= ext4_readpages,
+	.readahead		= ext4_readahead,
 	.writepage		= ext4_writepage,
 	.writepages		= ext4_writepages,
 	.write_begin		= ext4_write_begin,
@@ -3604,7 +3601,7 @@  static const struct address_space_operations ext4_aops = {
 
 static const struct address_space_operations ext4_journalled_aops = {
 	.readpage		= ext4_readpage,
-	.readpages		= ext4_readpages,
+	.readahead		= ext4_readahead,
 	.writepage		= ext4_writepage,
 	.writepages		= ext4_writepages,
 	.write_begin		= ext4_write_begin,
@@ -3620,7 +3617,7 @@  static const struct address_space_operations ext4_journalled_aops = {
 
 static const struct address_space_operations ext4_da_aops = {
 	.readpage		= ext4_readpage,
-	.readpages		= ext4_readpages,
+	.readahead		= ext4_readahead,
 	.writepage		= ext4_writepage,
 	.writepages		= ext4_writepages,
 	.write_begin		= ext4_da_write_begin,
diff --git a/fs/ext4/readpage.c b/fs/ext4/readpage.c
index c1769afbf799..66275f25235d 100644
--- a/fs/ext4/readpage.c
+++ b/fs/ext4/readpage.c
@@ -7,8 +7,8 @@ 
  *
  * This was originally taken from fs/mpage.c
  *
- * The intent is the ext4_mpage_readpages() function here is intended
- * to replace mpage_readpages() in the general case, not just for
+ * The ext4_mpage_readpages() function here is intended to
+ * replace mpage_readahead() in the general case, not just for
  * encrypted files.  It has some limitations (see below), where it
  * will fall back to read_block_full_page(), but these limitations
  * should only be hit when page_size != block_size.
@@ -222,8 +222,7 @@  static inline loff_t ext4_readpage_limit(struct inode *inode)
 }
 
 int ext4_mpage_readpages(struct address_space *mapping,
-			 struct list_head *pages, struct page *page,
-			 unsigned nr_pages, bool is_readahead)
+		struct readahead_control *rac, struct page *page)
 {
 	struct bio *bio = NULL;
 	sector_t last_block_in_bio = 0;
@@ -241,6 +240,7 @@  int ext4_mpage_readpages(struct address_space *mapping,
 	int length;
 	unsigned relative_block = 0;
 	struct ext4_map_blocks map;
+	unsigned int nr_pages = rac ? readahead_count(rac) : 1;
 
 	map.m_pblk = 0;
 	map.m_lblk = 0;
@@ -251,14 +251,9 @@  int ext4_mpage_readpages(struct address_space *mapping,
 		int fully_mapped = 1;
 		unsigned first_hole = blocks_per_page;
 
-		if (pages) {
-			page = lru_to_page(pages);
-
+		if (rac) {
+			page = readahead_page(rac);
 			prefetchw(&page->flags);
-			list_del(&page->lru);
-			if (add_to_page_cache_lru(page, mapping, page->index,
-				  readahead_gfp_mask(mapping)))
-				goto next_page;
 		}
 
 		if (page_has_buffers(page))
@@ -381,7 +376,7 @@  int ext4_mpage_readpages(struct address_space *mapping,
 			bio->bi_iter.bi_sector = blocks[0] << (blkbits - 9);
 			bio->bi_end_io = mpage_end_io;
 			bio_set_op_attrs(bio, REQ_OP_READ,
-						is_readahead ? REQ_RAHEAD : 0);
+						rac ? REQ_RAHEAD : 0);
 		}
 
 		length = first_hole << blkbits;
@@ -406,10 +401,9 @@  int ext4_mpage_readpages(struct address_space *mapping,
 		else
 			unlock_page(page);
 	next_page:
-		if (pages)
+		if (rac)
 			put_page(page);
 	}
-	BUG_ON(pages && !list_empty(pages));
 	if (bio)
 		submit_bio(bio);
 	return 0;