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Message-Id: <1181713875.6237.134.camel@localhost.localdomain>
Date: Wed, 13 Jun 2007 15:51:14 +1000
From: Rusty Russell <rusty@...tcorp.com.au>
To: Fengguang Wu <wfg@...l.ustc.edu.cn>
Cc: Andrew Morton <akpm@...l.org>, linux-kernel@...r.kernel.org,
Andi Kleen <andi@...stfloor.org>,
Jens Axboe <jens.axboe@...cle.com>,
Oleg Nesterov <oleg@...sign.ru>,
Steven Pratt <slpratt@...tin.ibm.com>,
Ram Pai <linuxram@...ibm.com>
Subject: Re: [PATCH 5/9] readahead: on-demand readahead logic
On Wed, 2007-06-13 at 12:00 +0800, Fengguang Wu wrote:
> On Wed, Jun 13, 2007 at 11:40:33AM +1000, Rusty Russell wrote:
> > +/* If page has PG_readahead flag set, call async readahead logic. */
> > +static inline void
> > +page_cache_check_readahead_page(struct address_space *mapping,
> > + struct file_ra_state *ra,
> > + struct file *filp,
> > + struct page *pg,
> > + pgoff_t offset,
> > + unsigned long size)
> > +{
> > + if (!PageReadahead(pg))
> > + return;
> > + page_cache_async_readahead(mapping, ra, filp, pg, offset, size);
> > +}
>
> This function might not be necessary?
> I guess the explicit code is clear enough.
Hi Fengguang,
Yes, I think you're right.
> > static unsigned long
> > ondemand_readahead(struct address_space *mapping,
> > struct file_ra_state *ra, struct file *filp,
> > - struct page *page, pgoff_t offset,
> > + bool hit_lookahead_marker, pgoff_t offset,
>
> Or use names like async/is_async for hit_lookahead_marker?
I wasn't sure. The argument says "we've hit a marker, so do readahead
even if doesn't look sequential.". Now, you and I know that only
happens if it's an async readahead, but that's not really relevant to
this function.
> Seems that you have accepted the 'lookahead' term ;)
Yes, but I shouldn't, because marker is still called PG_readahead 8) I
changed this to "hit_readahead_marker" instead.
> > +/**
> > + * page_cache_async_readahead - file readahead for marked pages
> > + * @mapping: address_space which holds the pagecache and I/O vectors
> > + * @ra: file_ra_state which holds the readahead state
> > + * @filp: passed on to ->readpage() and ->readpages()
> > + * @page: the page at @offset which has the PageReadahead flag set
>
> ^PG_readahead
Oh, yes.
> > + * @offset: start offset into @mapping, in PAGE_CACHE_SIZE units
> > + * @req_size: hint: total size of the read which the caller is performing in
> > + * PAGE_CACHE_SIZE units
>
> ^in pagecache pages?
> //Christoph is changing the fixed PAGE_CACHE_SIZE to variable ones.
This came from your patch, but it sounds like a good change. Also the
PAGE_CACHE_SIZE units above.
> I like this new comment!
Heh, maybe that means I finally understand the code 8).
OK, here is revised patch with your changes:
====
Split ondemand readahead interface into two functions. I think this
makes it a little clearer for non-readahead experts (like Rusty).
Internally they both call ondemand_readahead(), but the page argument
is changed to an obvious boolean flag.
Signed-off-by: Rusty Russell <rusty@...tcorp.com.au>
diff -r e96f11f61577 fs/ext3/dir.c
--- a/fs/ext3/dir.c Sun Jun 10 18:25:28 2007 +1000
+++ b/fs/ext3/dir.c Wed Jun 13 15:16:49 2007 +1000
@@ -139,10 +139,10 @@ static int ext3_readdir(struct file * fi
pgoff_t index = map_bh.b_blocknr >>
(PAGE_CACHE_SHIFT - inode->i_blkbits);
if (!ra_has_index(&filp->f_ra, index))
- page_cache_readahead_ondemand(
+ page_cache_sync_readahead(
sb->s_bdev->bd_inode->i_mapping,
&filp->f_ra, filp,
- NULL, index, 1);
+ index, 1);
filp->f_ra.prev_index = index;
bh = ext3_bread(NULL, inode, blk, 0, &err);
}
diff -r e96f11f61577 fs/ext4/dir.c
--- a/fs/ext4/dir.c Sun Jun 10 18:25:28 2007 +1000
+++ b/fs/ext4/dir.c Wed Jun 13 15:16:49 2007 +1000
@@ -138,10 +138,10 @@ static int ext4_readdir(struct file * fi
pgoff_t index = map_bh.b_blocknr >>
(PAGE_CACHE_SHIFT - inode->i_blkbits);
if (!ra_has_index(&filp->f_ra, index))
- page_cache_readahead_ondemand(
+ page_cache_sync_readahead(
sb->s_bdev->bd_inode->i_mapping,
&filp->f_ra, filp,
- NULL, index, 1);
+ index, 1);
filp->f_ra.prev_index = index;
bh = ext4_bread(NULL, inode, blk, 0, &err);
}
diff -r e96f11f61577 fs/splice.c
--- a/fs/splice.c Sun Jun 10 18:25:28 2007 +1000
+++ b/fs/splice.c Wed Jun 13 15:16:49 2007 +1000
@@ -312,8 +312,8 @@ __generic_file_splice_read(struct file *
*/
page = find_get_page(mapping, index);
if (!page) {
- page_cache_readahead_ondemand(mapping, &in->f_ra, in,
- NULL, index, nr_pages - spd.nr_pages);
+ page_cache_sync_readahead(mapping, &in->f_ra, in,
+ index, nr_pages - spd.nr_pages);
/*
* page didn't exist, allocate one.
@@ -361,7 +361,7 @@ __generic_file_splice_read(struct file *
page = pages[page_nr];
if (PageReadahead(page))
- page_cache_readahead_ondemand(mapping, &in->f_ra, in,
+ page_cache_async_readahead(mapping, &in->f_ra, in,
page, index, nr_pages - page_nr);
/*
diff -r e96f11f61577 include/linux/mm.h
--- a/include/linux/mm.h Sun Jun 10 18:25:28 2007 +1000
+++ b/include/linux/mm.h Wed Jun 13 15:16:49 2007 +1000
@@ -1146,12 +1146,20 @@ int do_page_cache_readahead(struct addre
pgoff_t offset, unsigned long nr_to_read);
int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
pgoff_t offset, unsigned long nr_to_read);
-unsigned long page_cache_readahead_ondemand(struct address_space *mapping,
- struct file_ra_state *ra,
- struct file *filp,
- struct page *page,
- pgoff_t offset,
- unsigned long size);
+
+void page_cache_sync_readahead(struct address_space *mapping,
+ struct file_ra_state *ra,
+ struct file *filp,
+ pgoff_t offset,
+ unsigned long size);
+
+void page_cache_async_readahead(struct address_space *mapping,
+ struct file_ra_state *ra,
+ struct file *filp,
+ struct page *pg,
+ pgoff_t offset,
+ unsigned long size);
+
unsigned long max_sane_readahead(unsigned long nr);
/* Do stack extension */
diff -r e96f11f61577 mm/filemap.c
--- a/mm/filemap.c Sun Jun 10 18:25:28 2007 +1000
+++ b/mm/filemap.c Wed Jun 13 15:16:49 2007 +1000
@@ -913,15 +913,15 @@ find_page:
find_page:
page = find_get_page(mapping, index);
if (!page) {
- page_cache_readahead_ondemand(mapping,
- &ra, filp, page,
+ page_cache_sync_readahead(mapping,
+ &ra, filp,
index, last_index - index);
page = find_get_page(mapping, index);
if (unlikely(page == NULL))
goto no_cached_page;
}
if (PageReadahead(page)) {
- page_cache_readahead_ondemand(mapping,
+ page_cache_async_readahead(mapping,
&ra, filp, page,
index, last_index - index);
}
@@ -1382,14 +1382,14 @@ retry_find:
*/
if (VM_SequentialReadHint(vma)) {
if (!page) {
- page_cache_readahead_ondemand(mapping, ra, file, page,
+ page_cache_sync_readahead(mapping, ra, file,
fdata->pgoff, 1);
page = find_lock_page(mapping, fdata->pgoff);
if (!page)
goto no_cached_page;
}
if (PageReadahead(page)) {
- page_cache_readahead_ondemand(mapping, ra, file, page,
+ page_cache_async_readahead(mapping, ra, file, page,
fdata->pgoff, 1);
}
}
diff -r e96f11f61577 mm/readahead.c
--- a/mm/readahead.c Sun Jun 10 18:25:28 2007 +1000
+++ b/mm/readahead.c Wed Jun 13 15:16:49 2007 +1000
@@ -351,7 +351,7 @@ static unsigned long
static unsigned long
ondemand_readahead(struct address_space *mapping,
struct file_ra_state *ra, struct file *filp,
- struct page *page, pgoff_t offset,
+ bool hit_readahead_marker, pgoff_t offset,
unsigned long req_size)
{
unsigned long max; /* max readahead pages */
@@ -379,7 +379,7 @@ ondemand_readahead(struct address_space
* Standalone, small read.
* Read as is, and do not pollute the readahead state.
*/
- if (!page && !sequential) {
+ if (!hit_readahead_marker && !sequential) {
return __do_page_cache_readahead(mapping, filp,
offset, req_size, 0);
}
@@ -400,7 +400,7 @@ ondemand_readahead(struct address_space
* E.g. interleaved reads.
* Not knowing its readahead pos/size, bet on the minimal possible one.
*/
- if (page) {
+ if (hit_readahead_marker) {
ra_index++;
ra_size = min(4 * ra_size, max);
}
@@ -413,50 +413,71 @@ fill_ra:
}
/**
- * page_cache_readahead_ondemand - generic file readahead
+ * page_cache_sync_readahead - generic file readahead
* @mapping: address_space which holds the pagecache and I/O vectors
* @ra: file_ra_state which holds the readahead state
* @filp: passed on to ->readpage() and ->readpages()
- * @page: the page at @offset, or NULL if non-present
- * @offset: start offset into @mapping, in PAGE_CACHE_SIZE units
+ * @offset: start offset into @mapping, in pagecache page-sized units
* @req_size: hint: total size of the read which the caller is performing in
- * PAGE_CACHE_SIZE units
- *
- * page_cache_readahead_ondemand() is the entry point of readahead logic.
- * This function should be called when it is time to perform readahead:
- * 1) @page == NULL
- * A cache miss happened, time for synchronous readahead.
- * 2) @page != NULL && PageReadahead(@page)
- * A look-ahead hit occured, time for asynchronous readahead.
- */
-unsigned long
-page_cache_readahead_ondemand(struct address_space *mapping,
- struct file_ra_state *ra, struct file *filp,
- struct page *page, pgoff_t offset,
- unsigned long req_size)
+ * pagecache pages
+ *
+ * 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 offset, unsigned long req_size)
{
/* no read-ahead */
if (!ra->ra_pages)
- return 0;
-
- if (page) {
- /*
- * It can be PG_reclaim.
- */
- if (PageWriteback(page))
- return 0;
-
- ClearPageReadahead(page);
-
- /*
- * Defer asynchronous read-ahead on IO congestion.
- */
- if (bdi_read_congested(mapping->backing_dev_info))
- return 0;
- }
+ return;
/* do read-ahead */
- return ondemand_readahead(mapping, ra, filp, page,
- offset, req_size);
-}
-EXPORT_SYMBOL_GPL(page_cache_readahead_ondemand);
+ ondemand_readahead(mapping, ra, filp, false, offset, req_size);
+}
+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
+ * @ra: file_ra_state which holds the readahead state
+ * @filp: passed on to ->readpage() and ->readpages()
+ * @page: the page at @offset which has the PG_readahead flag set
+ * @offset: start offset into @mapping, in pagecache page-sized units
+ * @req_size: hint: total size of the read which the caller is performing in
+ * pagecache pages
+ *
+ * page_cache_async_ondemand() should be called when a page is used which
+ * has the PG_readahead flag: 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 offset,
+ unsigned long req_size)
+{
+ /* no read-ahead */
+ if (!ra->ra_pages)
+ return;
+
+ /*
+ * Same bit is used for PG_readahead and PG_reclaim.
+ */
+ if (PageWriteback(page))
+ return;
+
+ ClearPageReadahead(page);
+
+ /*
+ * Defer asynchronous read-ahead on IO congestion.
+ */
+ if (bdi_read_congested(mapping->backing_dev_info))
+ return;
+
+ /* do read-ahead */
+ ondemand_readahead(mapping, ra, filp, true, offset, req_size);
+}
+EXPORT_SYMBOL_GPL(page_cache_async_readahead);
-
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