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SubjectRe: [PATCH 5/9] readahead: on-demand readahead logic
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On Tue, 2007-06-12 at 18:35 +0800, Fengguang Wu wrote:
> > This seems a little like two functions crammed into one. Do you think
> > page_cache_readahead_ondemand() should be split into
> > "page_cache_readahead()" which doesn't take a page*, and
> > "page_cache_check_readahead_page()" which is an inline which does the
> > PageReadahead(page) check as well?
>
> page_cache_check_readahead_page(..., page) is a good idea.
> But which part of the code should we put to page_cache_readahead()
> that does not take a page param?

OK, here's my attempt. I also made them return void, since none of the
callers seem to mind. I didn't change the internals much: I think
they're pretty clear and I didn't want to clash if you decided to rename
the "ra" fields. It compiles and boots.

Thoughts?

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

diff -r 3c8ae0c37063 fs/ext3/dir.c
--- a/fs/ext3/dir.c Tue Jun 12 17:17:25 2007 +1000
+++ b/fs/ext3/dir.c Wed Jun 13 11:17:42 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 3c8ae0c37063 fs/ext4/dir.c
--- a/fs/ext4/dir.c Tue Jun 12 17:17:25 2007 +1000
+++ b/fs/ext4/dir.c Wed Jun 13 11:19:56 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 3c8ae0c37063 fs/splice.c
--- a/fs/splice.c Tue Jun 12 17:17:25 2007 +1000
+++ b/fs/splice.c Wed Jun 13 11:18:38 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.
@@ -360,8 +360,7 @@ __generic_file_splice_read(struct file *
this_len = min_t(unsigned long, len, PAGE_CACHE_SIZE - loff);
page = pages[page_nr];

- if (PageReadahead(page))
- page_cache_readahead_ondemand(mapping, &in->f_ra, in,
+ page_cache_check_readahead_page(mapping, &in->f_ra, in,
page, index, nr_pages - page_nr);

/*
diff -r 3c8ae0c37063 include/linux/mm.h
--- a/include/linux/mm.h Tue Jun 12 17:17:25 2007 +1000
+++ b/include/linux/mm.h Wed Jun 13 11:25:10 2007 +1000
@@ -1146,12 +1146,34 @@ 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);
+
+/* 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);
+}
+
unsigned long max_sane_readahead(unsigned long nr);

/* Do stack extension */
diff -r 3c8ae0c37063 mm/filemap.c
--- a/mm/filemap.c Tue Jun 12 17:17:25 2007 +1000
+++ b/mm/filemap.c Wed Jun 13 11:22:04 2007 +1000
@@ -913,18 +913,16 @@ 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_check_readahead_page(mapping,
&ra, filp, page,
index, last_index - index);
- }
if (!PageUptodate(page))
goto page_not_up_to_date;
page_ok:
@@ -1382,16 +1380,14 @@ 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_check_readahead_page(mapping, ra, file, page,
fdata->pgoff, 1);
- }
}

if (!page) {
diff -r 3c8ae0c37063 mm/readahead.c
--- a/mm/readahead.c Tue Jun 12 17:17:25 2007 +1000
+++ b/mm/readahead.c Wed Jun 13 11:09:39 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_lookahead_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_lookahead_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_lookahead_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
* @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)
+ * 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 PageReadahead flag set
+ * @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
+ *
+ * page_cache_async_ondemand() should be called when a page is used which
+ * has the PageReadahead 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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