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Subject[PATCH 03/11] MM: improve cleanup when ->readpages doesn't process all pages.
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Date
If ->readpages doesn't process all the pages, then it is best to act as
though they weren't requested so that a subsequent readahead can try
again.
So:
- remove any 'ahead' pages from the page cache so they can be loaded
with ->readahead() rather then multiple ->read()s
- update the file_ra_state to reflect the reads that were actually
submitted.

This allows ->readpages() to abort early due e.g. to congestion, which
will then allow us to remove the inode_read_congested() test from
page_Cache_async_ra().

Signed-off-by: NeilBrown <neilb@suse.de>
---
mm/readahead.c | 19 +++++++++++++++++--
1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/mm/readahead.c b/mm/readahead.c
index c44b2957f59f..35a7ebfcb504 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -104,7 +104,13 @@
* for necessary resources (e.g. memory or indexing information) to
* become available. Pages in the final ``async_size`` may be
* considered less urgent and failure to read them is more acceptable.
- * They will eventually be read individually using ->readpage().
+ * In this case it is best to use delete_from_page_cache() to remove the
+ * pages from the page cache as is automatically done for pages that
+ * were not fetched with readahead_page(). This will allow a
+ * subsequent synchronous read ahead request to try them again. If they
+ * are left in the page cache, then they will be read individually using
+ * ->readpage().
+ *
*/

#include <linux/kernel.h>
@@ -226,8 +232,17 @@ static void read_pages(struct readahead_control *rac, struct list_head *pages,

if (aops->readahead) {
aops->readahead(rac);
- /* Clean up the remaining pages */
+ /*
+ * Clean up the remaining pages. The sizes in ->ra
+ * maybe be used to size next read-ahead, so make sure
+ * they accurately reflect what happened.
+ */
while ((page = readahead_page(rac))) {
+ rac->ra->size -= 1;
+ if (rac->ra->async_size > 0) {
+ rac->ra->async_size -= 1;
+ delete_from_page_cache(page);
+ }
unlock_page(page);
put_page(page);
}

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