mm: improve cleanup when ->readpages doesn't process all pages

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().

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: NeilBrown <[email protected]>
Cc: Anna Schumaker <[email protected]>
Cc: Chao Yu <[email protected]>
Cc: Darrick J. Wong <[email protected]>
Cc: Ilya Dryomov <[email protected]>
Cc: Jaegeuk Kim <[email protected]>
Cc: Jan Kara <[email protected]>
Cc: Jeff Layton <[email protected]>
Cc: Jens Axboe <[email protected]>
Cc: Lars Ellenberg <[email protected]>
Cc: Miklos Szeredi <[email protected]>
Cc: Paolo Valente <[email protected]>
Cc: Philipp Reisner <[email protected]>
Cc: Ryusuke Konishi <[email protected]>
Cc: Trond Myklebust <[email protected]>
Cc: Wu Fengguang <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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);
 		}