linux-ext4 - [PATCH 03/11] MM: improve cleanup when ->readpages doesn't process all pages.

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Date:   Thu, 10 Feb 2022 16:37:52 +1100
From:   NeilBrown <neilb@...e.de>
To:     Andrew Morton <akpm@...ux-foundation.org>, Jan Kara <jack@...e.cz>,
        Wu Fengguang <fengguang.wu@...el.com>,
        Jaegeuk Kim <jaegeuk@...nel.org>, Chao Yu <chao@...nel.org>,
        Jeff Layton <jlayton@...nel.org>,
        Ilya Dryomov <idryomov@...il.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        Ryusuke Konishi <konishi.ryusuke@...il.com>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Philipp Reisner <philipp.reisner@...bit.com>,
        Lars Ellenberg <lars.ellenberg@...bit.com>,
        Paolo Valente <paolo.valente@...aro.org>,
        Jens Axboe <axboe@...nel.dk>
Cc:     linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-nilfs@...r.kernel.org, linux-nfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, linux-ext4@...r.kernel.org,
        ceph-devel@...r.kernel.org, drbd-dev@...ts.linbit.com,
        linux-kernel@...r.kernel.org, linux-block@...r.kernel.org
Subject: [PATCH 03/11] 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().

Signed-off-by: NeilBrown <neilb@...e.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);
 		}