mm: document why we don't set PageReadahead

If the page is already in cache, we don't set PageReadahead on it.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: William Kucharski <william.kucharski@oracle.com>
Cc: Chao Yu <yuchao0@huawei.com>
Cc: Cong Wang <xiyou.wangcong@gmail.com>
Cc: Darrick J. Wong <darrick.wong@oracle.com>
Cc: Dave Chinner <dchinner@redhat.com>
Cc: Eric Biggers <ebiggers@google.com>
Cc: Gao Xiang <gaoxiang25@huawei.com>
Cc: Jaegeuk Kim <jaegeuk@kernel.org>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Joseph Qi <joseph.qi@linux.alibaba.com>
Cc: Junxiao Bi <junxiao.bi@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Cc: Miklos Szeredi <mszeredi@redhat.com>
Link: http://lkml.kernel.org/r/20200414150233.24495-15-willy@infradead.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/readahead.c b/mm/readahead.c
index ae231a5312cbe421af69e8cf5aca35edb2a2439b..73cb59ed5cffd0c25b74a2691e076c10b1df876b 100644 (file)
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -195,9 +195,12 @@ void page_cache_readahead_unbounded(struct address_space *mapping,
 
                if (page && !xa_is_value(page)) {
                        /*
-                        * Page already present?  Kick off the current batch of
-                        * contiguous pages before continuing with the next
-                        * batch.
+                        * Page already present?  Kick off the current batch
+                        * of contiguous pages before continuing with the
+                        * next batch.  This page may be the one we would
+                        * have intended to mark as Readahead, but we don't
+                        * have a stable reference to this page, and it's
+                        * not worth getting one just for that.
                         */
                        read_pages(&rac, &page_pool, true);
                        continue;