linux-ext4 - [PATCH v7 23/24] mm: Document why we don't set PageReadahead

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Message-Id: <20200219210103.32400-24-willy@infradead.org>
Date:   Wed, 19 Feb 2020 13:01:02 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     linux-fsdevel@...r.kernel.org
Cc:     "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-btrfs@...r.kernel.org, linux-erofs@...ts.ozlabs.org,
        linux-ext4@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net,
        cluster-devel@...hat.com, ocfs2-devel@....oracle.com,
        linux-xfs@...r.kernel.org
Subject: [PATCH v7 23/24] mm: Document why we don't set PageReadahead

From: "Matthew Wilcox (Oracle)" <willy@...radead.org>

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

Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
---
 mm/readahead.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/mm/readahead.c b/mm/readahead.c
index 453ef146de83..bbe7208fcc2d 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -196,9 +196,12 @@ 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);
 			continue;
-- 
2.25.0