linux-kernel - [PATCH] mm/filemap: do not allocate cache pages beyond end of file at read

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Date:   Mon, 28 Oct 2019 12:59:34 +0300
From:   Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
To:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org
Cc:     linux-fsdevel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Johannes Weiner <hannes@...xchg.org>
Subject: [PATCH] mm/filemap: do not allocate cache pages beyond end of file
 at read

Page cache could contain pages beyond end of file during write or
if read races with truncate. But generic_file_buffered_read() always
allocates unneeded pages beyond eof if somebody reads here and one
extra page at the end if file size is page-aligned.

Function generic_file_buffered_read() calls page_cache_sync_readahead()
if page not found in cache and then do another lookup. Readahead checks
file size in __do_page_cache_readahead() before allocating pages.
After that generic_file_buffered_read() falls back to slow path and
allocates page for ->readpage() without checking file size.

This patch checks file size before allocating page for ->readpage().

Signed-off-by: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
---
 mm/filemap.c |    4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mm/filemap.c b/mm/filemap.c
index 85b7d087eb45..92abf5f348a9 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2225,6 +2225,10 @@ static ssize_t generic_file_buffered_read(struct kiocb *iocb,
 		goto out;

 no_cached_page:
+		/* Do not allocate cache pages beyond end of file. */
+		if (((loff_t)index << PAGE_SHIFT) >= i_size_read(inode))
+			goto out;
+
 		/*
 		 * Ok, it wasn't cached, so we need to create a new
 		 * page..