[PATCH 3.19.y-ckt 096/164] fs, seqfile: always allow oom killer

[Kamal Mostafa]

3.19.8-ckt11 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Greg Thelen <gthelen at google.com>

commit 0f930902eb8806cff8dcaef9ff9faf3cfa5fd748 upstream.

Since 5cec38ac866b ("fs, seq_file: fallback to vmalloc instead of oom kill
processes") seq_buf_alloc() avoids calling the oom killer for PAGE_SIZE or
smaller allocations; but larger allocations can use the oom killer via
vmalloc().  Thus reads of small files can return ENOMEM, but larger files
use the oom killer to avoid ENOMEM.

The effect of this bug is that reads from /proc and other virtual
filesystems can return ENOMEM instead of the preferred behavior - oom
killing something (possibly the calling process).  I don't know of anyone
except Google who has noticed the issue.

I suspect the fix is more needed in smaller systems where there isn't any
reclaimable memory.  But these seem like the kinds of systems which
probably don't use the oom killer for production situations.

Memory overcommit requires use of the oom killer to select a victim
regardless of file size.

Enable oom killer for small seq_buf_alloc() allocations.

Fixes: 5cec38ac866b ("fs, seq_file: fallback to vmalloc instead of oom kill processes")
Signed-off-by: David Rientjes <rientjes at google.com>
Signed-off-by: Greg Thelen <gthelen at google.com>
Acked-by: Eric Dumazet <edumazet at google.com>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 fs/seq_file.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/fs/seq_file.c b/fs/seq_file.c
index dbf3a59..0950408 100644
--- a/fs/seq_file.c
+++ b/fs/seq_file.c
@@ -24,12 +24,17 @@ static void seq_set_overflow(struct seq_file *m)
 static void *seq_buf_alloc(unsigned long size)
 {
 	void *buf;
+	gfp_t gfp = GFP_KERNEL;
 
 	/*
-	 * __GFP_NORETRY to avoid oom-killings with high-order allocations -
-	 * it's better to fall back to vmalloc() than to kill things.
+	 * For high order allocations, use __GFP_NORETRY to avoid oom-killing -
+	 * it's better to fall back to vmalloc() than to kill things.  For small
+	 * allocations, just use GFP_KERNEL which will oom kill, thus no need
+	 * for vmalloc fallback.
 	 */
-	buf = kmalloc(size, GFP_KERNEL | __GFP_NORETRY | __GFP_NOWARN);
+	if (size > PAGE_SIZE)
+		gfp |= __GFP_NORETRY | __GFP_NOWARN;
+	buf = kmalloc(size, gfp);
 	if (!buf && size > PAGE_SIZE)
 		buf = vmalloc(size);
 	return buf;
-- 
1.9.1