[3.16.y-ckt stable] Patch "cfq-iosched: handle failure of cfq group allocation" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    cfq-iosched: handle failure of cfq group allocation

to the linux-3.16.y-queue branch of the 3.16.y-ckt extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.16.y-queue

This patch is scheduled to be released in version 3.16.7-ckt8.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.16.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From 3b3c7a2a4fc27e14c6fbc2e3d37ff4e62b01d889 Mon Sep 17 00:00:00 2001
From: Konstantin Khlebnikov <khlebnikov at yandex-team.ru>
Date: Mon, 9 Feb 2015 16:42:49 +0300
Subject: cfq-iosched: handle failure of cfq group allocation

commit 69abaffec7d47a083739b79e3066cb3730eba72e upstream.

Cfq_lookup_create_cfqg() allocates struct blkcg_gq using GFP_ATOMIC.
In cfq_find_alloc_queue() possible allocation failure is not handled.
As a result kernel oopses on NULL pointer dereference when
cfq_link_cfqq_cfqg() calls cfqg_get() for NULL pointer.

Bug was introduced in v3.5 in commit cd1604fab4f9 ("blkcg: factor
out blkio_group creation"). Prior to that commit cfq group lookup
had returned pointer to root group as fallback.

This patch handles this error using existing fallback oom_cfqq.

Signed-off-by: Konstantin Khlebnikov <khlebnikov at yandex-team.ru>
Acked-by: Tejun Heo <tj at kernel.org>
Acked-by: Vivek Goyal <vgoyal at redhat.com>
Fixes: cd1604fab4f9 ("blkcg: factor out blkio_group creation")
Signed-off-by: Jens Axboe <axboe at fb.com>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 block/cfq-iosched.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index f3627ca9303b..2c0585adfaea 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -3582,6 +3582,11 @@ retry:

 	blkcg = bio_blkcg(bio);
 	cfqg = cfq_lookup_create_cfqg(cfqd, blkcg);
+	if (!cfqg) {
+		cfqq = &cfqd->oom_cfqq;
+		goto out;
+	}
+
 	cfqq = cic_to_cfqq(cic, is_sync);

 	/*
@@ -3618,7 +3623,7 @@ retry:
 		} else
 			cfqq = &cfqd->oom_cfqq;
 	}
-
+out:
 	if (new_cfqq)
 		kmem_cache_free(cfq_pool, new_cfqq);