linux-kernel - [PATCH] block: make sure big bio is splitted into at most 256 bvecs

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Date:	Wed,  6 Apr 2016 01:44:06 +0800
From:	Ming Lei <ming.lei@...onical.com>
To:	Jens Axboe <axboe@...com>, linux-kernel@...r.kernel.org
Cc:	linux-block@...r.kernel.org, kent.overstreet@...il.com,
	Christoph Hellwig <hch@...radead.org>,
	Eric Wheeler <bcache@...ts.ewheeler.net>,
	Sebastian Roesner <sroesner-kernelorg@...sner-online.de>,
	Ming Lei <ming.lei@...onical.com>,
	stable@...r.kernel.org (4.2+), Shaohua Li <shli@...com>
Subject: [PATCH] block: make sure big bio is splitted into at most 256 bvecs

After arbitrary bio size is supported, the incoming bio may
be very big. We have to split the bio into small bios so that
each holds at most BIO_MAX_PAGES bvecs for safety reason, such
as bio_clone().

This patch fixes the following kernel crash:

> [  172.660142] BUG: unable to handle kernel NULL pointer dereference at
> 0000000000000028
> [  172.660229] IP: [<ffffffff811e53b4>] bio_trim+0xf/0x2a
> [  172.660289] PGD 7faf3e067 PUD 7f9279067 PMD 0
> [  172.660399] Oops: 0000 [#1] SMP
> [...]
> [  172.664780] Call Trace:
> [  172.664813]  [<ffffffffa007f3be>] ? raid1_make_request+0x2e8/0xad7 [raid1]
> [  172.664846]  [<ffffffff811f07da>] ? blk_queue_split+0x377/0x3d4
> [  172.664880]  [<ffffffffa005fb5f>] ? md_make_request+0xf6/0x1e9 [md_mod]
> [  172.664912]  [<ffffffff811eb860>] ? generic_make_request+0xb5/0x155
> [  172.664947]  [<ffffffffa0445c89>] ? prio_io+0x85/0x95 [bcache]
> [  172.664981]  [<ffffffffa0448252>] ? register_cache_set+0x355/0x8d0 [bcache]
> [  172.665016]  [<ffffffffa04497d3>] ? register_bcache+0x1006/0x1174 [bcache]

Fixes: 54efd50(block: make generic_make_request handle arbitrarily sized bios)
Reported-by: Sebastian Roesner <sroesner-kernelorg@...sner-online.de>
Reported-by: Eric Wheeler <bcache@...ts.ewheeler.net>
Cc: stable@...r.kernel.org (4.2+)
Cc: Shaohua Li <shli@...com>
Signed-off-by: Ming Lei <ming.lei@...onical.com>
---
I can reproduce the issue and verify the fix by the following approach:
	- create one raid1 over two virtio-blk 
	- build bcache device over the above raid1 and another cache device.
	- set cache mode as writeback
	- run random write over ext4 on the bcache device
	- then the crash can be triggered

 block/blk-merge.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/block/blk-merge.c b/block/blk-merge.c
index 2613531..9a8651f 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -79,6 +79,18 @@ static inline unsigned get_max_io_size(struct request_queue *q,
 	/* aligned to logical block size */
 	sectors &= ~(mask >> 9);
 
+	/*
+	 * With arbitrary bio size, the incoming bio may be very big.
+	 * We have to split the bio into small bios so that each holds
+	 * at most BIO_MAX_PAGES bvecs for safety reason, such as
+	 * bio_clone().
+	 *
+	 * In the future, the limit might be converted into per-queue
+	 * flag.
+	 */
+	sectors = min_t(unsigned, sectors, BIO_MAX_PAGES <<
+			(PAGE_CACHE_SHIFT - 9));
+
 	return sectors;
 }
 
-- 
1.9.1