Re: [PATCH] block: copy bi_vcnt in __bio_clone_fast

From: Ming Lei
Date: Thu Oct 09 2014 - 11:25:39 EST

On Thu, Oct 9, 2014 at 10:26 PM, Jeff Mahoney <jeffm@xxxxxxxx> wrote:
> Hash: SHA1
> On 10/9/14, 9:53 AM, Jeff Moyer wrote:
>> Jeff Mahoney <jeffm@xxxxxxxx> writes:
>>> Commit 05f1dd53152173 (block: add queue flag for disabling SG
>>> merging) uses bi_vcnt to assign bio->bi_phys_segments if sg
>>> merging is disabled. When using device mapper on top of a blk-mq
>>> device (virtio_blk in my test), we'd end up overflowing the
>>> scatterlist in __blk_bios_map_sg.
>>> __bio_clone_fast copies bi_iter and bi_io_vec but not bi_vcnt,
>>> so blk_recount_segments would report bi_phys_segments as 0.
>>> Since rq->nr_phys_segments is 0 as well, the checks to ensure
>>> that we don't exceed the queue's segment limit end up allowing
>>> more bios (and segments) to attach the a request until we finally
>>> map it. That also means we pass the BUG_ON at the beginning of
>>> virtio_queue_rq, ultimately causing memory corruption and a
>>> crash.
>>> If we copy bi_vcnt in __bio_clone_fast, the bios and requests
>>> properly report the number of segments and everything works as
>>> expected.
>>> Originally reported at
>> Hi, Jeff,
>> Did you manage to reproduce this problem with commit 0738854
>> (blk-merge: fix blk_recount_segments) applied? Or perhaps with
>> commit 200612e (dm table: propagate QUEUE_FLAG_NO_SG_MERGE)?
> Yep. I was able to reproduce it with 3.17. I did try 0738854 when I
> was still using 3.16 since it looked like a good candidate. Neither of
> those patches affect the problem here. bio->bi_phys_segments never
> gets a value set in the fast clone case and that translates to
> req->nr_phys_segments never getting properly accumulated. That might
> not be a problem except that the NO_SG_MERGE behavior bypasses the
> iteration that would come up with the correct value. In either case,

This patch may get incorrect rq->nr_phys_segments because
bio cloning is often used in case of I/O splitting, so could you
test if the attached patch fixes your problem?

Ming Lei
From d3946db93c8eb1da4a0c32249d603f415febf09e Mon Sep 17 00:00:00 2001
From: Ming Lei <ming.lei@xxxxxxxxxxxxx>
Date: Thu, 9 Oct 2014 23:17:35 +0800
Subject: [PATCH] blk-merge: don't compute bi_phys_segments from bi_vcnt for
cloned bio

It isn't correct to figure out req->bi_phys_segments from bio->bi_vcnt
if the bio is cloned.

Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxxxxx>
block/blk-merge.c | 1 +
1 file changed, 1 insertion(+)

diff --git a/block/blk-merge.c b/block/blk-merge.c
index f71bad3..d5cad1a 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -98,6 +98,7 @@ void blk_recalc_rq_segments(struct request *rq)
void blk_recount_segments(struct request_queue *q, struct bio *bio)
if (test_bit(QUEUE_FLAG_NO_SG_MERGE, &q->queue_flags) &&
+ !bio_flagged(bio, BIO_CLONED) &&
bio->bi_vcnt < queue_max_segments(q))
bio->bi_phys_segments = bio->bi_vcnt;
else {