Date:	Thu, 7 Apr 2016 23:29:07 +0000 (UTC)
From:	Eric Wheeler <bcache@...ts.ewheeler.net>
To:	Ming Lei <ming.lei@...onical.com>
cc:	Jens Axboe <axboe@...com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-block@...r.kernel.org,
	Kent Overstreet <kent.overstreet@...il.com>,
	Christoph Hellwig <hch@...radead.org>,
	Sebastian Roesner <sroesner-kernelorg@...sner-online.de>,
	"4.2+" <stable@...r.kernel.org>, Shaohua Li <shli@...com>
Subject: Re: [PATCH] block: make sure big bio is splitted into at most 256
 bvecs

On Thu, 7 Apr 2016, Ming Lei wrote:

> On Thu, Apr 7, 2016 at 9:56 AM, Eric Wheeler <bcache@...ts.ewheeler.net> wrote:
> > On Thu, 7 Apr 2016, Ming Lei wrote:
> >
> >> On Thu, Apr 7, 2016 at 9:36 AM, Eric Wheeler <bcache@...ts.ewheeler.net> wrote:
> >> > On Wed, 6 Apr 2016, Ming Lei wrote:
> >> >
> >> >> 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+)
> >> >
> >> > Ming Lei,
> >> >
> >> > get_max_io_size doesn't appear until 4.5 based on a quick LXR check so we
> >> > won't see it in stable I don't think.
> >> >
> >> > It would be nice to see this fixed in 4.1 (if affected there).  Is there
> >>
> >> The issue should be introduced to v4.3 via 54efd50
> >>
> >> > another place this could be applied to be a bit more backward compatible?
> >>
> >> The v1 needn't change to get_max_io_size(), and it should be simple enough
> >> to backport to previous stables, please try it:
> >>
> >> http://marc.info/?l=linux-block&m=145991422422927&w=2
> >
> > V1 changes blk_bio_segment_split() which doesn't exist until 4.3.
> >
> > How might you port this to v4.1.y?
> 
> Can you see the issue with v4.1?
> 
> You mentioned there are three reports:
> 
> > [2014-02-04] https://bugzilla.redhat.com/show_bug.cgi?id=1061339
> > [2016-01-13] http://www.spinics.net/lists/linux-bcache/msg03335.html
>              https://bugzilla.kernel.org/show_bug.cgi?id=110771
> > [2016-03-25] http://thread.gmane.org/gmane.linux.kernel.bcache.devel/3607 [this thread]
>              https://bugzilla.kernel.org/show_bug.cgi?id=114871
> 
> The first one has been fixed by '8423ae3 block: Fix cloning of
> discard/write same bios', as mentioned in bugzilla.
> 
> The other two are reported on v4.4 and v4.5.
> 
> If you look at the patch description, it is just needed for 4.3+.


Oh, that could be---but just to be sure:

I had thought perhaps this was an old issue since the first mention of 
this backtrace (but not bcache) was in 3.14 back in 2014 based on this 
post:
  https://bugzilla.redhat.com/show_bug.cgi?id=1061339

Is this relevant?

--
Eric Wheeler



> 
> Or I am wrong?
> 
> 
> Thanks,
> 
> >
> > --
> > Eric Wheeler
> >
> >
> >>
> >> Thanks,
> >>
> >> >
> >> >> 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
> >> >>
> >> >>
> >> > --
> >> > To unsubscribe from this list: send the line "unsubscribe linux-block" in
> >> > the body of a message to majordomo@...r.kernel.org
> >> > More majordomo info at  http://vger.kernel.org/majordomo-info.html
> >>
> 

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