linux-kernel - Re: [PATCH] Fix thinko in iov_iter_single_seg

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Message-ID: <20141113130509.GI7996@ZenIV.linux.org.uk>
Date:	Thu, 13 Nov 2014 13:05:09 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Paul Mackerras <paulus@...ba.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Fix thinko in iov_iter_single_seg_count

On Thu, Nov 13, 2014 at 08:15:23PM +1100, Paul Mackerras wrote:
> The branches of the if (i->type & ITER_BVEC) statement in
> iov_iter_single_seg_count() are the wrong way around; if ITER_BVEC is
> clear then we use i->bvec, when we should be using i->iov.  This fixes
> it.
> 
> In my case, the symptom that this caused was that a KVM guest doing
> filesystem operations on a virtual disk would result in one of qemu's
> threads on the host going into an infinite loop in
> generic_perform_write().  The loop would hit the copied == 0 case and
> call iov_iter_single_seg_count() to reduce the number of bytes to try
> to process, but because of the error, iov_iter_single_seg_count()
> would just return i->count and the loop made no progress and continued
> forever.

ACK.  Applied, will push today.
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