linux-kernel - Re: [PATCH 0/2] iter revert problems

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Message-ID: <a03abd9e-82c1-7a63-a0dc-c7319f0c0751@gmail.com>
Date:   Mon, 9 Aug 2021 19:56:21 +0100
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     linux-fsdevel@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
        io-uring@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] iter revert problems

On 8/9/21 4:52 PM, Al Viro wrote:
> On Mon, Aug 09, 2021 at 12:52:35PM +0100, Pavel Begunkov wrote:
>> For the bug description see 2/2. As mentioned there the current problems
>> is because of generic_write_checks(), but there was also a similar case
>> fixed in 5.12, which should have been triggerable by normal
>> write(2)/read(2) and others.
>>
>> It may be better to enforce reexpands as a long term solution, but for
>> now this patchset is quickier and easier to backport.
> 
> 	Umm...  Won't that screw the cases where we *are* doing proper
> reexpands?  AFAICS, with your patches that flag doesn't go away once
> it had been set...

In general, the userspace should already expecting and retrying on
EAGAIN, and it seems to me, truncates should be rare enough to not
care much about performance. However, it'd better to be more careful
with nowait attempts.

For instance, we can avoid failing reexpanded and reverted iters.

if (i->truncated && iov_iter_count(i) != orig_size)
	// fail;

Or even re-import iov+iter, if still in the right context.


Al, is that viable to you on the iov side?

-- 
Pavel Begunkov