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From: David Laight <David.Laight@ACULAB.COM>
To: 'Al Viro' <viro@zeniv.linux.org.uk>,
	Pavel Begunkov <asml.silence@gmail.com>
Cc: "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
	"Jens Axboe" <axboe@kernel.dk>,
	"io-uring@vger.kernel.org" <io-uring@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: RE: [PATCH 0/2] iter revert problems
Date: Tue, 10 Aug 2021 08:47:13 +0000	[thread overview]
Message-ID: <1fc5348f7598404088d4ecda3bbb397a@AcuMS.aculab.com> (raw)
In-Reply-To: <YRFPR25scNRYaRzW@zeniv-ca.linux.org.uk>

From: Al Viro
> Sent: 09 August 2021 16:53
> 
> 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...

From what I remember the pointer into the iov[] gets incremented
as it is processed - which makes 'backing up' hard.
The caller also has to remember the original pointer because
it might point to kmalloced memory.

So if the 'iter' always contained a pointer to the base of the iov[]
then various bits of code could be simplified.

Another useful change would be to embed the short iov_cache[8]
inside 'iter'.
Almost all the callers allocate both together (usually on stack)
so the stack use won't change.
I have local patches for most of this (somewhere) but the io_uring
changes start being non-trivial.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)


      parent reply	other threads:[~2021-08-10  8:47 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-09 11:52 Pavel Begunkov
2021-08-09 11:52 ` [PATCH 1/2] iov_iter: mark truncated iters Pavel Begunkov
2021-08-09 11:52 ` [PATCH 2/2] io_uring: don't retry with truncated iter Pavel Begunkov
2021-08-09 15:52 ` [PATCH 0/2] iter revert problems Al Viro
2021-08-09 18:56   ` Pavel Begunkov
2021-08-10  8:47   ` David Laight [this message]

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