linux-ext4 - Re: writev data loss bug in (at least) 2.6.31 and 2.6.32pre8 x86-64

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Date:	Thu, 3 Dec 2009 06:28:25 +0100
From:	Nick Piggin <npiggin@...e.de>
To:	Jan Kara <jack@...e.cz>
Cc:	Mike Galbraith <gleep@....de>, James Y Knight <foom@...m.net>,
	LKML <linux-kernel@...r.kernel.org>, linux-ext4@...r.kernel.org
Subject: Re: writev data loss bug in (at least) 2.6.31 and 2.6.32pre8 x86-64

On Wed, Dec 02, 2009 at 08:04:26PM +0100, Jan Kara wrote:
> >   When using writev, the page we copy from is not paged in (while when we
> > use ordinary write, it is paged in). This difference might be worth
> > investigation on its own (as it is likely to heavily impact performance of
> > writev) but is irrelevant for us now - we should handle this without data
> > corruption anyway.
>   I've looked into why writev fails reliably the writes. The reason is that
> iov_iter_fault_in_readable() faults in only the first IO buffer. Because
> this is just 600 bytes big, following iov_iter_copy_from_user_atomic copies
> only 600 bytes and block_write_end sets number of copied bytes to 0. Thus
> we restart the write and do it one iov per iteration which succeeds. So
> everything works as designed only it gets inefficient in this particular
> case.

Yep, this would be right. We could actually do more prefaulting; I
think I was being a little over conservative and worried about earlier
pages being unmapped before we were able to consume them... but I
think being too worried about that case is optimizing an unusual case
that is probably performing badly anyway at the expense of more common
patterns.

Anyway, what I was doing to test this code when I wrote it was to
inject random failures into user copy functions. I guess this could
be useful to merge in the error injection framework?

Thanks,
Nick

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