linux-ext4 - Re: Bug: Large writes can fail on ext4 if the write buffer is not empty

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Date:	Thu, 19 Apr 2012 22:56:24 +0800
From:	Zheng Liu <gnehzuil.liu@...il.com>
To:	Jouko Orava <jouko.orava@...sinki.fi>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: Bug: Large writes can fail on ext4 if the write buffer is not empty

On Thu, Apr 19, 2012 at 9:10 PM, Jouko Orava <jouko.orava@...sinki.fi> wrote:
> Hi Zheng,
>
> I can confirm the bug exists on latest RHEL 6 x86_64 kernels (2.6.32-220).
>
> On current mainline kernels all writes are limited to one page under 2GB,
> which masks the problem. I have not checked if mainline 2.6.32 has this
> limit or not. It does not matter: the limit is just a band-aid to paper
> over filesystem bugs, and should not mean you don't fix filesystem bugs.
>
> I can confirm the one line patch to fs/ext4/file.c does fix the problem.
> I have test kernels based on 2.6.32-220.7.1.el6.x86_64 with only
> the patch applied, at
>        http://www.helsinki.fi/~joorava/kernel/
> if anyone else is interested in testing.
>
> I did some (limited) testing on ext4 with the patch. It fixes the problem:
> large writes work very well too. No problems popped up in testing.
>
> Tested-by: Jouko Orava <jouko.orava@...sinki.fi>
>
>
> I'd also like to point at the real bug here, in Jouni's original strace:
>
>        writev(3, [{"\0\0\0\0\0\0\0\0", 8}, {"", 2147483648}], 2) = -2147483640
>
> The syscall returns a negative value, which is not the actual number of
> bytes written (since it is 32-bit wrapped), and errno has not been
> changed. There is no way for userspace to handle this result correctly!
>
> There is no way anyone sane should just gloss over this saying
> "programmer fault, you're doing it wrong".
> This is a real bug, and deserves fixing sooner rather than later.
>
> Thanks,
>   Jouko Orava

Hi Jouko,

Thanks for testing.  I don't test this bug in redhat kernel.  As Eric said, he
will fix it if it exists.  I think that it might be in stable kernel.
So I will dig it
in stable kernel.

Regards,
Zheng
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