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

[Eric Sandeen <sandeen@redhat.com>]

On 4/19/12 8:10 AM, Jouko Orava 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.

FWIW, we tried fairly hard to get the limit lifted in the vfs, to no avail.

> 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@helsinki.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.

Agreed, I think Dave was a little to quick on the draw on his reply.  :)

-Eric

> Thanks,
>    Jouko Orava
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