linux-kernel - Re: Bug with "fix partial page writes" [3.2-rc regression]

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Message-ID: <4EDF9CAB.3050502@linux.vnet.ibm.com>
Date:	Wed, 07 Dec 2011 10:04:43 -0700
From:	Allison Henderson <achender@...ux.vnet.ibm.com>
To:	Yongqiang Yang <xiaoqiangnk@...il.com>
CC:	Hugh Dickins <hughd@...gle.com>, "Ted Ts'o" <tytso@....edu>,
	Curt Wohlgemuth <curtw@...gle.com>,
	Surbhi Palande <csurbhi@...il.com>,
	Rafael Wysocki <rjw@...k.pl>, linux-ext4@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: Bug with "fix partial page writes" [3.2-rc regression]

On 12/07/2011 01:28 AM, Yongqiang Yang wrote:
> Hi Allison and Hugh,
>
>   I think I found the problem and it has nothing to do with punching
> hole.  The patch [ext4: let ext4_bio_write_page handle EOF correctly]
> would fix up the problem.
>
> I post the patch so that it can be tested as early as possible.  The
> problem has not appeared on my machine since the patch is applied.
>
> Yongqiang.

Great!  I will try it out with your other set in my sandbox and let you 
know what happens.  Thx!

Allison Henderson

> On Wed, Dec 7, 2011 at 5:15 AM, Allison Henderson
> <achender@...ux.vnet.ibm.com>  wrote:
>> On 12/06/2011 01:55 AM, Hugh Dickins wrote:
>>>
>>> On Mon, 5 Dec 2011, Allison Henderson wrote:
>>>>
>>>> On 12/05/2011 04:38 PM, Hugh Dickins wrote:
>>>>>
>>>>>
>>>>> This has been outstanding for a month now, and we've heard no progress:
>>>>> please revert commit 02fac1297eb3 "ext4: fix partial page writes" for
>>>>> rc5.
>>>>>
>>>>> The problems appear on a 1k-blocksize filesystem under memory pressure:
>>>>> the hunk in ext4_da_write_end() causes oops, because it's playing with
>>>>> a page after generic_write_end() dropped our last reference to it; and
>>>>> backing out the hunk in ext4_da_write_begin() is then found to stop
>>>>> rare data corruption seen when kbuilding.
>>>>>
>>>>> Although I earlier reported that backing out the patch caused an fsx
>>>>> test to fail earlier, I've since found great variation in how soon it
>>>>> fails, and seen it fail just as quickly with 02fac1297eb3 still in.
>>>>> I also reported that I had to go back to 2.6.38 for fsx not to fail
>>>>> under memory pressure: you won't be surprised that that turned out to
>>>>> be because 2.6.38 defaults nomblk_io_submit but 2.6.39 mblk_io_submit.
>>>>
>>>>
>>>> Have you tried Yongqiang's patch "[PATCH 1/2] ext4: let mpage_submit_io
>>>> works well when blocksize<    pagesize" ?  I have tried it and it does seem
>>>> to
>>>> help, but I am still running into some failures that I am trying to
>>>> debug,
>>>> but let please let us know if it helps the issues that you are seeing.
>>>>   Thx!
>>>
>>>
>>> That 1/2, or the 2/2 "ext4: let ext4_discard_partial_buffers handle
>>> pages without buffers correctly"?  The latter is mostly a reversion
>>> of your 02fac1297eb3, so that's the one I need to fix the oops and
>>> rare data corruption.  Perhaps you're suggesting 1/2 for fsx failures
>>> under memory pressure?
>>>
>>> I've now tried the fsx test on three machines, with both 1/2 and 2/2
>>> applied to 3.2-rc4.  On one machine, with ext2 on loop on tmpfs, the
>>> fsx test failed in a couple of minutes with those patches; on another
>>> machine, with ext2 on loop on tmpfs, it failed after about 40 minutes
>>> with  the patches; on this laptop, with ext2 on SSD, it's just now
>>> failed after 35 minutes with the patches.
>>>
>>> That's not to say that Yongqiang's patches aren't good; but I cannot
>>> detect whether they make any improvement or not, since lasting for 2 or
>>> 40 minutes is typical for fsx under memory pressure with recent kernels.
>>
>>
>>
>> Well, initially I meant to just try the whole set, but now that I try just
>> one of them, I find that I get further with only the first one.  I think
>> Yongqiang and I have a similar set up because I get the hang if I dont have
>> the first patch, and I get the fsx write failure (in about 20 or so minutes)
>> if I have the second one.  But I think Yongqiang's right, we need to figure
>> out why the page is uptodate when it shouldn't be.
>>
>>
>>>
>>> Hugh
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>>
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>
>
>

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