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Date:	Mon, 25 Apr 2011 16:20:42 -0700
From:	Curt Wohlgemuth <curtw@...gle.com>
To:	Andreas Dilger <adilger@...ger.ca>
Cc:	linux-ext4@...r.kernel.org, jim@...ering.net, cmm@...ibm.com,
	hughd@...gle.com, tytso@....edu
Subject: Re: [PATCH v3] ext4: Don't set PageUptodate in ext4_end_bio()

Hi again Andreas:

On Mon, Apr 25, 2011 at 3:45 PM, Curt Wohlgemuth <curtw@...gle.com> wrote:
> Hi Andreas:
>
> On Mon, Apr 25, 2011 at 3:40 PM, Andreas Dilger <adilger@...ger.ca> wrote:
>> On 2011-04-25, at 2:23 PM, Curt Wohlgemuth wrote:
>>> In the bio completion routine, we should not be setting
>>> PageUptodate at all -- it's set at sys_write() time, and is
>>> unaffected by success/failure of the write to disk.
>>>
>>> This can cause a page corruption bug when
>>>
>>>    block size < page size
>>>
>>> @@ -203,46 +203,29 @@ static void ext4_end_bio(struct bio *bio, int error)
>>> -             /*
>>> -              * If this is a partial write which happened to make
>>> -              * all buffers uptodate then we can optimize away a
>>> -              * bogus readpage() for the next read(). Here we
>>> -              * 'discover' whether the page went uptodate as a
>>> -              * result of this (potentially partial) write.
>>> -              */
>>> -             if (!partial_write)
>>> -                     SetPageUptodate(page);
>>> -
>>
>> I think this is the important part of the code - if there is a read-after-write for a file that was written in "blocksize" units (blocksize < pagesize), does the page get set uptodate when all of the blocks have been written and/or the writing is at EOF?  Otherwise, a read-after-write will always cause data to be fetched from disk needlessly, even though the uptodate information is already in cache.
>
> Hmm, that's a good question.  I would kind of doubt that the page
> would be marked uptodate when the final block was written, and this
> might be what the code above was trying to do.  It wasn't doing it
> correctly :-), but it might have possibly avoided the extra read when
> it there was no error.
>
> I'll look at this some more, and see if I can't test for your scenario
> above.  Perhaps at least checking that all BHs in the page are mapped
> + uptodate => SetPageUptodate would not be out of line.

My testing is now showing the read coming through after writing to the
4 blocks of a 4K file, using 1K blocksize.  And it seems to me that
this is taken care of in __block_commit_write(), which is called from
all the .write_end callbacks for ext4, at least.

Thanks,
Curt

>
> Thanks,
> Curt
>
>
>
>>
>> Cheers, Andreas
>>
>>
>>
>>
>>
>>
>
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