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Date:	Mon, 25 Apr 2011 15:45:59 -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 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.

Thanks,
Curt



>
> Cheers, Andreas
>
>
>
>
>
>
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