linux-ext4 - Re: [PATCH v3] ext4: Don't set PageUptodate in ext4_end

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Message-ID: <BANLkTikJjHKchRTt3t2jW+O_1wba9v8jmg@mail.gmail.com>
Date:	Tue, 26 Apr 2011 15:41:47 +0800
From:	Yongqiang Yang <xiaoqiangnk@...il.com>
To:	Andreas Dilger <adilger@...ger.ca>
Cc:	Curt Wohlgemuth <curtw@...gle.com>, 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()

This function is only called from write path which flushes pages to
disk,  actually, pages' state have been set right at time when
write_end() is called.  Why did we handle pages' state in this
function?


On Tue, Apr 26, 2011 at 6:40 AM, 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.
>
> Cheers, Andreas
>
>
>
>
>
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-- 
Best Wishes
Yongqiang Yang
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