linux-ext4 - Re: Questions on ext4 and writeback

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Message-ID: <6601abe90908180957u6ebe424q878229d2df9ffa94@mail.gmail.com>
Date:	Tue, 18 Aug 2009 09:57:49 -0700
From:	Curt Wohlgemuth <curtw@...gle.com>
To:	Theodore Tso <tytso@....edu>
Cc:	ext4 development <linux-ext4@...r.kernel.org>
Subject: Re: Questions on ext4 and writeback

Hi Ted:

On Tue, Aug 18, 2009 at 9:39 AM, Theodore Tso<tytso@....edu> wrote:
> On Thu, Aug 13, 2009 at 09:09:58AM -0700, Curt Wohlgemuth wrote:
>> But the actual submittal of I/O for the previous extent might cause
>> pages_skipped to be bumped, right?  Removing these increments might cause
>> the accounting to be incorrect, it seems to me.
>
> I don't see where the submission of an extent of pages for I/O would
> cause page_skipped to be incremented or changed --- am I missing
> something?

Probably not.  But it seems to me that a call order of

  write_cache_pages -> __mpage_da_writepage -> mpage_da_submit_io ->
ext4_writepage

can cause pages_skipped to be incremented, either directly in
ext4_writepage() (page has delayed/unwritted buffers) or in
__block_write_full_page() (buffer already locked).

In fact, in mpage_da_submit_io(), pages_written is only incremented if
pages_skipped hasn't been bumped -- so that routine already knows that
pages_skipped might be changed on I/O submit.

If this happens, ext4_da_writepages() will wipe out the fact that
pages_skipped was changed during submittal, won't it?

Thanks,
Curt
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