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Date:	Tue, 05 Aug 2008 14:35:54 -0700
From:	Mingming Cao <cmm@...ibm.com>
To:	Hisashi Hifumi <hifumi.hisashi@....ntt.co.jp>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, jack@...e.cz,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] jbd jbd2: fix dio write returning EIO
	whentry_to_release_page fails


在 2008-08-05二的 11:36 +0900,Hisashi Hifumi写道:
> >> 
> >> diff -Nrup linux-2.6.27-rc1.org/fs/jbd/transaction.c 
> >linux-2.6.27-rc1/fs/jbd/transaction.c
> >> --- linux-2.6.27-rc1.org/fs/jbd/transaction.c	2008-07-29 
> >19:28:47.000000000 +0900
> >> +++ linux-2.6.27-rc1/fs/jbd/transaction.c	2008-07-29 20:40:12.000000000 +0900
> >> @@ -1764,6 +1764,12 @@ int journal_try_to_free_buffers(journal_
> >>  	*/
> >>  	if (ret == 0 && (gfp_mask & __GFP_WAIT) && (gfp_mask & __GFP_FS)) {
> >>  		journal_wait_for_transaction_sync_data(journal);
> >> +
> >> +		bh = head;
> >> +		do {
> >> +			while (atomic_read(&bh->b_count))
> >> +				schedule();
> >> +		} while ((bh = bh->b_this_page) != head);
> >>  		ret = try_to_free_buffers(page);
> >>  	}
> >
> >The loop is problematic.  If the scheduler decides to keep running this
> >task then we have a busy loop.  If this task has realtime policy then
> >it might even lock up the kernel.
> > 
> >Perhaps we can use wait_on_page_writeback()?
> >
> 
> We cannot use wait_on_page_writeback() to wait for releasing bh ref because
> in ext3_ordered_writepage() bh ref is grabbed and released through walk_page_buffers
> so between both walk_page_buffers, it remains taking a bh ref even if end_page_writeback
> is performed.
>       ->ext3_ordered_writepage()
>      	   walk_page_buffers() <- take a bh ref
>  	   block_write_full_page() <- unlock_page
> 		: <- end_page_writeback
>                 : <- race! (dio write->try_to_release_page fails): ---> remains taking a bh ref
>       	   walk_page_buffers() <-release a bh ref
> 

Okay, I see the race window, DIO could come in before
walk_page_buffers() release the bh reference. So far I don't see a nicer
way to sync between background  writeout with DIO path yet...

Mingming

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