Re: [PATCH] jbd jbd2: fix dio write returning EIO whentry_to_release_page fails

[Mingming Cao <cmm@us.ibm.com>]

在 2008-08-05二的 12:17 -0400，Chris Mason写道：
> On Tue, 2008-08-05 at 13:51 +0900, Hisashi Hifumi wrote:
> > >> > 
> > >> > 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.
> > >> 
> > >
> > >ocfs2 calls journal_try_to_free_buffers too, looping on b_count might
> > >not be the best idea there either.
> > >
> > >This code gets called from releasepage, which is used other places than
> > >the O_DIRECT invalidation paths, I'd be worried about performance
> > >problems here.
> > >
> > 
> > try_to_release_page has gfp_mask parameter. So when try_to_releasepage
> > is called from performance sensitive part, gfp_mask should not be set.
> > b_count check loop is inside of (gfp_mask & __GFP_WAIT) && (gfp_mask & __GFP_FS) check.
> 
> Looks like try_to_free_pages will go into releasepage with wait & fs
> both set.  This kind of change would make me very nervous.
> 

Hi Chris,

The gfp_mask try_to_free_pages() takes from it's caller will past it
down to try_to_release_page().  Based on the meaning of __GFP_WAIT and
GFP_FS, if the upper level caller set these two flags,  I assume the
upper level caller expect delay and wait for fs to finish?


But I agree that using a loop in journal_try_to_free_buffers() to wait
for the busy bh release the counter is expensive...
Mingming
> -chris
> 
> 
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