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

[Hisashi Hifumi <hifumi.hisashi@oss.ntt.co.jp>]

>> > >> > 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...

I modified my patch.
I do not change Checking b_count in a loop, but introduce 
set_current_state(TASK_UNINTERRUPTIBLE) to mitigate the loop. I think this can 
lead to avoid busy loop.
I used the same approach of do_sync_read()->wait_on_retry_sync_kiocb or some drivers(qla2xxx).

Signed-off-by: Hisashi Hifumi <hifumi.hisashi@oss.ntt.co.jp>

diff -Nrup linux-2.6.27-rc1.org/fs/jbd/transaction.c linux-2.6.27-rc1.jbdfix/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.jbdfix/fs/jbd/transaction.c	2008-08-06 13:35:37.000000000 +0900
@@ -1764,6 +1764,15 @@ 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)) {
+				set_current_state(TASK_UNINTERRUPTIBLE);
+				schedule();
+				__set_current_state(TASK_RUNNING);
+			}
+		} while ((bh = bh->b_this_page) != head);
 		ret = try_to_free_buffers(page);
 	}