Re: splice: move balance_dirty_pages_ratelimited() outside of splice actor

[Jens Axboe]

On Tue, Jun 12 2007, Jens Axboe wrote:
> On Tue, Jun 12 2007, Jens Axboe wrote:
> > On Mon, Jun 11 2007, Andrew Morton wrote:
> > > On Mon, 11 Jun 2007 21:59:15 GMT
> > > Linux Kernel Mailing List <linux-kernel@xxxxxxxxxxxxxxx> wrote:
> > > 
> > > > Gitweb:     http://git.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=20d698db67059a63d217030dfd02872cb5f88dfb
> > > > Commit:     20d698db67059a63d217030dfd02872cb5f88dfb
> > > > Parent:     17374ff1aa9ce2a0597416a16729474b538af443
> > > > Author:     Jens Axboe <jens.axboe@xxxxxxxxxx>
> > > > AuthorDate: Tue Jun 5 11:05:11 2007 +0200
> > > > Committer:  Jens Axboe <jens.axboe@xxxxxxxxxx>
> > > > CommitDate: Fri Jun 8 08:33:59 2007 +0200
> > > > 
> > > >     splice: move balance_dirty_pages_ratelimited() outside of splice actor
> > > >     
> > > >     I've seen inode related deadlocks, so move this call outside of the
> > > >     actor itself, which may hold the inode lock.
> > > >     
> > > 
> > > eh?  If the pipe_to_file() caller holds inode_lock, our problems are large.
> > > 
> > > I doubt if that's true, so what problem is this patch really fixing??
> > 
> > I can repeatedly lock up balance_dirty_pages() if it's called inside the
> > double lock of splice_from_pipe(). I'll fire up the test box and
> > reproduce, then post the backtraces.
> 
> OK, here it is. Running
> 
> $ ./splice-fromnet 9999 | ./splice-out xxx
> 
> on one box (receive from network port 9999, store in file xxx) and a
> simple
> 
> $ cat SUSE-Linux-10.1-GM-DVD-i386.iso | netcat centera 9999
> 
> on another box. After ~180mb has been transferred, splice-out gets
> stuck. With the balancing moved outside of the splice actor, everything
> works perfectly regardless of memory pressure.
> 
> splice-fromne S 00000000     0  9829   9778 (NOTLB)
>    efe63d4c 00000046 f524cb20 00000000 efe63cfc c0144ae3 00000000 f524cb20
>    c03c33fa f1b9ee0c 00000009 f524cb20 02ad51ce 00000024 00002406 f524cc30
>    c1cf3980 00000001 efe63d44 00000246 f6692040 00000282 efe63d54 f2f83600
> Call Trace:
>  [<c01809bd>] pipe_wait+0x6d/0x90
>  [<c0198006>] splice_to_pipe+0x196/0x240
>  [<c035eb4d>] skb_splice_bits+0xcd/0xe0
>  [<c038157b>] tcp_splice_data_recv+0x2b/0x40
>  [<c0381501>] tcp_read_sock+0x131/0x180
>  [<c0381f55>] tcp_splice_read+0x85/0x1e0
>  [<c03572f5>] sock_splice_read+0x25/0x30
>  [<c019743e>] do_splice_to+0x5e/0x80
>  [<c0198beb>] sys_splice+0x1eb/0x200
>  [<c010511e>] sysenter_past_esp+0x5f/0x99
>  =======================
> splice-out    D EF423C98     0  9830   9778 (NOTLB)
>    ef423cac 00000046 00000002 ef423c98 ef423c94 00000000 00000046 f760ba98
>    f760ba98 f760b9a0 00000006 f7cba0f0 02ca49b4 00000024 001cf7e6 f7cba200
>    c1cf3980 00000001 c0144c73 c1cdaf78 f55c64c0 00000046 c1cdaf78 fffd5d5b
> Call Trace:
>  [<c03c272d>] io_schedule+0x1d/0x30
>  [<c0154979>] sync_page+0x39/0x50
>  [<c03c2d20>] __wait_on_bit_lock+0x40/0x70
>  [<c0154929>] __lock_page+0x69/0x70
>  [<c015a9c5>] write_cache_pages+0x105/0x2f0
>  [<c015abd3>] generic_writepages+0x23/0x30
>  [<c015ac29>] do_writepages+0x49/0x50
>  [<c01960a4>] __writeback_single_inode+0x94/0x370
>  [<c0196783>] sync_sb_inodes+0x1b3/0x270
>  [<c0196aa8>] writeback_inodes+0xb8/0xf0
>  [<c015afed>] balance_dirty_pages_ratelimited_nr+0x9d/0x1b0
>  [<c01978b3>] pipe_to_file+0x163/0x260
>  [<c0197a0f>] __splice_from_pipe+0x5f/0x250
>  [<c0197d18>] splice_from_pipe+0x58/0x80
>  [<c0197dc4>] generic_file_splice_write+0x54/0x100
>  [<c01974bf>] do_splice_from+0x5f/0x80
>  [<c0198bce>] sys_splice+0x1ce/0x200
>  [<c010511e>] sysenter_past_esp+0x5f/0x99

Alright, I think the "mystery" is solved - it's not the inode locks
(pheew), it's pipe_to_file() calling balance_dirty_pages() with the page
lock still held. The rest is self-explanatory, we end up blocking on the
page lock.

Would you prefer this change, then? I'd prefer keeping the current code,
unless it's absolutely critical that we call
balance_dirty_pages_ratelimited() for each and every page instead of eg
every 16 pages here.

diff --git a/fs/splice.c b/fs/splice.c
index 25ec9c8..21e6b92 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -660,6 +699,8 @@ find_page:
 out:
 	page_cache_release(page);
 	unlock_page(page);
+	if (ret)
+		balance_dirty_pages_ratelimited(mapping);
 out_ret:
 	return ret;
 }
@@ -857,7 +898,6 @@ generic_file_splice_write_nolock(struct pipe_inode_info *pipe, struct file *out,
 			if (err)
 				ret = err;
 		}
-		balance_dirty_pages_ratelimited(mapping);
 	}
 
 	return ret;
@@ -913,7 +953,6 @@ generic_file_splice_write(struct pipe_inode_info *pipe, struct file *out,
 			if (err)
 				ret = err;
 		}
-		balance_dirty_pages_ratelimited(mapping);
 	}
 
 	return ret;

-- 
Jens Axboe

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