Re: Temporary lockup on loopback block device

[Andrew Morton <akpm@linux-foundation.org>]

On Sat, 10 Nov 2007 20:51:31 +0100 (CET) Mikulas Patocka <mikulas@artax.karlin.mff.cuni.cz> wrote:

> Hi
> 
> I am experiencing a transient lockup in 'D' state with loopback device. It 
> happens when process writes to a filesystem in loopback with command like
> dd if=/dev/zero of=/s/fill bs=4k 
> 
> CPU is idle, disk is idle too, yet the dd process is waiting in 'D' in 
> congestion_wait called from balance_dirty_pages.
> 
> After about 30 seconds, the lockup is gone and dd resumes, but it locks up 
> soon again.
> 
> I added a printk to the balance_dirty_pages
> printk("wait: nr_reclaimable %d, nr_writeback %d, dirty_thresh %d, 
> pages_written %d, write_chunk %d\n", nr_reclaimable, 
> global_page_state(NR_WRITEBACK), dirty_thresh, pages_written, 
> write_chunk);
> 
> and it shows this during the lockup:
> 
> wait: nr_reclaimable 3099, nr_writeback 0, dirty_thresh 2985, 
> pages_written 1021, write_chunk 1522
> wait: nr_reclaimable 3099, nr_writeback 0, dirty_thresh 2985, 
> pages_written 1021, write_chunk 1522
> wait: nr_reclaimable 3099, nr_writeback 0, dirty_thresh 2985, 
> pages_written 1021, write_chunk 1522
> 
> What apparently happens:
> 
> writeback_inodes syncs inodes only on the given wbc->bdi, however 
> balance_dirty_pages checks against global counts of dirty pages. So if 
> there's nothing to sync on a given device, but there are other dirty pages 
> so that the counts are over the limit, it will loop without doing any 
> work.
> 
> To reproduce it, you need totally idle machine (no GUI, etc.) -- if 
> something writes to the backing device, it flushes the dirty pages 
> generated by the loopback and the lockup is gone. If you add printk, don't 
> forget to stop klogd, otherwise logging would end the lockup.

erk.

> The hotfix (that I verified to work) is to not set wbc->bdi, so that all 
> devices are flushed ... but the code probably needs some redesign (i.e. 
> either account per-device and flush per-device, or account-global and 
> flush-global).
> 
> Mikulas
> 
> 
> diff -u -r ../x/linux-2.6.23.1/mm/page-writeback.c mm/page-writeback.c
> --- ../x/linux-2.6.23.1/mm/page-writeback.c     2007-10-12 18:43:44.000000000 +0200
> +++ mm/page-writeback.c 2007-11-10 20:32:43.000000000 +0100
> @@ -214,7 +214,6 @@
> 
> 	for (;;) {
> 		struct writeback_control wbc = {
> -			.bdi            = bdi,
> 			.sync_mode      = WB_SYNC_NONE,
> 			.older_than_this = NULL,
> 			.nr_to_write    = write_chunk,

Arguably we just have the wrong backing-device here, and what we should do
is to propagate the real backing device's pointer through up into the
filesystem.  There's machinery for this which things like DM stacks use.

I wonder if the post-2.6.23 changes happened to make this problem go away.

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