Messages in this thread | | | Subject | Re: Temporary lockup on loopback block device | From | Peter Zijlstra <> | Date | Sun, 11 Nov 2007 00:02:32 +0100 |
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On Sat, 2007-11-10 at 14:54 -0800, Andrew Morton wrote: > 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.
known issue.
> > 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).
.24 will have the per-device solution.
> > > > 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.
The per BDI dirty stuff in 24 should make this work, I just checked and loopback thingies seem to have their own BDI, so all should be well.
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