Re: dirty balancing deadlock

[Chris Mason]

On Mon, Feb 19, 2007 at 01:54:31AM +0100, Miklos Szeredi wrote:
> > > > > > If so, writes to B will decrease the dirty memory threshold.
> > > > > 
> > > > > Yes, but not by enough.  Say A dirties a 1100 pages, limit is 1000.
> > > > > Some pages queued for writeback (doesn't matter how much).  B writes
> > > > > back 1, 1099 dirty remain in A, zero in B.  balance_dirty_pages() for
> > > > > B doesn't know that there's nothing more to write back for B, it's
> > > > > just waiting there for those 1099, which'll never get written.
> > > > 
> > > > hm, OK, arguable.  I guess something like this..
> > > 
> > > Doesn't help the fuse case, but does seem to help the loopback mount
> > > one.
> > > 
> > > For fuse it's worse with the patch: now the write triggered by the
> > > balance recurses into fuse, with disastrous results, since the fuse
> > > writeback is now blocked on the userspace queue.
> > > 
> > > fusexmp_fh_no D 40136678     0   505    494           506   504 (NOTLB)
> > > 08982b78 00000001 00000000 08f9f9b4 0805d8cb 089a75f8 08982b78 08f98000
> > >        08f98000 08f9f9dc 0805a38a 089a7100 08982680 08f9f9cc 08f98000 08f98000
> > >        085d8300 08982680 089a7100 08f9fa34 08183006 089a7100 08982680 089a7100 Call Trace:
> > > 08f9f9a0:  [<0805d8cb>] switch_to_skas+0x3b/0x83
> > > 08f9f9b8:  [<0805a38a>] _switch_to+0x49/0x99
> > > 08f9f9e0:  [<08183006>] schedule+0x246/0x547
> > > 08f9fa38:  [<08103c7e>] fuse_get_req_wp+0xe9/0x14a
> > > 08f9fa70:  [<08103d2e>] fuse_writepage+0x4f/0x12c
> > 
> > In general, writepage is supposed to do work without blocking on
> > expensive locks that will get pdflush and dirty reclaim stuck in this
> > fashion.  You'll probably have to take the same approach reiserfs does
> > in data=journal mode, which is leaving the page dirty if fuse_get_req_wp
> > is going to block without making progress.
> 
> Pdflush, and dirty reclaim set wbc->nonblocking to true.
> balance_dirty_pages and fsync don't.  The problem here is that
> Andrew's patch is wrong to let balance_dirty_pages() try to write back
> pages from a different queue.

async or sync, writepage is supposed to either make progress or bail.
loopback aside, if the fuse call is blocking long term, you're going to
run into problems.

-chris
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