Re: dirty balancing deadlock

[Chris Mason]

On Mon, Feb 19, 2007 at 02:14:15AM +0100, Miklos Szeredi wrote:
> > > > 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.
> 
> Hmm, like what?

Something a little different from what you're seeing.  Basically if the
PF_MEMALLOC paths end up waiting on a filesystem transaction, and that
transaction is waiting for more ram, the system will eventually grind to
a halt.  data=journal is the easiest way to hit it, since writepage
always logs at least 4k.

WB_SYNC_NONE and wbc->nonblocking aren't a great test, in reiser I
resorted to testing PF_MEMALLOC.

-chris

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