Re: [PATCH] mm: Fix assertion mapping->nrpages == 0 in end_writeback()

[Jan Kara <jack@suse.cz>]

On Mon 13-06-11 15:14:01, Andrew Morton wrote:
> On Tue, 14 Jun 2011 00:01:44 +0200
> Jan Kara <jack@suse.cz> wrote:
> > On Wed 08-06-11 18:36:43, Jan Kara wrote:
> > > On Tue 07-06-11 14:33:01, Andrew Morton wrote:
> > > > On Tue, 07 Jun 2011 07:46:37 +0200
> > > > Miklos Szeredi <mszeredi@suse.cz> wrote:
> > > > 
> > > > > > Either way, I don't think that the uglypatch expresses a full
> > > > > > understanding of te bug ;)
> > > > > 
> > > > > I don't see a better way, how would we make nrpages update atomically
> > > > > wrt the radix-tree while using only RCU?
> > > > > 
> > > > > The question is, does it matter that those two can get temporarily out
> > > > > of sync?
> > > > > 
> > > > > In case of inode eviction it does, not only because of that BUG_ON, but
> > > > > because page reclaim must be somehow synchronised with eviction.
> > > > > Otherwise it may access tree_lock on the mapping of an already freed
> > > > > inode.
> > > > > 
> > > > > In other cases?  AFAICS it doesn't matter.  Most ->nrpages accesses
> > > > > weren't under tree_lock before Nick's RCUification, so their use were
> > > > > just optimization.   
> > > > 
> > > > Gee, we've made a bit of a mess here.
> > > > 
> > > > Rather than bodging around particualr codesites where that mess exposes
> > > > itself, how about we step back and work out what our design is here,
> > > > then implement it and check that all sites comply with it?
> > > > 
> > > > What is the relationship between the radix-tree and nrpages?  What are
> > > > the locking rules?  Can anyone come up with a one-sentence proposal?
> > > AFAIU, nrpages and radix-tree are consistent under tree_lock.
> > > 
> > > nrpages is only used (well, apart from shmfs and other filesystems which
> > > use the value as a guess how much should they expect to write or similar
> > > heuristics) to test mapping->nrpages == 0 and the test is performed without
> > > any synchronization which looks natural because we later do only
> > > rcu-protected lookups anyway. So it seems it's expected the test is
> > > unreliable and we just use it to make things faster. The same race as with
> > > nrpages test can happen during the radix tree lookup anyway...
> > > 
> > > I went through the tests and the only place which seems to really care
> > > about the races with __add_to_page_cache() or __delete_from_page_cache()
> > > is when the inode should be removed from memory. There we have to be
> > > careful. Races with __add_to_page_cache() cannot happen because there is
> > > noone who could trigger addition of new page to the inode being evicted.
> > > Races with __delete_from_page_cache() are possible though...
> >   Andrew, any opinion on this? I'd like to get the bug fixed... I'll
> > happily move the nrpages check in end_writeback() under the spinlock if
> > people find that nicer. That place really looks like the only one which
> > depends on nrpages being consistent and uptodate.
> 
> That seems a cleaner way of avoiding one manifestation of the bug.
  OK.

> But what *is* the bug?  That we've made nrpages incoherent with the
> state of the tree?  Or is it simply that the rule has always been "you
> must hold tree_lock to access nrpages", and the rcuification exposed
> that?
> 
> I want to actually fix this stuff up and get a good clear design which
> we can describe and understand.  No band-aids, please.  Not in here.
  OK, I belive the rule is "you must hold tree_lock to access nrpages" but
there are plenty of places which don't hold tree_lock and still peek at
nrpages to see if they have anything to do (and they were there even before
radix tree was rcuified). These are inherently racy and usually they don't
care - but possibly each such place should carry a comment explaining why
this racy check does not matter...

								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR

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