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

[Jan Kara <jack@suse.cz>]

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...

But I don't feel to be expert in this code so maybe I missed something.

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

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