Re: [PATCH v2] mm: Fix assertion mapping->nrpages == 0 in end_writeback()

[Jan Kara <jack@suse.cz>]

On Tue 21-06-11 08:47:49, Miklos Szeredi wrote:
> On Mon, 2011-06-20 at 17:18 -0700, Andrew Morton wrote:
> > On Wed, 15 Jun 2011 17:37:13 +0200
> > Jan Kara <jack@suse.cz> wrote:
> > 
> > > Under heavy memory and filesystem load, users observe the assertion
> > > mapping->nrpages == 0 in end_writeback() trigger. This can be caused
> > > by page reclaim reclaiming the last page from a mapping in the following
> > > race:
> > > 	CPU0				CPU1
> > >   ...
> > >   shrink_page_list()
> > >     __remove_mapping()
> > >       __delete_from_page_cache()
> > >         radix_tree_delete()
> > > 					evict_inode()
> > > 					  truncate_inode_pages()
> > > 					    truncate_inode_pages_range()
> > > 					      pagevec_lookup() - finds nothing
> > > 					  end_writeback()
> > > 					    mapping->nrpages != 0 -> BUG
> > >         page->mapping = NULL
> > >         mapping->nrpages--
> > > 
> > > Fix the problem by doing a reliable check of mapping->nrpages under
> > > mapping->tree_lock in end_writeback().
> > > 
> > > Analyzed by Jay <jinshan.xiong@whamcloud.com>, lost in LKML, and dug
> > > out by Miklos Szeredi <mszeredi@suse.de>.
> > > 
> > > CC: Jay <jinshan.xiong@whamcloud.com>
> > > CC: Miklos Szeredi <mszeredi@suse.de>
> > > Signed-off-by: Jan Kara <jack@suse.cz>
> > > ---
> > >  fs/inode.c         |    7 +++++++
> > >  include/linux/fs.h |    1 +
> > >  mm/truncate.c      |    5 +++++
> > >  3 files changed, 13 insertions(+), 0 deletions(-)
> > > 
> > >   Andrew, does this look better?
> > 
> > spose so.
> > 
> > > diff --git a/fs/inode.c b/fs/inode.c
> > > index 33c963d..1133cb0 100644
> > > --- a/fs/inode.c
> > > +++ b/fs/inode.c
> > > @@ -467,7 +467,14 @@ EXPORT_SYMBOL(remove_inode_hash);
> > >  void end_writeback(struct inode *inode)
> > >  {
> > >  	might_sleep();
> > > +	/*
> > > +	 * We have to cycle tree_lock here because reclaim can be still in the
> > > +	 * process of removing the last page (in __delete_from_page_cache())
> > > +	 * and we must not free mapping under it.
> > > +	 */
> > > +	spin_lock(&inode->i_data.tree_lock);
> > >  	BUG_ON(inode->i_data.nrpages);
> > > +	spin_unlock(&inode->i_data.tree_lock);
> > 
> > That's an expensive assertion.  We might want to wrap all this in
> > CONFIG_DEBUG_VM.
> > 
> > Or we could do
> > 
> > 	if (unlikely(inode->i_data.nrpages)) {
> > 		/* comment goes here */
> > 		spin_lock(&inode->i_data.tree_lock);
> > 		BUG_ON(inode->i_data.nrpages);
> > 		spin_unlock(&inode->i_data.tree_lock);
> > 	}
> > 
> 
> It's not *just* the assertion that needs locking.   Suppose that we are
> in __remove_mapping() just before the 
> spin_unlock_irq(&mapping->tree_lock) and the inode is freed along with
> the mapping at that point in evict().  In that case the spin_unlock
> would be touching freed memory.
  Exactly. That's why I added the comment before the spin_lock. If it's not
clear enough, feel free to change it to a better wording.

> truncate_inode_pages() used to synchronize page reclaim with inode
> eviction, but now that synchronization is gone.
  Yes, but strictly speaking I don't see a real reason for synchronization
other then evict_inode() (the page that is in __delete_from_page_cache()
is really almost dead and page->mapping is NULL for anyone who reliably
reads that). So I think synchronizing in evict_inode() should be OK.

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

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