Re: [rfc][patch] mm: lockdep page lock

[Jan Kara <jack@suse.cz>]

On Tue 16-03-10 13:21:53, Nick Piggin wrote:
> On Mon, Mar 15, 2010 at 07:08:00PM +0100, Jan Kara wrote:
> >   Hi,
> > 
> > On Tue 16-03-10 02:58:59, Nick Piggin wrote:
> > > This patch isn't totally complete. Needs some nesting annotations for
> > > filesystems like ntfs, and some async lock release annotations for other
> > > end-io handlers, also page migration code needs to set the page lock
> > > class. But the core of it is working nicely and is a pretty small patch.
> > > 
> > > It is a bit different to one Peter posted a while back, with differences.
> > > I don't care so much about bloating struct page with a few more bytes.
> > > lockdep can't run on a production kernel so I think it's preferable to be
> > > catching more complex errors than avoiding overhead. I also set the page
> > > lock class at the time it is added to pagecache when we have the mapping
> > > pinned to the page.
> > > 
> > > One issue I wonder about is if the lock class is changed while some other
> > > page locker is waiting to get the lock but has already called
> > > lock_acquire for the old class. Possibly it could be solved if lockdep
> > > has different primitives to say the caller is contending for a lock
> > > versus if it has been granted the lock?
> > > 
> > > Do you think it would be useful?  --
> > > 
> > > Page lock has very complex dependencies, so it would be really nice to
> > > add lockdep support for it.
> > > 
> > > For example: add_to_page_cache_locked(GFP_KERNEL) (called with page
> > > locked) -> page reclaim performs a trylock_page -> page reclaim performs
> > > a writepage -> writepage performs a get_block -> get_block reads
> > > buffercache -> buffercache read requires grow_dev_page -> grow_dev_page
> > > locks buffercache page -> if writepage fails, page reclaim calls
> > > handle_write_error -> handle_write_error performs a lock_page
> > > 
> > > So before even considering any other locks or more complex nested
> > > filesystems, we can hold at least 3 different page locks at once. Should
> > > be safe because we have an fs->bdev page lock ordering, and because
> > > add_to_page_cache* tend to be called on new (non-LRU) pages that can't be
> > > locked elsewhere, however a notable exception is tmpfs which moves live
> > > pages in and out of pagecache.
> > > 
> > > So lockdepify the page lock. Each filesystem type gets a unique key, to
> > > handle inter-filesystem nesting (like regular filesystem -> buffercache,
> > > or ecryptfs -> lower). Newly allocated pages get a default lock class,
> > > and it is reassigned to their filesystem type when being added to page
> > > cache.
> >   You'll probably soon notice that quite some filesystems (ext4, xfs,
> > ocfs2, ...) lock several pages at once in their writepages function. The
> 
> Yes indeed. This is what I had meant about nesting with NTFS, but I
> understand that others do it too.
> 
> 
> > locking rule here is that we always lock pages in index increasing order. I
> > don't think lockdep will be able to handle something like that. Probably we
> > can just avoid lockdep checking in these functions (or just acquire the
> > page lock class for the first page) but definitely there will be some
> 
> You are right, I don't think lockdep would work with that, so just
> checking the lock for the first page should be better than nothing.
> It might require some lockdep support in order to add context so it
> doesn't go mad when unlock_page is called (would rather not add any
> page flags to track that).
> 
> If we were really clever and able to get back to the address of
> struct page that _is_ holding the lock, we could just do a simple
> check to ensure its index is < the index of the page we are trying
> to take.
> 
> That would give reasonable nesting checking without requiring lockdep
> to track new chains for every page (obviously not feasible).
  This is an interesting idea. We could store a pointer to the
first locked page (which is attached to some mapping) in task_struct.
That should work fine.
								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR

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