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

[Jan Kara]

  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
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
filesystem work needed. So it would be useful to allow filesystems to
opt-out from page lock checking (until fs maintainers are able to audit
their page locking) so that people can still use lockdep to verify other
things (when lockdep detects some issue, it turns itself off so if people
would hit pagelock problems with their fs, they'd be basically unable to
use lockdep for anything).

								Honza
-- 
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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