Re: [RFC 1/2] page-flags: Make page lock operation atomic

[Matthew Wilcox]

On Mon, Feb 11, 2019 at 06:48:46PM +0100, Jan Kara wrote:
> On Mon 11-02-19 13:59:24, Linux Upstream wrote:
> > > 
> > >> Signed-off-by: Chintan Pandya <chintan.pandya@xxxxxxxxxxx>
> > > 
> > > NAK.
> > > 
> > > This is bound to regress some stuff. Now agreed that using non-atomic
> > > ops is tricky, but many are in places where we 'know' there can't be
> > > concurrency.
> > > 
> > > If you can show any single one is wrong, we can fix that one, but we're
> > > not going to blanket remove all this just because.
> > 
> > Not quite familiar with below stack but from crash dump, found that this
> > was another stack running on some other CPU at the same time which also
> > updates page cache lru and manipulate locks.
> > 
> > [84415.344577] [20190123_21:27:50.786264]@1 preempt_count_add+0xdc/0x184
> > [84415.344588] [20190123_21:27:50.786276]@1 workingset_refault+0xdc/0x268
> > [84415.344600] [20190123_21:27:50.786288]@1 add_to_page_cache_lru+0x84/0x11c
> > [84415.344612] [20190123_21:27:50.786301]@1 ext4_mpage_readpages+0x178/0x714
> > [84415.344625] [20190123_21:27:50.786313]@1 ext4_readpages+0x50/0x60
> > [84415.344636] [20190123_21:27:50.786324]@1 
> > __do_page_cache_readahead+0x16c/0x280
> > [84415.344646] [20190123_21:27:50.786334]@1 filemap_fault+0x41c/0x588
> > [84415.344655] [20190123_21:27:50.786343]@1 ext4_filemap_fault+0x34/0x50
> > [84415.344664] [20190123_21:27:50.786353]@1 __do_fault+0x28/0x88
> > 
> > Not entirely sure if it's racing with the crashing stack or it's simply
> > overrides the the bit set by case 2 (mentioned in 0/2).
> 
> So this is interesting. Looking at __add_to_page_cache_locked() nothing
> seems to prevent __SetPageLocked(page) in add_to_page_cache_lru() to get
> reordered into __add_to_page_cache_locked() after page is actually added to
> the xarray. So that one particular instance might benefit from atomic
> SetPageLocked or a barrier somewhere between __SetPageLocked() and the
> actual addition of entry into the xarray.

There's a write barrier when you add something to the XArray, by virtue
of the call to rcu_assign_pointer().