Re: [PATCH] mm: cachestat: avoid bogus workingset test during swapping & invalidation races

[Johannes Weiner]

On Fri, Mar 15, 2024 at 11:16:35AM +0800, Chengming Zhou wrote:
> On 2024/3/15 00:49, Johannes Weiner wrote:
> > When cachestat against shmem races with swapping and invalidation, the
> > shadow entry might not exist: swapout IO is still in progress and
> > we're before __remove_mapping; or swapin/invalidation/swapoff has
> > removed the shadow from swapcache after we saw a shmem swap entry.
> > 
> > This will send a NULL to workingset_test_recent(). The latter purely
> > operates on pointer bits, so it won't crash - node 0, memcg ID 0,
> > eviction timestamp 0, etc. are all valid inputs - but it's a bogus
> > test. In theory that could result in a false "recently evicted" count.
> 
> Good catch!
> 
> > 
> > Such a false positive wouldn't be the end of the world. But for code
> > clarity and (future) robustness, be explicit about this case.
> > 
> > Fixes: cf264e1329fb ("cachestat: implement cachestat syscall")
> > Reported-by: Jann Horn <jannh@xxxxxxxxxx>
> > Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> > ---
> >  mm/filemap.c | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/mm/filemap.c b/mm/filemap.c
> > index 222adac7c9c5..a07c27df7eab 100644
> > --- a/mm/filemap.c
> > +++ b/mm/filemap.c
> > @@ -4199,6 +4199,9 @@ static void filemap_cachestat(struct address_space *mapping,
> >  				swp_entry_t swp = radix_to_swp_entry(folio);
> >  
> 
> IIUC, we should first check if it's a real swap entry using non_swap_entry(), right?
> Since there maybe other types of entries in shmem.

Good point, it could be a poisoned entry. I'll add the
non_swap_entry() check on swp.

> And need to get_swap_device() to prevent concurrent swapoff here,
> get_shadow_from_swap_cache() won't do it for us.

We're holding rcu_read_lock() for the xarray iteration, so if we see
the swap entry in the shmem mapping, it means we beat shmem_unuse()
and swapoff hasn't run synchronize_rcu() yet.

So it's safe. But I think it could use a comment. Maybe the
documentation of get_swap_device() should mention this option too?