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?