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

[Chengming Zhou]

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

Thanks.

>  				shadow = get_shadow_from_swap_cache(swp);
> +				/* can race with swapping & invalidation */
> +				if (!shadow)
> +					goto resched;
>  			}
>  #endif
>  			if (workingset_test_recent(shadow, true, &workingset))