Re: [PATCH v5] mm/memcg: clear folio memcg after changing per memcg stats

On Wed, Sep 09, 2026 at 10:59:04PM -0700, Andrew Morton wrote:

On Thu, 10 Sep 2026 11:46:58 +0800 Bingfang Guo via B4 Relay [off-list ref] wrote:

quoted

I notice extremely high swapcached count in the per memcg level
memory.stat when running tests with cgroupv1 setup by swapping pages in
and out.  It seems that the counter never gets decreased so the value is
rather useless and confusing to users reading it. So I think fixing it
so that the value can reflect the actual swapcache usage correctly could
be helpful.

__memcg1_swapout() transfers the memsw charge of a folio to its swap
entry and clears folio->memcg_data as part of that.  In the vmscan
swapout path it runs before __swap_cache_del_folio(), which then
decrements the swapcache stats through lruvec_stat_mod_folio().  Since
folio->memcg_data has already been cleared, folio_memcg() returns NULL
and the NR_SWAPCACHE decrement only updates the node-level counter
instead of the memcg's lruvec, leaking the per-memcg swapcache count.

Move the __memcg1_swapout() call into __swap_cache_del_folio(), after
the NR_FILE_PAGES and NR_SWAPCACHE updates but before
__swap_cache_do_del_folio() removes the folio from the swap cache.  This
keeps the stats attributed to the folio's memcg while still recording
the swap cgroup with a valid folio->swap.  Add a swapout parameter so
the plain swap_cache_del_folio() path is left unchanged.

Thanks.

quoted

Fixes: 2732acda82c9 ("mm, swap: use swap cache as the swap in synchronize layer")

January this year.

How does this affect users?  Should we backport the fix into earlier
kernels?  If so, why?

Thanks.

Users using swaps will read totally meaningless swapcache value
from per memcg memory.stat, like 200GB of swapcache on a 64GB
setup, which is quite confusing and may trigger monitoring
alerts, if any.

So I think we should backport this patch.  Although I don't
really know user space programs relying on this value to work, I
think it's always good to make sure that values provided by the
kernel are helpful and mean something.

Regards,
Bingfang