Re: [PATCH] mm/swap: fix swap cache memcg accounting

[Kairui Song]

On Fri, Mar 20, 2026 at 06:05:59AM +0800, Alexandre Ghiti wrote:
> The swap readahead path was recently refactored and while doing this,
> the order between the charging of the folio in the memcg and the addition
> of the folio in the swap cache was inverted.
> 
> Since the accounting of the folio is done while adding the folio to the
> swap cache and the folio is not charged in the memcg yet, the accounting
> is then done at the node level, which is wrong.
> 
> Fix this by charging the folio in the memcg before adding it to the swap cache.
> 
> Fixes: 2732acda82c9 ("mm, swap: use swap cache as the swap in synchronize layer")
> Signed-off-by: Alexandre Ghiti <alex@xxxxxxxx>
> ---
>  mm/swap_state.c | 9 ++++-----
>  1 file changed, 4 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/swap_state.c b/mm/swap_state.c
> index 6d0eef7470be..48aff2c917c0 100644
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -494,6 +494,10 @@ static struct folio *__swap_cache_prepare_and_add(swp_entry_t entry,
>  
>  	__folio_set_locked(folio);
>  	__folio_set_swapbacked(folio);
> +
> +	if (!charged && mem_cgroup_swapin_charge_folio(folio, NULL, gfp, entry))
> +		goto failed;
> +

Thanks! I was worrying charging it first could cause thrashing, raced
swapin will cause multiple charge and failed one will uncharge, so
I put the charge after insert into cache, completely forgot about
the lruvec static info part.

For a quick fix this this is clean and good enough, and it
doesn't effect the most performance sensitive part, mTHP and
ZRAM would have `charged == true` here so they are not effected.

Later we can move the lruvec static info out of
__swap_cache_add_folio after the charge, maybe I'll do this with
someother follow up optimzation together, then everything should
be perfect.

And I think we need Cc stable here.

Acked-by: Kairui Song <kasong@xxxxxxxxxxx>