linux-kernel - Re: [PATCH v2 4/9] mm/swap: always account swapped in page into current memcg

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Date: Fri, 05 Jan 2024 15:14:49 +0800
From: "Huang, Ying" <ying.huang@...el.com>
To: Kairui Song <ryncsn@...il.com>
Cc: linux-mm@...ck.org,  Kairui Song <kasong@...cent.com>,  Andrew Morton
 <akpm@...ux-foundation.org>,  Chris Li <chrisl@...nel.org>,  Hugh Dickins
 <hughd@...gle.com>,  Johannes Weiner <hannes@...xchg.org>,  Matthew Wilcox
 <willy@...radead.org>,  Michal Hocko <mhocko@...e.com>,  Yosry Ahmed
 <yosryahmed@...gle.com>,  David Hildenbrand <david@...hat.com>,
  linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 4/9] mm/swap: always account swapped in page into
 current memcg

Kairui Song <ryncsn@...il.com> writes:

> From: Kairui Song <kasong@...cent.com>
>
> Currently, mem_cgroup_swapin_charge_folio is always called with
> mm argument as NULL, except in swapin_direct.
>
> swapin_direct is used when swapin should skip readahead and
> swapcache (SWP_SYNCHRONOUS_IO). Other caller paths of
> mem_cgroup_swapin_charge_folio are for swapin that should
> not skip readahead and cache.
>
> This could cause swapin charging to behave differently depending
> on swap device. This currently didn't happen because the only call
> path of swapin_direct is the direct anon page fault path, where mm
> equals to current->mm, but will no longer be true if swapin_direct
> is shared and have other callers (eg, swapoff).
>
> So make swapin_direct also passes NULL for mm, no feature change.
>
> Signed-off-by: Kairui Song <kasong@...cent.com>
> ---
>  mm/swap_state.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/swap_state.c b/mm/swap_state.c
> index 6130de8d5226..d39c5369da21 100644
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -881,7 +881,7 @@ struct folio *swapin_direct(swp_entry_t entry, gfp_t gfp_mask,
>  	folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0,
>  				vma, vmf->address, false);
>  	if (folio) {
> -		if (mem_cgroup_swapin_charge_folio(folio, vma->vm_mm,
> +		if (mem_cgroup_swapin_charge_folio(folio, NULL,
>  						   GFP_KERNEL, entry)) {
>  			folio_put(folio);
>  			return NULL;

I think that why not provide "mm" when it's available?  For
swapin_direct() called by do_swap_page(), mm can be provided.  While,
for swapin_direct() called by shmem swapin, mm will be NULL.  We can
even provide "mm" for __read_swap_cache_async() for VMA based swapin and
for the fault address for cluster swapin.

--
Best Regards,
Huang, Ying