Re: [PATCH 1/5] ksm: reinstate memcg charge on copied pages

[Shakeel Butt]

On Sun, Aug 30, 2020 at 1:59 PM Hugh Dickins <hughd@xxxxxxxxxx> wrote:
>
> In 5.8 some instances of memcg charging in do_swap_page() and unuse_pte()
> were removed, on the understanding that swap cache is now already charged
> at those points; but a case was missed, when ksm_might_need_to_copy() has
> decided it must allocate a substitute page: such pages were never charged.
> Fix it inside ksm_might_need_to_copy().
>
> This was discovered by Alex Shi's prospective commit "mm/memcg: warning
> on !memcg after readahead page charged".
>
> But there is a another surprise: this also fixes some rarer uncharged
> PageAnon cases, when KSM is configured in, but has never been activated.
> ksm_might_need_to_copy()'s anon_vma->root and linear_page_index() check
> sometimes catches a case which would need to have been copied if KSM
> were turned on.  Or that's my optimistic interpretation (of my own old
> code), but it leaves some doubt as to whether everything is working as
> intended there - might it hint at rare anon ptes which rmap cannot find?
> A question not easily answered: put in the fix for missed memcg charges.
>
> Fixes: 4c6355b25e8b ("mm: memcontrol: charge swapin pages on instantiation")
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx # v5.8

Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx>