linux-kernel - Re: [PATCH unstable] mm: rmap: abstract updating per-node and per-memcg stats fix

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Date: Tue, 11 Jun 2024 23:52:37 -0700
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Hugh Dickins <hughd@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, David Hildenbrand <david@...hat.com>, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH unstable] mm: rmap: abstract updating per-node and
 per-memcg stats fix

On Tue, Jun 11, 2024 at 10:10 PM Hugh Dickins <hughd@...gle.com> wrote:
>
> /proc/meminfo is showing ridiculously large numbers on some lines:
> __folio_remove_rmap()'s __folio_mod_stat() should be subtracting!
>
> Signed-off-by: Hugh Dickins <hughd@...gle.com>

Reviewed-by: Yosry Ahmed <yosryahmed@...gle.com>

Thanks a lot for fixing this! I was just looking at a test failure
report by the kernel robot caused by this [1].

Just to document my own stupidity here:

1. In [2], I sent a fix to use  __mod_node_page_state() instead of
__lruvec_stat_mod_folio() in __folio_remove_rmap(). I made the same
mistake of replacing subtraction with addition.

2. In [3], I sent a v2 of that fix that kept the subtraction in
__folio_remove_rmap() correctly.

3. In [4], I sent a cleanup on top of the fix, and that cleanup
replaced the subtraction in __folio_remove_rmap() with an addition,
again.

Apparently, I just suck at subtraction :)

[1]https://lore.kernel.org/linux-mm/202406121026.579593f2-oliver.sang@intel.com/
[2]https://lore.kernel.org/lkml/20240506170024.202111-1-yosryahmed@google.com/
[3]https://lore.kernel.org/lkml/20240506192924.271999-1-yosryahmed@google.com/
[4]https://lore.kernel.org/lkml/20240506211333.346605-1-yosryahmed@google.com/

> ---
> A fix for folding into mm-unstable, not needed for 6.10-rc.
>
>  mm/rmap.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1567,7 +1567,7 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
>                     list_empty(&folio->_deferred_list))
>                         deferred_split_folio(folio);
>         }
> -       __folio_mod_stat(folio, nr, nr_pmdmapped);
> +       __folio_mod_stat(folio, -nr, -nr_pmdmapped);
>
>         /*
>          * It would be tidy to reset folio_test_anon mapping when fully
> --
> 2.35.3
>