Re: [PATCH 6.11.y 2/3] mm/codetag: fix pgalloc_tag_split()

[Greg KH <gregkh@linuxfoundation.org>]

On Thu, Oct 17, 2024 at 02:58:03PM -0700, chrisl@kernel.org wrote:
> From: Yu Zhao <yuzhao@google.com>
> 
> [ Upstream commit 95599ef684d01136a8b77c16a7c853496786e173 ]
> 
> The current assumption is that a large folio can only be split into
> order-0 folios.  That is not the case for hugeTLB demotion, nor for THP
> split: see commit c010d47f107f ("mm: thp: split huge page to any lower
> order pages").
> 
> When a large folio is split into ones of a lower non-zero order, only the
> new head pages should be tagged.  Tagging tail pages can cause imbalanced
> "calls" counters, since only head pages are untagged by pgalloc_tag_sub()
> and the "calls" counts on tail pages are leaked, e.g.,
> 
>   # echo 2048kB >/sys/kernel/mm/hugepages/hugepages-1048576kB/demote_size
>   # echo 700 >/sys/kernel/mm/hugepages/hugepages-1048576kB/nr_hugepages
>   # time echo 700 >/sys/kernel/mm/hugepages/hugepages-1048576kB/demote
>   # echo 0 >/sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages
>   # grep alloc_gigantic_folio /proc/allocinfo
> 
> Before this patch:
>   0  549427200  mm/hugetlb.c:1549 func:alloc_gigantic_folio
> 
>   real  0m2.057s
>   user  0m0.000s
>   sys   0m2.051s
> 
> After this patch:
>   0          0  mm/hugetlb.c:1549 func:alloc_gigantic_folio
> 
>   real  0m1.711s
>   user  0m0.000s
>   sys   0m1.704s
> 
> Not tagging tail pages also improves the splitting time, e.g., by about
> 15% when demoting 1GB hugeTLB folios to 2MB ones, as shown above.
> 
> Link: https://lkml.kernel.org/r/20240906042108.1150526-2-yuzhao@google.com
> Fixes: be25d1d4e822 ("mm: create new codetag references during page splitting")
> Signed-off-by: Yu Zhao <yuzhao@google.com>
> Acked-by: Suren Baghdasaryan <surenb@google.com>
> Cc: Kent Overstreet <kent.overstreet@linux.dev>
> Cc: Muchun Song <muchun.song@linux.dev>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

You did not sign off on this backport, so even if I wanted to take it, I
couldn't :(

Please fix this, and patch 3/3 up, and just send those.

sorry,

greg k-h