linux-kernel - Re: [PATCH v1 5/6] memcg-v1: no need for memcg locking for MGLRU

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Message-ID: <CAOUHufYCPkUH0ysujoXZaw3PSrPvaw356-Pb97=LPGVRu_7FNQ@mail.gmail.com>
Date: Fri, 25 Oct 2024 21:55:38 -0600
From: Yu Zhao <yuzhao@...gle.com>
To: Shakeel Butt <shakeel.butt@...ux.dev>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Johannes Weiner <hannes@...xchg.org>, 
	Michal Hocko <mhocko@...nel.org>, Roman Gushchin <roman.gushchin@...ux.dev>, 
	Muchun Song <muchun.song@...ux.dev>, Hugh Dickins <hughd@...gle.com>, 
	Yosry Ahmed <yosryahmed@...gle.com>, linux-mm@...ck.org, cgroups@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
	linux-doc@...r.kernel.org, Meta kernel team <kernel-team@...a.com>
Subject: Re: [PATCH v1 5/6] memcg-v1: no need for memcg locking for MGLRU

On Thu, Oct 24, 2024 at 7:23 PM Shakeel Butt <shakeel.butt@...ux.dev> wrote:
>
> While updating the generation of the folios, MGLRU requires that the
> folio's memcg association remains stable. With the charge migration
> deprecated, there is no need for MGLRU to acquire locks to keep the
> folio and memcg association stable.
>
> Signed-off-by: Shakeel Butt <shakeel.butt@...ux.dev>
> ---
>  mm/vmscan.c | 11 -----------
>  1 file changed, 11 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 29c098790b01..fd7171658b63 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -3662,10 +3662,6 @@ static void walk_mm(struct mm_struct *mm, struct lru_gen_mm_walk *walk)
>                 if (walk->seq != max_seq)
>                         break;

Please remove the lingering `struct mem_cgroup *memcg` as well as
folio_memcg_rcu(). Otherwise it causes both build and lockdep
warnings.

> -               /* folio_update_gen() requires stable folio_memcg() */
> -               if (!mem_cgroup_trylock_pages(memcg))
> -                       break;
> -
>                 /* the caller might be holding the lock for write */
>                 if (mmap_read_trylock(mm)) {
>                         err = walk_page_range(mm, walk->next_addr, ULONG_MAX, &mm_walk_ops, walk);
> @@ -3673,8 +3669,6 @@ static void walk_mm(struct mm_struct *mm, struct lru_gen_mm_walk *walk)
>                         mmap_read_unlock(mm);
>                 }
>
> -               mem_cgroup_unlock_pages();
> -
>                 if (walk->batched) {
>                         spin_lock_irq(&lruvec->lru_lock);
>                         reset_batch_size(walk);
> @@ -4096,10 +4090,6 @@ bool lru_gen_look_around(struct page_vma_mapped_walk *pvmw)
>                 }
>         }
>
> -       /* folio_update_gen() requires stable folio_memcg() */
> -       if (!mem_cgroup_trylock_pages(memcg))
> -               return true;
> -
>         arch_enter_lazy_mmu_mode();
>
>         pte -= (addr - start) / PAGE_SIZE;
> @@ -4144,7 +4134,6 @@ bool lru_gen_look_around(struct page_vma_mapped_walk *pvmw)
>         }
>
>         arch_leave_lazy_mmu_mode();
> -       mem_cgroup_unlock_pages();
>
>         /* feedback from rmap walkers to page table walkers */
>         if (mm_state && suitable_to_scan(i, young))
> --
> 2.43.5
>
>