linux-kernel - Re: [PATCH v2 3/3] memcg: no irq disable for memcg stock lock

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Message-ID: <CAADnVQJ-XEEwVppk-qY2mmGB4R18_nqH-wdv5nuJf2LST5=Aaw@mail.gmail.com>
Date: Fri, 2 May 2025 11:29:16 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.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>, Alexei Starovoitov <ast@...nel.org>, linux-mm <linux-mm@...ck.org>, 
	"open list:CONTROL GROUP (CGROUP)" <cgroups@...r.kernel.org>, bpf <bpf@...r.kernel.org>, 
	LKML <linux-kernel@...r.kernel.org>, Meta kernel team <kernel-team@...a.com>, 
	Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH v2 3/3] memcg: no irq disable for memcg stock lock

On Thu, May 1, 2025 at 5:18 PM Shakeel Butt <shakeel.butt@...ux.dev> wrote:
>
> There is no need to disable irqs to use memcg per-cpu stock, so let's
> just not do that. One consequence of this change is if the kernel while
> in task context has the memcg stock lock and that cpu got interrupted.
> The memcg charges on that cpu in the irq context will take the slow path
> of memcg charging. However that should be super rare and should be fine
> in general.
>
> Signed-off-by: Shakeel Butt <shakeel.butt@...ux.dev>
> Acked-by: Vlastimil Babka <vbabka@...e.cz>
> ---
>  mm/memcontrol.c | 17 +++++++----------
>  1 file changed, 7 insertions(+), 10 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index cd81c70d144b..f8b9c7aa6771 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1858,7 +1858,6 @@ static bool consume_stock(struct mem_cgroup *memcg, unsigned int nr_pages,
>  {
>         struct memcg_stock_pcp *stock;
>         uint8_t stock_pages;
> -       unsigned long flags;
>         bool ret = false;
>         int i;
>
> @@ -1866,8 +1865,8 @@ static bool consume_stock(struct mem_cgroup *memcg, unsigned int nr_pages,
>                 return ret;
>
>         if (gfpflags_allow_spinning(gfp_mask))
> -               local_lock_irqsave(&memcg_stock.lock, flags);
> -       else if (!local_trylock_irqsave(&memcg_stock.lock, flags))
> +               local_lock(&memcg_stock.lock);
> +       else if (!local_trylock(&memcg_stock.lock))
>                 return ret;

I don't think it works.
When there is a normal irq and something doing regular GFP_NOWAIT
allocation gfpflags_allow_spinning() will be true and
local_lock() will reenter and complain that lock->acquired is
already set... but only with lockdep on.