linux-kernel - Re: [PATCH 1/2] mm: Charge current memcg when no mm is set

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Message-ID: <20200206152704.GA24735@cmpxchg.org>
Date:   Thu, 6 Feb 2020 10:27:04 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Dan Schatzberg <dschatzberg@...com>
Cc:     Jens Axboe <axboe@...nel.dk>, Tejun Heo <tj@...nel.org>,
        Li Zefan <lizefan@...wei.com>,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        "open list:BLOCK LAYER" <linux-block@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        "open list:CONTROL GROUP (CGROUP)" <cgroups@...r.kernel.org>,
        "open list:CONTROL GROUP - MEMORY RESOURCE CONTROLLER (MEMCG)" 
        <linux-mm@...ck.org>
Subject: Re: [PATCH 1/2] mm: Charge current memcg when no mm is set

On Wed, Feb 05, 2020 at 02:33:47PM -0800, Dan Schatzberg wrote:
> This modifies the shmem and mm charge logic so that now if there is no
> mm set (as in the case of tmpfs backed loop device), we charge the
> current memcg, if set.
> 
> Signed-off-by: Dan Schatzberg <dschatzberg@...com>

Acked-by: Johannes Weiner <hannes@...xchg.org>

It's a dependency for 2/2, but it's also an overdue cleanup IMO: it's
always been a bit weird that memalloc_use_memcg() worked for kernel
allocations but was silently ignored for user pages.

This patch establishes a precedence order for who gets charged:

1. If there is a memcg associated with the page already, that memcg is
   charged. This happens during swapin.

2. If an explicit mm is passed, mm->memcg is charged. This happens
   during page faults, which can be triggered in remote VMs (eg gup).

3. Otherwise consult the current process context. If it has configured
   a current->active_memcg, use that. Otherwise, current->mm->memcg.

Thanks Dan