linux-kernel - Re: [PATCH 1/6] mm: Replace PF_MEMALLOC_NOIO with memalloc

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Message-ID: <20200625123418.GB7703@casper.infradead.org>
Date:   Thu, 25 Jun 2020 13:34:18 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-xfs@...r.kernel.org, dm-devel@...hat.com,
        Mikulas Patocka <mpatocka@...hat.com>,
        Jens Axboe <axboe@...nel.dk>, NeilBrown <neilb@...e.de>
Subject: Re: [PATCH 1/6] mm: Replace PF_MEMALLOC_NOIO with memalloc_noio

On Thu, Jun 25, 2020 at 02:22:39PM +0200, Michal Hocko wrote:
> On Thu 25-06-20 12:31:17, Matthew Wilcox wrote:
> > We're short on PF_* flags, so make memalloc_noio its own bit where we
> > have plenty of space.
> 
> I do not mind moving that outside of the PF_* space. Unless I
> misremember all flags in this space were intented to be set only on the
> current which rules out any RMW races and therefore they can be
> lockless. I am not sure this holds for the bitfield you are adding this
> to. At least in_memstall seem to be set on external task as well. But
> this would require double checking. Maybe that is not really intended or
> just a bug.

I was going from the comment:

        /* Unserialized, strictly 'current' */
(which you can't see from the context of the diff, but is above the block)

The situation with ->flags is a little more ambiguous:

/*
 * Only the _current_ task can read/write to tsk->flags, but other
 * tasks can access tsk->flags in readonly mode for example
 * with tsk_used_math (like during threaded core dumping).
 * There is however an exception to this rule during ptrace
 * or during fork: the ptracer task is allowed to write to the
 * child->flags of its traced child (same goes for fork, the parent
 * can write to the child->flags), because we're guaranteed the
 * child is not running and in turn not changing child->flags
 * at the same time the parent does it.
 */

but it wasn't unsafe to use the PF_ flags in the way that you were.
It's just crowded.

If in_memstall is set on other tasks, then it should be moved to the
PFA flags, which there are plenty of.

But a quick grep shows it only being read on other tasks and always
set on current:

kernel/sched/psi.c:     *flags = current->in_memstall;
kernel/sched/psi.c:      * in_memstall setting & accounting needs to be atomic wrt
kernel/sched/psi.c:     current->in_memstall = 1;
kernel/sched/psi.c:      * in_memstall clearing & accounting needs to be atomic wrt
kernel/sched/psi.c:     current->in_memstall = 0;
kernel/sched/psi.c:     if (task->in_memstall)
kernel/sched/stats.h:           if (p->in_memstall)
kernel/sched/stats.h:           if (p->in_memstall)
kernel/sched/stats.h:   if (unlikely(p->in_iowait || p->in_memstall)) {
kernel/sched/stats.h:           if (p->in_memstall)
kernel/sched/stats.h:   if (unlikely(rq->curr->in_memstall))

so I think everything is fine.