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From: Michal Hocko <mhocko@kernel.org>
To: Matthew Wilcox <willy@infradead.org>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	linux-xfs@vger.kernel.org, dm-devel@redhat.com,
	Mikulas Patocka <mpatocka@redhat.com>,
	Jens Axboe <axboe@kernel.dk>, NeilBrown <neilb@suse.de>
Subject: Re: [PATCH 1/6] mm: Replace PF_MEMALLOC_NOIO with memalloc_noio
Date: Thu, 25 Jun 2020 14:42:34 +0200	[thread overview]
Message-ID: <20200625124234.GM1320@dhcp22.suse.cz> (raw)
In-Reply-To: <20200625123418.GB7703@casper.infradead.org>

On Thu 25-06-20 13:34:18, Matthew Wilcox wrote:
> 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.
>  */

OK, I have obviously missed that.

> 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)

Have a look at cgroup_move_task. So I believe this is something to be
fixed but independent on your change.

Feel free to add
Acked-by: Michal Hocko <mhocko@suse.com>

> 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.

-- 
Michal Hocko
SUSE Labs


  reply	other threads:[~2020-06-25 12:42 UTC|newest]

Thread overview: 41+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-06-25 11:31 [PATCH 0/6] Overhaul memalloc_no* Matthew Wilcox (Oracle)
2020-06-25 11:31 ` [PATCH 1/6] mm: Replace PF_MEMALLOC_NOIO with memalloc_noio Matthew Wilcox (Oracle)
2020-06-25 12:22   ` Michal Hocko
2020-06-25 12:34     ` Matthew Wilcox
2020-06-25 12:42       ` Michal Hocko [this message]
2020-06-25 11:31 ` [PATCH 2/6] mm: Add become_kswapd and restore_kswapd Matthew Wilcox (Oracle)
2020-06-25 12:31   ` Michal Hocko
2020-06-25 11:31 ` [PATCH 3/6] xfs: Convert to memalloc_nofs_save Matthew Wilcox (Oracle)
2020-06-25 11:31 ` [PATCH 4/6] mm: Replace PF_MEMALLOC_NOFS with memalloc_nofs Matthew Wilcox (Oracle)
2020-06-25 13:35   ` Michal Hocko
2020-06-25 11:31 ` [PATCH 5/6] mm: Replace PF_MEMALLOC_NOIO with memalloc_nocma Matthew Wilcox (Oracle)
2020-06-25 11:31 ` [PATCH 6/6] mm: Add memalloc_nowait Matthew Wilcox (Oracle)
2020-06-25 12:40   ` Michal Hocko
2020-06-25 13:10     ` Matthew Wilcox
2020-06-25 13:34       ` Michal Hocko
2020-06-25 19:05   ` kernel test robot
2020-06-25 23:51   ` kernel test robot
2020-06-29  5:08   ` Mike Rapoport
2020-06-29 12:18     ` Matthew Wilcox
2020-06-29 12:52       ` Michal Hocko
2020-06-29 13:45         ` Mike Rapoport
2020-06-29 21:28           ` Matthew Wilcox
2020-06-30  6:34             ` Michal Hocko
2020-07-01  4:12               ` Matthew Wilcox
2020-07-01  5:53                 ` Michal Hocko
2020-07-01  7:04                   ` Mike Rapoport
2020-09-24  0:39   ` Mike Snitzer
2020-09-24  1:10     ` Matthew Wilcox
2020-10-23 14:49   ` Daniel Vetter
2020-06-25 18:48 ` [PATCH 0/6] Overhaul memalloc_no* Darrick J. Wong
2020-06-25 20:34   ` Matthew Wilcox
2020-06-25 20:36   ` Michal Hocko
2020-06-25 20:40     ` Matthew Wilcox
2020-06-26 15:02 ` Mikulas Patocka
2020-06-26 23:08   ` Dave Chinner
2020-06-27 13:09     ` Mikulas Patocka
2020-06-29  0:35       ` Dave Chinner
2020-06-29 13:43         ` Mikulas Patocka
2020-06-29 22:34           ` Dave Chinner
2020-07-03 14:26             ` [PATCH] dm-bufio: do cleanup from a workqueue Mikulas Patocka
2020-06-29  8:22     ` [PATCH 0/6] Overhaul memalloc_no* Michal Hocko

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