Messages in this thread | | | Date | Thu, 25 Jun 2020 13:34:18 +0100 | From | Matthew Wilcox <> | Subject | Re: [PATCH 1/6] mm: Replace PF_MEMALLOC_NOIO with memalloc_noio |
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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.
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