Messages in this thread | | | Date | Sat, 16 Dec 2017 12:33:29 +0100 | From | Michal Hocko <> | Subject | Re: [patch v2 1/2] mm, mmu_notifier: annotate mmu notifiers with blockable invalidate callbacks |
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On Sat 16-12-17 15:21:51, Tetsuo Handa wrote: > On 2017/12/16 1:25, Michal Hocko wrote: > >> struct mmu_notifier_ops { > >> + /* > >> + * Flags to specify behavior of callbacks for this MMU notifier. > >> + * Used to determine which context an operation may be called. > >> + * > >> + * MMU_INVALIDATE_DOES_NOT_BLOCK: invalidate_{start,end} does not > >> + * block > >> + */ > >> + int flags; > > > > This should be more specific IMHO. What do you think about the following > > wording? > > > > invalidate_{start,end,range} doesn't block on any locks which depend > > directly or indirectly (via lock chain or resources e.g. worker context) > > on a memory allocation. > > I disagree. It needlessly complicates validating the correctness.
But it makes it clear what is the actual semantic.
> What if the invalidate_{start,end} calls schedule_timeout_idle(10 * HZ) ?
Let's talk seriously about a real code. Any mmu notifier doing this is just crazy and should be fixed.
> schedule_timeout_idle() will not block on any locks which depend directly or > indirectly on a memory allocation, but we are already blocking other memory > allocating threads at mutex_trylock(&oom_lock) in __alloc_pages_may_oom().
Then the reaper will block and progress would be slower.
> This is essentially same with "sleeping forever due to schedule_timeout_killable(1) by > SCHED_IDLE thread with oom_lock held" versus "looping due to mutex_trylock(&oom_lock) > by all other allocating threads" lockup problem. The OOM reaper does not want to get > blocked for so long.
Yes, it absolutely doesn't want to do that. MMu notifiers should be reasonable because they are called from performance sensitive call paths.
-- Michal Hocko SUSE Labs
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