Re: [PATCH 9/9] mm, page_alloc: optionally disable pcplists during page isolation

Michal Hocko
2020-10-05T14:05:35Z

On Fri 25-09-20 13:10:05, Vlastimil Babka wrote:
> On 9/25/20 12:54 PM, David Hildenbrand wrote:
> >>> --- a/mm/page_isolation.c
> >>> +++ b/mm/page_isolation.c
> >>> @@ -15,6 +15,22 @@
> >>> #define CREATE_TRACE_POINTS
> >>> #include <trace/events/page_isolation.h>
> >>>
> >>> +void zone_pcplist_disable(struct zone *zone)
> >>> +{
> >>> + down_read(&pcp_batch_high_lock);
> >>> + if (atomic_inc_return(&zone->pcplist_disabled) == 1) {
> >>> + zone_update_pageset_high_and_batch(zone, 0, 1);
> >>> + __drain_all_pages(zone, true);
> >>> + }
> >> Hm, if one CPU is still inside the if-clause, the other one would
> >> continue, however pcp wpould not be disabled and zones not drained when
> >> returning.
>
> Ah, well spotted, thanks!
>
> >> (while we only allow a single Offline_pages() call, it will be different
> >> when we use the function in other context - especially,
> >> alloc_contig_range() for some users)
> >>
> >> Can't we use down_write() here? So it's serialized and everybody has to
> >> properly wait. (and we would not have to rely on an atomic_t)
> > Sorry, I meant down_write only temporarily in this code path. Not
> > keeping it locked in write when returning (I remember there is a way to
> > downgrade).
>
> Hmm that temporary write lock would still block new callers until previous
> finish with the downgraded-to-read lock.
>
> But I guess something like this would work:
>
> retry:
> if (atomic_read(...) == 0) {
> // zone_update... + drain
> atomic_inc(...);
> else if (atomic_inc_return == 1)
> // atomic_cmpxchg from 0 to 1; if that fails, goto retry
>
> Tricky, but races could only read to unnecessary duplicated updates + flushing
> but nothing worse?
>
> Or add another spinlock to cover this part instead of the temp write lock...

Do you plan to post a new version or should I review this one?

--
Michal Hocko
SUSE Labs