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

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Date:   Fri, 25 Sep 2020 13:10:05 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     David Hildenbrand <david@...hat.com>, linux-mm@...ck.org
Cc:     linux-kernel@...r.kernel.org, Michal Hocko <mhocko@...nel.org>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Oscar Salvador <osalvador@...e.de>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH 9/9] mm, page_alloc: optionally disable pcplists during
 page isolation

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...