Re: [PATCH] mm/rmap: do not add fully unmapped large folio to deferred split list

[Zi Yan <ziy@nvidia.com>]

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On 12 Apr 2024, at 18:29, Yang Shi wrote:

> On Fri, Apr 12, 2024 at 2:06 PM Zi Yan <ziy@nvidia.com> wrote:
>>
>> On 12 Apr 2024, at 15:32, David Hildenbrand wrote:
>>
>>> On 12.04.24 16:35, Zi Yan wrote:
>>>> On 11 Apr 2024, at 11:46, David Hildenbrand wrote:
>>>>
>>>>> On 11.04.24 17:32, Zi Yan wrote:
>>>>>> From: Zi Yan <ziy@nvidia.com>
>>>>>>
>>>>>> In __folio_remove_rmap(), a large folio is added to deferred split list
>>>>>> if any page in a folio loses its final mapping. It is possible that
>>>>>> the folio is unmapped fully, but it is unnecessary to add the folio
>>>>>> to deferred split list at all. Fix it by checking folio mapcount before
>>>>>> adding a folio to deferred split list.
>>>>>>
>>>>>> Signed-off-by: Zi Yan <ziy@nvidia.com>
>>>>>> ---
>>>>>>    mm/rmap.c | 9 ++++++---
>>>>>>    1 file changed, 6 insertions(+), 3 deletions(-)
>>>>>>
>>>>>> diff --git a/mm/rmap.c b/mm/rmap.c
>>>>>> index 2608c40dffad..d599a772e282 100644
>>>>>> --- a/mm/rmap.c
>>>>>> +++ b/mm/rmap.c
>>>>>> @@ -1494,7 +1494,7 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
>>>>>>                    enum rmap_level level)
>>>>>>    {
>>>>>>            atomic_t *mapped = &folio->_nr_pages_mapped;
>>>>>> -  int last, nr = 0, nr_pmdmapped = 0;
>>>>>> +  int last, nr = 0, nr_pmdmapped = 0, mapcount = 0;
>>>>>>            enum node_stat_item idx;
>>>>>>            __folio_rmap_sanity_checks(folio, page, nr_pages, level);
>>>>>> @@ -1506,7 +1506,8 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
>>>>>>                            break;
>>>>>>                    }
>>>>>>   -                atomic_sub(nr_pages, &folio->_large_mapcount);
>>>>>> +          mapcount = atomic_sub_return(nr_pages,
>>>>>> +                                       &folio->_large_mapcount) + 1;
>>>>>
>>>>> That becomes a new memory barrier on some archs. Rather just re-read it below. Re-reading should be fine here.
>>>>
>>>> Would atomic_sub_return_relaxed() work? Originally I was using atomic_read(mapped)
>>>> below, but to save an atomic op, I chose to read mapcount here.
>>>
>>> Some points:
>>>
>>> (1) I suggest reading about atomic get/set vs. atomic RMW vs. atomic
>>> RMW that return a value -- and how they interact with memory barriers.
>>> Further, how relaxed variants are only optimized on some architectures.
>>>
>>> atomic_read() is usually READ_ONCE(), which is just an "ordinary" memory
>>> access that should not be refetched. Usually cheaper than most other stuff
>>> that involves atomics.
>>
>> I should have checked the actual implementation instead of being fooled
>> by the name. Will read about it. Thanks.
>>
>>>
>>> (2) We can either use folio_large_mapcount() == 0 or !atomic_read(mapped)
>>> to figure out if the folio is now completely unmapped.
>>>
>>> (3) There is one fundamental issue: if we are not batch-unmapping the whole
>>> thing, we will still add the folios to the deferred split queue. Migration
>>> would still do that, or if there are multiple VMAs covering a folio.
>>>
>>> (4) We should really avoid making common operations slower only to make
>>> some unreliable stats less unreliable.
>>>
>>>
>>> We should likely do something like the following, which might even be a bit
>>> faster in some cases because we avoid a function call in case we unmap
>>> individual PTEs by checking _deferred_list ahead of time
>>>
>>> diff --git a/mm/rmap.c b/mm/rmap.c
>>> index 2608c40dffad..356598b3dc3c 100644
>>> --- a/mm/rmap.c
>>> +++ b/mm/rmap.c
>>> @@ -1553,9 +1553,11 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
>>>                  * page of the folio is unmapped and at least one page
>>>                  * is still mapped.
>>>                  */
>>> -               if (folio_test_large(folio) && folio_test_anon(folio))
>>> -                       if (level == RMAP_LEVEL_PTE || nr < nr_pmdmapped)
>>> -                               deferred_split_folio(folio);
>>> +               if (folio_test_large(folio) && folio_test_anon(folio) &&
>>> +                   (level == RMAP_LEVEL_PTE || nr < nr_pmdmapped) &&
>>> +                   atomic_read(mapped) &&
>>> +                   data_race(list_empty(&folio->_deferred_list)))
>>
>> data_race() might not be needed, as Ryan pointed out[1]
>>
>>> +                       deferred_split_folio(folio);
>>>         }
>>>
>>> I also thought about handling the scenario where we unmap the whole
>>> think in smaller chunks. We could detect "!atomic_read(mapped)" and
>>> detect that it is on the deferred split list, and simply remove it
>>> from that list incrementing an THP_UNDO_DEFERRED_SPLIT_PAGE event.
>>>
>>> But it would be racy with concurrent remapping of the folio (might happen with
>>> anon folios in corner cases I guess).
>>>
>>> What we can do is the following, though:
>>>
>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>> index dc30139590e6..f05cba1807f2 100644
>>> --- a/mm/huge_memory.c
>>> +++ b/mm/huge_memory.c
>>> @@ -3133,6 +3133,8 @@ void folio_undo_large_rmappable(struct folio *folio)
>>>         ds_queue = get_deferred_split_queue(folio);
>>>         spin_lock_irqsave(&ds_queue->split_queue_lock, flags);
>>>         if (!list_empty(&folio->_deferred_list)) {
>>> +               if (folio_test_pmd_mappable(folio))
>>> +                       count_vm_event(THP_UNDO_DEFERRED_SPLIT_PAGE);
>>>                 ds_queue->split_queue_len--;
>>>                 list_del_init(&folio->_deferred_list);
>>>         }
>>>
>>> Adding the right event of course.
>>>
>>>
>>> Then it's easy to filter out these "temporarily added to the list, but never split
>>> before the folio was freed" cases.
>>
>> So instead of making THP_DEFERRED_SPLIT_PAGE precise, use
>> THP_DEFERRED_SPLIT_PAGE - THP_UNDO_DEFERRED_SPLIT_PAGE instead? That should work.
>
> It is definitely possible that the THP on the deferred split queue are
> freed instead of split. For example, 1M is unmapped for a 2M THP, then
> later the remaining 1M is unmapped, or the process exits before memory
> pressure happens. So how come we can tell it is "temporarily added to
> list"? Then THP_DEFERRED_SPLIT_PAGE - THP_UNDO_DEFERRED_SPLIT_PAGE
> actually just counts how many pages are still on deferred split queue.
> It may be useful. However the counter is typically used to estimate
> how many THP are partially unmapped during a period of time. So we
> just need to know the initial value and the value when we read it
> again.
>
>>
>> I wonder what THP_DEFERRED_SPLIT_PAGE counts. If it counts THP deferred
>> splits, why not just move the counter to deferred_split_scan(), where the actual
>> split happens. Or the counter has a different meaning?
>
> The deferred_split_scan() / deferred_split_count() just can return the
> number of pages on a specific queue (a specific node with a specific
> memcg). But THP_DEFERRED_SPLIT_PAGE is a global counter. Did I miss
> something? Or you mean traverse all memcgs and all nodes? It sounds
> too overkilling.

I mean instead of increasing THP_DEFERRED_SPLIT_PAGE when a folio is added
to the split list, increase it when a folio is split in deferred_split_scan(),
regardless which list the folio is on.

--
Best Regards,
Yan, Zi

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