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 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.

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?



[1] https://lore.kernel.org/linux-mm/e3e14098-eade-483e-a459-e43200b87941@arm.com/

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Best Regards,
Yan, Zi

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