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

[David Hildenbrand <david@redhat.com>]

On 11.04.24 21:01, Yang Shi wrote:
> On Thu, Apr 11, 2024 at 8:46 AM David Hildenbrand <david@redhat.com> 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.
>>
>>>                do {
>>>                        last = atomic_add_negative(-1, &page->_mapcount);
>>>                        if (last) {
>>> @@ -1554,7 +1555,9 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
>>>                 * is still mapped.
>>>                 */
>>>                if (folio_test_large(folio) && folio_test_anon(folio))
>>> -                     if (level == RMAP_LEVEL_PTE || nr < nr_pmdmapped)
>>> +                     if ((level == RMAP_LEVEL_PTE &&
>>> +                          mapcount != 0) ||
>>> +                         (level == RMAP_LEVEL_PMD && nr < nr_pmdmapped))
>>>                                deferred_split_folio(folio);
>>>        }
>>
>> But I do wonder if we really care? Usually the folio will simply get
>> freed afterwards, where we simply remove it from the list.
>>
>> If it's pinned, we won't be able to free or reclaim, but it's rather a
>> corner case ...
>>
>> Is it really worth the added code? Not convinced.
> 
> It is actually not only an optimization, but also fixed the broken
> thp_deferred_split_page counter in /proc/vmstat.
> 
> The counter actually counted the partially unmapped huge pages (so
> they are on deferred split queue), but it counts the fully unmapped
> mTHP as well now. For example, when a 64K THP is fully unmapped, the
> thp_deferred_split_page is not supposed to get inc'ed, but it does
> now.
> 
> The counter is also useful for performance analysis, for example,
> whether a workload did a lot of partial unmap or not. So fixing the
> counter seems worthy. Zi Yan should have mentioned this in the commit
> log.

Yes, all that is information that is missing from the patch description. 
If it's a fix, there should be a "Fixes:".

Likely we want to have a folio_large_mapcount() check in the code below. 
(I yet have to digest the condition where this happens -- can we have an 
example where we'd use to do the wrong thing and now would do the right 
thing as well?)
-- 
Cheers,

David / dhildenb