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

[Zi Yan]

On 11 Apr 2024, at 17:59, Yang Shi wrote:

> On Thu, Apr 11, 2024 at 2:15 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>>
>> On 11.04.24 21:01, Yang Shi wrote:
>>> On Thu, Apr 11, 2024 at 8:46 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>>>>
>>>> On 11.04.24 17:32, Zi Yan wrote:
>>>>> From: Zi Yan <ziy@xxxxxxxxxx>
>>>>>
>>>>> 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@xxxxxxxxxx>
>>>>> ---
>>>>>    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?)
>
> For example, map 1G memory with 64K mTHP, then unmap the whole 1G or
> some full 64K areas, you will see thp_deferred_split_page increased,
> but it shouldn't.
>
> It looks __folio_remove_rmap() incorrectly detected whether the mTHP
> is fully unmapped or partially unmapped by comparing the number of
> still-mapped subpages to ENTIRELY_MAPPED, which should just work for
> PMD-mappable THP.
>
> However I just realized this problem was kind of workaround'ed by commit:
>
> commit 98046944a1597f3a02b792dbe9665e9943b77f28
> Author: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> Date:   Fri Mar 29 14:59:33 2024 +0800
>
>     mm: huge_memory: add the missing folio_test_pmd_mappable() for THP
> split statistics
>
>     Now the mTHP can also be split or added into the deferred list, so add
>     folio_test_pmd_mappable() validation for PMD mapped THP, to avoid
>     confusion with PMD mapped THP related statistics.
>
>     Link: https://lkml.kernel.org/r/a5341defeef27c9ac7b85c97f030f93e4368bbc1.1711694852.git.baolin.wang@xxxxxxxxxxxxxxxxx
>     Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
>     Acked-by: David Hildenbrand <david@xxxxxxxxxx>
>     Cc: Muchun Song <muchun.song@xxxxxxxxx>
>     Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>
> This commit made thp_deferred_split_page didn't count mTHP anymore, it
> also made thp_split_page didn't count mTHP anymore.
>
> However Zi Yan's patch does make the code more robust and we don't
> need to worry about the miscounting issue anymore if we will add
> deferred_split_page and split_page counters for mTHP in the future.

Actually, the patch above does not fix everything. A fully unmapped
PTE-mapped order-9 THP is also added to deferred split list and
counted as THP_DEFERRED_SPLIT_PAGE without my patch, since nr is 512
(non zero), level is RMAP_LEVEL_PTE, and inside deferred_split_folio()
the order-9 folio is folio_test_pmd_mappable().

I will add this information in the next version.

--
Best Regards,
Yan, Zi

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