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

[Zi Yan]

On 12 Apr 2024, at 10:21, Zi Yan wrote:

> 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.

It might
Fixes: b06dc281aa99 ("mm/rmap: introduce folio_remove_rmap_[pte|ptes|pmd]()"),
but before this commit fully unmapping a PTE-mapped order-9 THP still increased
THP_DEFERRED_SPLIT_PAGE, because PTEs are unmapped individually and first PTE
unmapping adds the THP into the deferred split list. This means commit b06dc281aa99
did not change anything and before that THP_DEFERRED_SPLIT_PAGE increase is
due to implementation. I will add this to the commit log as well without Fixes
tag.


--
Best Regards,
Yan, Zi

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