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

[David Hildenbrand <david@redhat.com>]

On 12.04.24 16:31, Zi Yan wrote:
> 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@redhat.com> wrote:
>>>>
>>>> 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?)
>>>
>>> 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@linux.alibaba.com>
>>> 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@linux.alibaba.com
>>>      Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>>>      Acked-by: David Hildenbrand <david@redhat.com>
>>>      Cc: Muchun Song <muchun.song@linux.dev>
>>>      Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
>>>
>>> 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.

Right, so it's always been a problem for PTE-mapped PMD-sized THP and 
only with batching we can now do "better". But not fix it completely.

I'll reply separately to your other mail.

-- 
Cheers,

David / dhildenb