On Fri, Apr 26, 2024 at 11:28 AM Barry Song <21cnbao@xxxxxxxxx> wrote:
>
> On Fri, Apr 26, 2024 at 10:50 AM Zi Yan <ziy@xxxxxxxxxx> wrote:
> >
> > On 25 Apr 2024, at 22:23, Barry Song wrote:
> >
> > > On Fri, Apr 26, 2024 at 9:55 AM Zi Yan <ziy@xxxxxxxxxx> wrote:
> > >>
> > >> On 25 Apr 2024, at 21:45, Barry Song wrote:
> > >>
> > >>> On Fri, Apr 26, 2024 at 5:11 AM Zi Yan <zi.yan@xxxxxxxx> 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. But it is possible that
> > >>>> the folio is fully unmapped and adding it to deferred split list is
> > >>>> unnecessary.
> > >>>>
> > >>>> For PMD-mapped THPs, that was not really an issue, because removing the
> > >>>> last PMD mapping in the absence of PTE mappings would not have added the
> > >>>> folio to the deferred split queue.
> > >>>>
> > >>>> However, for PTE-mapped THPs, which are now more prominent due to mTHP,
> > >>>> they are always added to the deferred split queue. One side effect
> > >>>> is that the THP_DEFERRED_SPLIT_PAGE stat for a PTE-mapped folio can be
> > >>>> unintentionally increased, making it look like there are many partially
> > >>>> mapped folios -- although the whole folio is fully unmapped stepwise.
> > >>>>
> > >>>> Core-mm now tries batch-unmapping consecutive PTEs of PTE-mapped THPs
> > >>>> where possible starting from commit b06dc281aa99 ("mm/rmap: introduce
> > >>>> folio_remove_rmap_[pte|ptes|pmd]()"). When it happens, a whole PTE-mapped
> > >>>> folio is unmapped in one go and can avoid being added to deferred split
> > >>>> list, reducing the THP_DEFERRED_SPLIT_PAGE noise. But there will still be
> > >>>> noise when we cannot batch-unmap a complete PTE-mapped folio in one go
> > >>>> -- or where this type of batching is not implemented yet, e.g., migration.
> > >>>>
> > >>>> To avoid the unnecessary addition, folio->_nr_pages_mapped is checked
> > >>>> to tell if the whole folio is unmapped. If the folio is already on
> > >>>> deferred split list, it will be skipped, too.
> > >>>>
> > >>>> Note: commit 98046944a159 ("mm: huge_memory: add the missing
> > >>>> folio_test_pmd_mappable() for THP split statistics") tried to exclude
> > >>>> mTHP deferred split stats from THP_DEFERRED_SPLIT_PAGE, but it does not
> > >>>> fix the above issue. A fully unmapped PTE-mapped order-9 THP was still
> > >>>> added to deferred split list and counted as THP_DEFERRED_SPLIT_PAGE,
> > >>>> 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().
> > >>>>
> > >>>> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
> > >>>> Reviewed-by: Yang Shi <shy828301@xxxxxxxxx>
> > >>>> ---
> > >>>> mm/rmap.c | 8 +++++---
> > >>>> 1 file changed, 5 insertions(+), 3 deletions(-)
> > >>>>
> > >>>> diff --git a/mm/rmap.c b/mm/rmap.c
> > >>>> index a7913a454028..220ad8a83589 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) &&
> > >>>> + list_empty(&folio->_deferred_list) &&
> > >>>> + ((level == RMAP_LEVEL_PTE && atomic_read(mapped)) ||
> > >>>> + (level == RMAP_LEVEL_PMD && nr < nr_pmdmapped)))
> > >>>> + deferred_split_folio(folio);
> > >>>
> > >>> Hi Zi Yan,
> > >>> in case a mTHP is mapped by two processed (forked but not CoW yet), if we
> > >>> unmap the whole folio by pte level in one process only, are we still adding this
> > >>> folio into deferred list?
> > >>
> > >> No. Because the mTHP is still fully mapped by the other process. In terms of code,
> > >> nr will be 0 in that case and this if condition is skipped. nr is only increased
> > >> from 0 when one of the subpages in the mTHP has no mapping, namely page->_mapcount
> > >> becomes negative and last is true in the case RMAP_LEVEL_PTE.
> > >
> > > Ok. i see, so "last" won't be true?
> > >
> > > case RMAP_LEVEL_PTE:
> > > do {
> > > last = atomic_add_negative(-1, &page->_mapcount);
> > > if (last && folio_test_large(folio)) {
> > > last = atomic_dec_return_relaxed(mapped);
> > > last = (last < ENTIRELY_MAPPED);
> > > }
> > >
> > > if (last)
> > > nr++;
> > > } while (page++, --nr_pages > 0);
> > > break;
> >
> > Right, because for every subpage its corresponding
> > last = atomic_add_negative(-1, &page->_mapcount); is not true after the unmapping.2
>
> if a mTHP is mapped only by one process, and we unmap it entirely, we will
> get nr > 0, then we are executing adding it into deferred_list? so it seems
> atomic_read(mapped) is preventing this case from adding deferred_list?
>
> I wonder if it is possible to fixup nr to 0 from the first place?
> for example
> /* we are doing an entire unmapping */
> if (page==&folio->page && nr_pages == folio_nr_pages(folio))
or maybe
case RMAP_LEVEL_PTE:
...
+ if (!atomic_read(mapped))
+ nr = 0;
break;
> ...
>
> >
> >
> > --
> > Best Regards,
> > Yan, Zi
