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

[Yang Shi]

On Fri, Apr 12, 2024 at 7:31 AM Zi Yan <ziy@xxxxxxxxxx> 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@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.

Thanks for digging deeper. The problem may be not that obvious before
mTHP because PMD-mappable THP is converted to PTE-mapped due to
partial unmap in most cases. But mTHP is always PTE-mapped in the
first place. The other reason is batched rmap remove was not supported
before David's optimization.

Now we do have reasonable motivation to make it precise and it is also
easier to do so than before.

>
>
> --
> Best Regards,
> Yan, Zi