Re: [PATCH v10 12/13] khugepaged: add per-order mTHP khugepaged stats

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

One small nitty note - be super helpful if you could add a newline before/after
your reply or soething like this as

>> blah blah blah
> blah
>> blah blah blah

Is harder to read than:

>> blah blah blah
>
> blah
>
>> blah blah blah

Thanks :)

On Tue, Sep 09, 2025 at 12:36:54AM -0600, Nico Pache wrote:
> On Thu, Aug 21, 2025 at 8:49 AM Lorenzo Stoakes
> <lorenzo.stoakes@oracle.com> wrote:
> >
> > On Tue, Aug 19, 2025 at 08:16:10AM -0600, Nico Pache wrote:
> > > With mTHP support inplace, let add the per-order mTHP stats for
> > > exceeding NONE, SWAP, and SHARED.
> > >
> >
> > This is really not enough of a commit message. Exceeding what, where, why,
> > how? What does 'exceeding' mean here, etc. etc. More words please :)
> Ok I will add more in the next version

Thanks

> >
> > > Signed-off-by: Nico Pache <npache@redhat.com>
> > > ---
> > >  Documentation/admin-guide/mm/transhuge.rst | 17 +++++++++++++++++
> > >  include/linux/huge_mm.h                    |  3 +++
> > >  mm/huge_memory.c                           |  7 +++++++
> > >  mm/khugepaged.c                            | 16 +++++++++++++---
> > >  4 files changed, 40 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
> > > index 7ccb93e22852..b85547ac4fe9 100644
> > > --- a/Documentation/admin-guide/mm/transhuge.rst
> > > +++ b/Documentation/admin-guide/mm/transhuge.rst
> > > @@ -705,6 +705,23 @@ nr_anon_partially_mapped
> > >         an anonymous THP as "partially mapped" and count it here, even though it
> > >         is not actually partially mapped anymore.
> > >
> > > +collapse_exceed_swap_pte
> > > +       The number of anonymous THP which contain at least one swap PTE.
> >
> > The number of anonymous THP what? Pages? Let's be specific.
> ack

Thanks

> >
> > > +       Currently khugepaged does not support collapsing mTHP regions that
> > > +       contain a swap PTE.
> >
> > Wait what? So we have a counter for something that's unsupported? That
> > seems not so useful?
> The current implementation does not support swapped out or shared
> pages. However these counters allow us to monitor when a mTHP collapse
> fails due to exceeding the threshold (ie 0, hitting any swapped out or
> shared page)

So the collapse counters are not measuring collapses? That seems a bit
confusing. Or actually is this implied in the 'exceed' bit? Because that'd make
sense actually.

But let's obviously document this carefully.


> >
> > > +
> > > +collapse_exceed_none_pte
> > > +       The number of anonymous THP which have exceeded the none PTE threshold.
> >
> > THP pages. What's the 'none PTE threshold'? Do you mean
> > /sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_none ?
> ack, I will expand these descriptions

Thanks.

> >
> > Let's spell that out please, this is far too vague.
> >
> > > +       With mTHP collapse, a bitmap is used to gather the state of a PMD region
> > > +       and is then recursively checked from largest to smallest order against
> > > +       the scaled max_ptes_none count. This counter indicates that the next
> > > +       enabled order will be checked.
> >
> > I think you really need to expand upon this as this is confusing and vague.
> >
> > I also don't think saying 'recursive' here really benefits anything, Just
> > saying that we try to collapse the largest mTHP size we can in each
> > instance, and then give a more 'words-y' explanation as to how
> > max_ptes_none is (in effect) converted to a ratio of a PMD, and then that
> > ratio is applied to the mTHP sizes.
> >
> > You can then go on to say that this counter measures the number of
> > occasions in which this occurred.
> ack I will clean it up

Thanks

> >
> > > +
> > > +collapse_exceed_shared_pte
> > > +       The number of anonymous THP which contain at least one shared PTE.
> >
> > anonymous THP pages right? :)
> regions?

I don't understand what regions would mean here?

So what are you actually measuring? The number of anonymous THP mappings? If so
I think 'mappings' is probably better. Or 'The number of anonymous THP page
table ranges...' perhaps?

> >
> > > +       Currently khugepaged does not support collapsing mTHP regions that
> > > +       contain a shared PTE.
> >
> > Again I don't really understand the purpose of creating a counter for
> > something we don't support.

> see above

Ack

> >
> > Let's add it when we support it.
> >
> > I also in this case and the exceed swap case don't understand what you mean
> > by exceed here, you need to spell this out clearly.
> >
> > Perhaps the context missing here is that you _also_ count THP events in
> > these counters.
> >
> > But again, given we have THP_... counters for the stats mTHP doesn't do
> > yet, I'd say adding these is pointless.
> >
> > > +
> > >  As the system ages, allocating huge pages may be expensive as the
> > >  system uses memory compaction to copy data around memory to free a
> > >  huge page for use. There are some counters in ``/proc/vmstat`` to help
> > > diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> > > index 4ada5d1f7297..6f1593d0b4b5 100644
> > > --- a/include/linux/huge_mm.h
> > > +++ b/include/linux/huge_mm.h
> > > @@ -144,6 +144,9 @@ enum mthp_stat_item {
> > >       MTHP_STAT_SPLIT_DEFERRED,
> > >       MTHP_STAT_NR_ANON,
> > >       MTHP_STAT_NR_ANON_PARTIALLY_MAPPED,
> > > +     MTHP_STAT_COLLAPSE_EXCEED_SWAP,
> > > +     MTHP_STAT_COLLAPSE_EXCEED_NONE,
> > > +     MTHP_STAT_COLLAPSE_EXCEED_SHARED,
> >
> > Wh do we put 'collapse' here but not in the THP equivalents?

> to indicate they come from the collapse functionality. I can shorten
> it by removing COLLAPSE if youd like

Hmm, if this is actually giving information then fine to keep.

> >
> > >       __MTHP_STAT_COUNT
> > >  };
> > >
> > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > > index 20d005c2c61f..9f0470c3e983 100644
> > > --- a/mm/huge_memory.c
> > > +++ b/mm/huge_memory.c
> > > @@ -639,6 +639,10 @@ DEFINE_MTHP_STAT_ATTR(split_failed, MTHP_STAT_SPLIT_FAILED);
> > >  DEFINE_MTHP_STAT_ATTR(split_deferred, MTHP_STAT_SPLIT_DEFERRED);
> > >  DEFINE_MTHP_STAT_ATTR(nr_anon, MTHP_STAT_NR_ANON);
> > >  DEFINE_MTHP_STAT_ATTR(nr_anon_partially_mapped, MTHP_STAT_NR_ANON_PARTIALLY_MAPPED);
> > > +DEFINE_MTHP_STAT_ATTR(collapse_exceed_swap_pte, MTHP_STAT_COLLAPSE_EXCEED_SWAP);
> > > +DEFINE_MTHP_STAT_ATTR(collapse_exceed_none_pte, MTHP_STAT_COLLAPSE_EXCEED_NONE);
> > > +DEFINE_MTHP_STAT_ATTR(collapse_exceed_shared_pte, MTHP_STAT_COLLAPSE_EXCEED_SHARED);
> > > +
> > >
> > >  static struct attribute *anon_stats_attrs[] = {
> > >       &anon_fault_alloc_attr.attr,
> > > @@ -655,6 +659,9 @@ static struct attribute *anon_stats_attrs[] = {
> > >       &split_deferred_attr.attr,
> > >       &nr_anon_attr.attr,
> > >       &nr_anon_partially_mapped_attr.attr,
> > > +     &collapse_exceed_swap_pte_attr.attr,
> > > +     &collapse_exceed_none_pte_attr.attr,
> > > +     &collapse_exceed_shared_pte_attr.attr,
> > >       NULL,
> > >  };
> > >
> > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > > index c13bc583a368..5a3386043f39 100644
> > > --- a/mm/khugepaged.c
> > > +++ b/mm/khugepaged.c
> > > @@ -594,7 +594,9 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> > >                               continue;
> > >                       } else {
> > >                               result = SCAN_EXCEED_NONE_PTE;
> > > -                             count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
> >
> > Hm so wait you were miscounting statistics in patch 10/13 when you turned
> > all this one? That's not good.
> >
> > This should be in place _first_ before enabling the feature.
> Ok I can move them around.

Thanks

> >
> > > +                             if (order == HPAGE_PMD_ORDER)
> > > +                                     count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
> > > +                             count_mthp_stat(order, MTHP_STAT_COLLAPSE_EXCEED_NONE);
> > >                               goto out;
> > >                       }
> > >               }
> > > @@ -633,10 +635,17 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> > >                        * shared may cause a future higher order collapse on a
> > >                        * rescan of the same range.
> > >                        */
> > > -                     if (order != HPAGE_PMD_ORDER || (cc->is_khugepaged &&
> > > -                         shared > khugepaged_max_ptes_shared)) {
> > > +                     if (order != HPAGE_PMD_ORDER) {
> >
> > Hm wait what? I dont understand what's going on here? You're no longer
> > actually doing any check except order != HPAGE_PMD_ORDER?... am I missnig
> > something?
> >
> > Again why we are bothering to maintain a counter that doesn't mean anything
> > I don't know? I may be misinterpreting somehow however.

I guess answered by rest.

> >
> > > +                             result = SCAN_EXCEED_SHARED_PTE;
> > > +                             count_mthp_stat(order, MTHP_STAT_COLLAPSE_EXCEED_SHARED);
> > > +                             goto out;
> > > +                     }
> > > +
> > > +                     if (cc->is_khugepaged &&
> > > +                         shared > khugepaged_max_ptes_shared) {
> > >                               result = SCAN_EXCEED_SHARED_PTE;
> > >                               count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
> > > +                             count_mthp_stat(order, MTHP_STAT_COLLAPSE_EXCEED_SHARED);
> > >                               goto out;
> > >                       }
> > >               }
> > > @@ -1084,6 +1093,7 @@ static int __collapse_huge_page_swapin(struct mm_struct *mm,
> > >                * range.
> > >                */
> > >               if (order != HPAGE_PMD_ORDER) {
> > > +                     count_mthp_stat(order, MTHP_STAT_COLLAPSE_EXCEED_SWAP);
> >
> > This again seems surely to not be testing for what it claims to be
> > tracking? I may again be missing context here.
> We are bailing out of the mTHP collapse due to it having a SWAP page.
> In turn exceeding our threshold of 0.

OK.

>
> Cheers,
> -- Nico
> >
> > >                       pte_unmap(pte);
> > >                       mmap_read_unlock(mm);
> > >                       result = SCAN_EXCEED_SWAP_PTE;
> > > --
> > > 2.50.1
> > >
> >
>