Message-ID: <d8932d68-1c43-479e-8167-6ab7b1b92cb6@lucifer.local>
Date: Fri, 22 Aug 2025 11:21:44 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Nico Pache <npache@...hat.com>
Cc: linux-mm@...ck.org, linux-doc@...r.kernel.org,
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Subject: Re: [PATCH v10 02/13] introduce collapse_single_pmd to unify
 khugepaged and madvise_collapse

On Wed, Aug 20, 2025 at 10:35:57AM -0600, Nico Pache wrote:
> On Wed, Aug 20, 2025 at 5:22 AM Lorenzo Stoakes
> <lorenzo.stoakes@...cle.com> wrote:
> >
> > On Tue, Aug 19, 2025 at 07:41:54AM -0600, Nico Pache wrote:
> > > The khugepaged daemon and madvise_collapse have two different
> > > implementations that do almost the same thing.
> > >
> > > Create collapse_single_pmd to increase code reuse and create an entry
> > > point to these two users.
> > >
> > > Refactor madvise_collapse and collapse_scan_mm_slot to use the new
> > > collapse_single_pmd function. This introduces a minor behavioral change
> > > that is most likely an undiscovered bug. The current implementation of
> > > khugepaged tests collapse_test_exit_or_disable before calling
> > > collapse_pte_mapped_thp, but we weren't doing it in the madvise_collapse
> > > case. By unifying these two callers madvise_collapse now also performs
> > > this check.
> > >
> > > Reviewed-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
> > > Acked-by: David Hildenbrand <david@...hat.com>
> > > Signed-off-by: Nico Pache <npache@...hat.com>
> > > ---
> > >  mm/khugepaged.c | 94 ++++++++++++++++++++++++++-----------------------
> > >  1 file changed, 49 insertions(+), 45 deletions(-)
> > >
> > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > > index 0e7bbadf03ee..b7b98aebb670 100644
> > > --- a/mm/khugepaged.c
> > > +++ b/mm/khugepaged.c
> > > @@ -2382,6 +2382,50 @@ static int collapse_scan_file(struct mm_struct *mm, unsigned long addr,
> > >       return result;
> > >  }
> > >
> > > +/*
> > > + * Try to collapse a single PMD starting at a PMD aligned addr, and return
> > > + * the results.
> > > + */
> > > +static int collapse_single_pmd(unsigned long addr,
> > > +                             struct vm_area_struct *vma, bool *mmap_locked,
> > > +                             struct collapse_control *cc)
> > > +{
> > > +     int result = SCAN_FAIL;
> >
> > You assign result in all branches, so this can be uninitialised.
> ack, thanks.
> >
> > > +     struct mm_struct *mm = vma->vm_mm;
> > > +
> > > +     if (!vma_is_anonymous(vma)) {
> > > +             struct file *file = get_file(vma->vm_file);
> > > +             pgoff_t pgoff = linear_page_index(vma, addr);
> > > +
> > > +             mmap_read_unlock(mm);
> > > +             *mmap_locked = false;
> > > +             result = collapse_scan_file(mm, addr, file, pgoff, cc);
> > > +             fput(file);
> > > +             if (result == SCAN_PTE_MAPPED_HUGEPAGE) {
> > > +                     mmap_read_lock(mm);
> > > +                     *mmap_locked = true;
> > > +                     if (collapse_test_exit_or_disable(mm)) {
> > > +                             mmap_read_unlock(mm);
> > > +                             *mmap_locked = false;
> > > +                             result = SCAN_ANY_PROCESS;
> > > +                             goto end;
> >
> > Don't love that in e.g. collapse_scan_mm_slot() we are using the mmap lock being
> > disabled as in effect an error code.
> >
> > Is SCAN_ANY_PROCESS correct here? Because in collapse_scan_mm_slot() you'd
> > previously:
> https://lore.kernel.org/lkml/a881ed65-351a-469f-b625-a3066d0f1d5c@linux.alibaba.com/
> Baolin brought up a good point a while back that if
> collapse_test_exit_or_disable returns true we will be breaking out of
> the loop and should change the return value to indicate this. So to
> combine the madvise breakout and the scan_slot breakout we drop the
> lock and return SCAN_ANY_PROCESS.

Let's document in commit msg, as Liam's pointed out it's really important to
track things, and part of that as well is detailing in the commit message what
you're doing + why.

With the THP code being as 'organically grown' as it is shall we say :) it's
even more mportant to be specific.

> >
> >         if (*result == SCAN_PTE_MAPPED_HUGEPAGE) {
> >                 mmap_read_lock(mm);
> >                 if (collapse_test_exit_or_disable(mm))
> >                         goto breakouterloop;
> >                 ...
> >         }
> >
> > But now you're setting result = SCAN_ANY_PROCESS rather than
> > SCAN_PTE_MAPPED_HUGEPAGE in this instance?
> >
> > You don't mention that you're changing this, or at least explicitly enough,
> > the commit message should state that you're changing this and explain why
> > it's ok.
> I do state it but perhaps I need to be more verbose! I will update the
> message to state we are also changing the result value too.

Thanks!

> >
> > This whole file is horrid, and it's kinda an aside, but I really wish we
> > had some comment going through each of the scan_result cases and explaining
> > what each one meant.
> Yeah its been a huge pain to have to investigate what everything is
> supposed to mean, and I often have to go searching to confirm things.
> include/trace/events/huge_memory.h has a "good" summary of them
> >
> > Also I think:
> >
> >         return SCAN_ANY_PROCESS;
> >
> > Is better than:
> >
> >         result = SCAN_ANY_PROCESS;
> >         goto end;
> I agree! I will change that :)
> >         ...
> > end:
> >         return result;
> >
> > > +                     }
> > > +                     result = collapse_pte_mapped_thp(mm, addr,
> > > +                                                      !cc->is_khugepaged);
> >
> > Hm another change here, in the original code in collapse_scan_mm_slot()
> > this is:
> >
> >         *result = collapse_pte_mapped_thp(mm,
> >                 khugepaged_scan.address, false);
> >
> > Presumably collapse_scan_mm_slot() is only ever invoked with
> > cc->is_khugepaged?
> Correct, but the madvise_collapse calls this with true, hence why it
> now depends on the is_khugepaged variable. No functional change here.
> >
> > Maybe worth adding a VM_WARN_ON_ONCE(!cc->is_khugepaged) at the top of
> > collapse_scan_mm_slot() to assert this (and other places where your change
> > assumes this to be the case).
> Ok I will investigate doing that but it would take a huge mistake to
> hit that assertion.
> >
> >
> > > +                     if (result == SCAN_PMD_MAPPED)
> > > +                             result = SCAN_SUCCEED;
> > > +                     mmap_read_unlock(mm);
> > > +                     *mmap_locked = false;
> > > +             }
> > > +     } else {
> > > +             result = collapse_scan_pmd(mm, vma, addr, mmap_locked, cc);
> > > +     }
> > > +     if (cc->is_khugepaged && result == SCAN_SUCCEED)
> > > +             ++khugepaged_pages_collapsed;
> >
> > Similarly, presumably because collapse_scan_mm_slot() only ever invoked
> > khugepaged case this didn't have the cc->is_khugepaged check?
> Correct, we only increment this when its khugepaged, so we need to
> guard it so madvise collapse wont increment this.

You know what I'm going to say :) commit message please!

> >
> > > +end:
> > > +     return result;
> > > +}
> >
> > There's a LOT of nesting going on here, I think we can simplify this a
> > lot. If we make the change I noted above re: returning SCAN_ANY_PROCESS< we
> > can move the end label up a bit and avoid a ton of nesting, e.g.:
> Ah I like this much more, I will try to implement/test it.
> >
> > static int collapse_single_pmd(unsigned long addr,
> >                                 struct vm_area_struct *vma, bool *mmap_locked,
> >                                 struct collapse_control *cc)
> > {
> >         struct mm_struct *mm = vma->vm_mm;
> >         struct file *file;
> >         pgoff_t pgoff;
> >         int result;
> >
> >         if (vma_is_anonymous(vma)) {
> >                 result = collapse_scan_pmd(mm, vma, addr, mmap_locked, cc);
> >                 goto end:
> >         }
> >
> >         file = get_file(vma->vm_file);
> >         pgoff = linear_page_index(vma, addr);
> >
> >         mmap_read_unlock(mm);
> >         *mmap_locked = false;
> >         result = collapse_scan_file(mm, addr, file, pgoff, cc);
> >         fput(file);
> >         if (result != SCAN_PTE_MAPPED_HUGEPAGE)
> >                 goto end;
> >
> >         mmap_read_lock(mm);
> >         *mmap_locked = true;
> >         if (collapse_test_exit_or_disable(mm)) {
> >                 mmap_read_unlock(mm);
> >                 *mmap_locked = false;
> >                 return SCAN_ANY_PROCESS;
> >         }
> >         result = collapse_pte_mapped_thp(mm, addr, !cc->is_khugepaged);
> >         if (result == SCAN_PMD_MAPPED)
> >                 result = SCAN_SUCCEED;
> >         mmap_read_unlock(mm);
> >         *mmap_locked = false;
> >
> > end:
> >         if (cc->is_khugepaged && result == SCAN_SUCCEED)
> >                 ++khugepaged_pages_collapsed;
> >
> >         return result;
> > }
> >
> > (untested, thrown together so do double check!)

This suggested refactoring work for you?

> >
> > > +
> > >  static unsigned int collapse_scan_mm_slot(unsigned int pages, int *result,
> > >                                           struct collapse_control *cc)
> > >       __releases(&khugepaged_mm_lock)
> > > @@ -2455,34 +2499,9 @@ static unsigned int collapse_scan_mm_slot(unsigned int pages, int *result,
> > >                       VM_BUG_ON(khugepaged_scan.address < hstart ||
> > >                                 khugepaged_scan.address + HPAGE_PMD_SIZE >
> > >                                 hend);
> > > -                     if (!vma_is_anonymous(vma)) {
> > > -                             struct file *file = get_file(vma->vm_file);
> > > -                             pgoff_t pgoff = linear_page_index(vma,
> > > -                                             khugepaged_scan.address);
> > > -
> > > -                             mmap_read_unlock(mm);
> > > -                             mmap_locked = false;
> > > -                             *result = collapse_scan_file(mm,
> > > -                                     khugepaged_scan.address, file, pgoff, cc);
> > > -                             fput(file);
> > > -                             if (*result == SCAN_PTE_MAPPED_HUGEPAGE) {
> > > -                                     mmap_read_lock(mm);
> > > -                                     if (collapse_test_exit_or_disable(mm))
> > > -                                             goto breakouterloop;
> > > -                                     *result = collapse_pte_mapped_thp(mm,
> > > -                                             khugepaged_scan.address, false);
> > > -                                     if (*result == SCAN_PMD_MAPPED)
> > > -                                             *result = SCAN_SUCCEED;
> > > -                                     mmap_read_unlock(mm);
> > > -                             }
> > > -                     } else {
> > > -                             *result = collapse_scan_pmd(mm, vma,
> > > -                                     khugepaged_scan.address, &mmap_locked, cc);
> > > -                     }
> > > -
> > > -                     if (*result == SCAN_SUCCEED)
> > > -                             ++khugepaged_pages_collapsed;
> > >
> > > +                     *result = collapse_single_pmd(khugepaged_scan.address,
> > > +                                                     vma, &mmap_locked, cc);
> > >                       /* move to next address */
> > >                       khugepaged_scan.address += HPAGE_PMD_SIZE;
> > >                       progress += HPAGE_PMD_NR;
> > > @@ -2799,34 +2818,19 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
> > >               mmap_assert_locked(mm);
> > >               memset(cc->node_load, 0, sizeof(cc->node_load));
> > >               nodes_clear(cc->alloc_nmask);
> > > -             if (!vma_is_anonymous(vma)) {
> > > -                     struct file *file = get_file(vma->vm_file);
> > > -                     pgoff_t pgoff = linear_page_index(vma, addr);
> > >
> > > -                     mmap_read_unlock(mm);
> > > -                     mmap_locked = false;
> > > -                     result = collapse_scan_file(mm, addr, file, pgoff, cc);
> > > -                     fput(file);
> > > -             } else {
> > > -                     result = collapse_scan_pmd(mm, vma, addr,
> > > -                                                &mmap_locked, cc);
> > > -             }
> > > +             result = collapse_single_pmd(addr, vma, &mmap_locked, cc);
> > > +
> >
> > Ack the fact you noted the behaviour change re:
> > collapse_test_exit_or_disable() that seems fine.
> >
> > >               if (!mmap_locked)
> > >                       *lock_dropped = true;
> > >
> > > -handle_result:
> > >               switch (result) {
> > >               case SCAN_SUCCEED:
> > >               case SCAN_PMD_MAPPED:
> > >                       ++thps;
> > >                       break;
> > > -             case SCAN_PTE_MAPPED_HUGEPAGE:
> > > -                     BUG_ON(mmap_locked);
> > > -                     mmap_read_lock(mm);
> > > -                     result = collapse_pte_mapped_thp(mm, addr, true);
> > > -                     mmap_read_unlock(mm);
> > > -                     goto handle_result;
> >
> > One thing that differs with new code her is we filter SCAN_PMD_MAPPED to
> > SCAN_SUCCEED.
> >
> > I was about to say 'but ++thps - is this correct' but now I realise this
> > was looping back on itself with a goto to do just that... ugh ye gads.
> >
> > Anwyay that's fine because it doesn't change anything.
> >
> > Re: switch statement in general, again would be good to always have each
> > scan possibility in switch statements, but perhaps given so many not
> > practical :)
>
> Yeah it may be worth investigating for future changes I have for
> khugepaged (including the new switch statement I implement later and
> you commented on)

Ack yeah this can be one for the future!

> >
> > (that way the compiler warns on missing a newly added enum val)
> >
> > >               /* Whitelisted set of results where continuing OK */
> > > +             case SCAN_PTE_MAPPED_HUGEPAGE:
> > >               case SCAN_PMD_NULL:
> > >               case SCAN_PTE_NON_PRESENT:
> > >               case SCAN_PTE_UFFD_WP:
> > > --
>
> Thanks for the review :)

No probs, to underline as well - the critique is to make sure we get this right,
my aim here is to get your series landed in as good a form as possible :)

>
> -- Nico
> > > 2.50.1
> > >
> >
>

Cheers, Lorenzo

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