Re: [PATCH v10 02/13] introduce collapse_single_pmd to unify khugepaged and madvise_collapse

Nico Pache
2025-08-20T16:36:31Z

On Wed, Aug 20, 2025 at 5:22 AM Lorenzo Stoakes
<lorenzo.stoakes@oracle.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@linux.alibaba.com>
> > Acked-by: David Hildenbrand <david@redhat.com>
> > Signed-off-by: Nico Pache <npache@redhat.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.
>
> 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.
>
> 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.
>
> > +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!)
>
> > +
> > 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)
>
> (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 :)

-- Nico
> > 2.50.1
> >
>