Re: [PATCH mm-unstable v14 03/16] introduce collapse_single_pmd to unify khugepaged and madvise_collapse

[Nico Pache <npache@redhat.com>]

On Thu, Jan 22, 2026 at 10:08 PM Lance Yang <lance.yang@linux.dev> wrote:
>
>
>
> On 2026/1/23 03:28, 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. We also modify the return value to be SCAN_ANY_PROCESS which
> > properly indicates that this process is no longer valid to operate on.
> >
> > We also guard the khugepaged_pages_collapsed variable to ensure its only
> > incremented for khugepaged.
> >
> > Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
> > Reviewed-by: Lance Yang <lance.yang@linux.dev>
> > Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> > Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> > Reviewed-by: Zi Yan <ziy@nvidia.com>
> > Acked-by: David Hildenbrand <david@redhat.com>
> > Signed-off-by: Nico Pache <npache@redhat.com>
> > ---
>
> I think this patch introduces some functional changes compared to previous
> version[1] ...
>
> Maybe we should drop the r-b tags and let folks take another look?
>
> There might be an issue with the vma access in madvise_collapse(). See
> below:
>
> [1]
> https://lore.kernel.org/linux-mm/20251201174627.23295-3-npache@redhat.com/
>
> >   mm/khugepaged.c | 106 +++++++++++++++++++++++++++---------------------
> >   1 file changed, 60 insertions(+), 46 deletions(-)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index fefcbdca4510..59e5a5588d85 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -2394,6 +2394,54 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm, unsigned long a
> >       return result;
> >   }
> >
> > +/*
> > + * Try to collapse a single PMD starting at a PMD aligned addr, and return
> > + * the results.
> > + */
> > +static enum scan_result 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;
> > +     enum scan_result result;
> > +     struct file *file;
> > +     pgoff_t pgoff;
> > +
> > +     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 = try_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;
> > +}
> > +
> >   static unsigned int collapse_scan_mm_slot(unsigned int pages, enum scan_result *result,
> >                                           struct collapse_control *cc)
> >       __releases(&khugepaged_mm_lock)
> > @@ -2466,34 +2514,9 @@ static unsigned int collapse_scan_mm_slot(unsigned int pages, enum scan_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 = try_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;
> > @@ -2799,6 +2822,7 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
> >                       cond_resched();
> >                       mmap_read_lock(mm);
> >                       mmap_locked = true;
> > +                     *lock_dropped = true;
> >                       result = hugepage_vma_revalidate(mm, addr, false, &vma,
> >                                                        cc);
> >                       if (result  != SCAN_SUCCEED) {
> > @@ -2809,17 +2833,17 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
> >                       hend = min(hend, vma->vm_end & HPAGE_PMD_MASK);
> >               }
> >               mmap_assert_locked(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_single_pmd(addr, vma, &mmap_locked, cc);
> > +
> > +             if (!mmap_locked)
> >                       *lock_dropped = true;
> > -                     result = collapse_scan_file(mm, addr, file, pgoff, cc);
> >
> > -                     if (result == SCAN_PAGE_DIRTY_OR_WRITEBACK && !triggered_wb &&
> > -                         mapping_can_writeback(file->f_mapping)) {
> > +             if (result == SCAN_PAGE_DIRTY_OR_WRITEBACK && !triggered_wb) {
> > +                     struct file *file = get_file(vma->vm_file);
> > +                     pgoff_t pgoff = linear_page_index(vma, addr);
>
>
> After collapse_single_pmd() returns, mmap_lock might have been released.
> Between
> that unlock and here, another thread could unmap/remap the VMA, making
> the vma
> pointer stale when we access vma->vm_file?

+ Shivank, I thought they were on the CC list.

Hey! I thought of this case, but then figured it was no different than
what is currently implemented for the writeback-retry logic, since the
mmap lock is dropped and not revalidated. BUT I failed to consider
that the file reference is held throughout that time.

I thought of moving the functionality into collapse_single_pmd(), but
figured I'd keep it in madvise_collapse() as it's the sole user of
that functionality. Given the potential file ref issue, that may be
the best solution, and I dont think it should be too difficult. I'll
queue that up, and also drop the r-b tags as you suggested.

Ok, here's my solution, does this look like the right approach?:
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 59e5a5588d85..dda9fdc35767 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -2418,6 +2418,14 @@ static enum scan_result
collapse_single_pmd(unsigned long addr,
        mmap_read_unlock(mm);
        *mmap_locked = false;
        result = collapse_scan_file(mm, addr, file, pgoff, cc);
+
+       if (!cc->is_khugepaged && result == SCAN_PAGE_DIRTY_OR_WRITEBACK &&
+           mapping_can_writeback(file->f_mapping)) {
+               loff_t lstart = (loff_t)pgoff << PAGE_SHIFT;
+               loff_t lend = lstart + HPAGE_PMD_SIZE - 1;
+
+               filemap_write_and_wait_range(file->f_mapping, lstart, lend);
+       }
        fput(file);

        if (result != SCAN_PTE_MAPPED_HUGEPAGE)
@@ -2840,19 +2848,8 @@ int madvise_collapse(struct vm_area_struct
*vma, unsigned long start,
                        *lock_dropped = true;

                if (result == SCAN_PAGE_DIRTY_OR_WRITEBACK && !triggered_wb) {
-                       struct file *file = get_file(vma->vm_file);
-                       pgoff_t pgoff = linear_page_index(vma, addr);
-
-                       if (mapping_can_writeback(file->f_mapping)) {
-                               loff_t lstart = (loff_t)pgoff << PAGE_SHIFT;
-                               loff_t lend = lstart + HPAGE_PMD_SIZE - 1;
-
-
filemap_write_and_wait_range(file->f_mapping, lstart, lend);
-                               triggered_wb = true;
-                               fput(file);
-                               goto retry;
-                       }
-                       fput(file);
+                       triggered_wb = true;
+                       goto retry;
                }

                switch (result) {



-- Nico


>
> Would it be safer to get the file reference before calling
> collapse_single_pmd()?
> Or we need to revalidate the VMA after getting the lock back?
>
>
> Thanks,
> Lance
>
> > +
> > +                     if (mapping_can_writeback(file->f_mapping)) {
> >                               loff_t lstart = (loff_t)pgoff << PAGE_SHIFT;
> >                               loff_t lend = lstart + HPAGE_PMD_SIZE - 1;
> >
> > @@ -2829,26 +2853,16 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
> >                               goto retry;
> >                       }
> >                       fput(file);
> > -             } else {
> > -                     result = collapse_scan_pmd(mm, vma, addr, &mmap_locked, cc);
> >               }
> > -             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 = try_collapse_pte_mapped_thp(mm, addr, true);
> > -                     mmap_read_unlock(mm);
> > -                     goto handle_result;
> >               /* Whitelisted set of results where continuing OK */
> >               case SCAN_NO_PTE_TABLE:
> > +             case SCAN_PTE_MAPPED_HUGEPAGE:
> >               case SCAN_PTE_NON_PRESENT:
> >               case SCAN_PTE_UFFD_WP:
> >               case SCAN_LACK_REFERENCED_PAGE:
>