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

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

On Fri, Jan 23, 2026 at 04:26:09PM -0700, Nico Pache wrote:
> 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 properly indicates that this process is no longer valid to operate on.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.

You obviously can't manipulate or reference a pointer to a VMA in any way
if is no longer stabilised, that'd be a potential UAF.

>
> 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;

NIT, but Let's const-ify these.

Also credit to Baolin for having suggested taking the approach of putting
here! :)


> +
> +               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;

OK this looks correct I agree with Lance.

Could you send this in reply to the parent, i.e. [0], as a fix-patch and
ask Andrew to apply it?

Can then review that there.

[0]:https://lore.kernel.org/all/20260122192841.128719-4-npache@redhat.com/


>                 }
>
>                 switch (result) {
>
>
>
> -- Nico
>

Cheers, Lorenzo