Re: [PATCH mm-unstable v4 5/5] mm/khugepaged: unify khugepaged and madv_collapse with collapse_single_pmd()

Nico Pache
2026-03-31T20:00:14Z

On Tue, Mar 31, 2026 at 8:01 AM Lorenzo Stoakes (Oracle) <ljs@kernel.org> wrote:
>
> OK we need a fairly urgent fix for this as this has triggered a syzbot. See [0]
> for an analysis.
>
> I show inline where the issue is, and attach a fix-patch for the bug.
>
> [0]: https://lore.kernel.org/all/e1cb33b8-c1f7-4972-8628-3a2169077d6e@lucifer.local/
>
> See below for details.
>
> Cheers, Lorenzo
>
> On Wed, Mar 25, 2026 at 05:40:22AM -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. To help reduce confusion around the
> > mmap_locked variable, we rename mmap_locked to lock_dropped in the
> > collapse_scan_mm_slot() function, and remove the redundant mmap_locked
> > in madvise_collapse(); this further unifies the code readiblity. the
> > SCAN_PTE_MAPPED_HUGEPAGE enum is no longer reachable in the
> > madvise_collapse() function, so we drop it from the list of "continuing"
> > enums.
> >
> > 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 process is no longer valid to operate on.
> >
> > By moving the madvise_collapse writeback-retry logic into the helper
> > function we can also avoid having to revalidate the VMA.
> >
> > We guard the khugepaged_pages_collapsed variable to ensure its only
> > incremented for khugepaged.
> >
> > As requested we also convert a VM_BUG_ON to a VM_WARN_ON.
> >
> > Reviewed-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
> > Reviewed-by: Lance Yang <lance.yang@linux.dev>
> > Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> > Acked-by: David Hildenbrand (Arm) <david@kernel.org>
> > Signed-off-by: Nico Pache <npache@redhat.com>
> > ---
> > mm/khugepaged.c | 142 ++++++++++++++++++++++++------------------------
> > 1 file changed, 72 insertions(+), 70 deletions(-)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index 3728a2cf133c..d06d84219e1b 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -1257,7 +1257,7 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
> >
> > static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
> > struct vm_area_struct *vma, unsigned long start_addr,
> > - bool *mmap_locked, struct collapse_control *cc)
> > + bool *lock_dropped, struct collapse_control *cc)
> > {
> > pmd_t *pmd;
> > pte_t *pte, *_pte;
> > @@ -1432,7 +1432,7 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
> > result = collapse_huge_page(mm, start_addr, referenced,
> > unmapped, cc);
> > /* collapse_huge_page will return with the mmap_lock released */
> > - *mmap_locked = false;
> > + *lock_dropped = true;
> > }
> > out:
> > trace_mm_khugepaged_scan_pmd(mm, folio, referenced,
> > @@ -2424,6 +2424,67 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm,
> > 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 *lock_dropped,
> > + struct collapse_control *cc)
> > +{
> > + struct mm_struct *mm = vma->vm_mm;
> > + bool triggered_wb = false;
> > + enum scan_result result;
> > + struct file *file;
> > + pgoff_t pgoff;
> > +
> > + mmap_assert_locked(mm);
> > +
> > + if (vma_is_anonymous(vma)) {
> > + result = collapse_scan_pmd(mm, vma, addr, lock_dropped, cc);
> > + goto end;
> > + }
> > +
> > + file = get_file(vma->vm_file);
> > + pgoff = linear_page_index(vma, addr);
> > +
> > + mmap_read_unlock(mm);
> > + *lock_dropped = true;
> > +retry:
> > + result = collapse_scan_file(mm, addr, file, pgoff, cc);
> > +
> > + /*
> > + * For MADV_COLLAPSE, when encountering dirty pages, try to writeback,
> > + * then retry the collapse one time.
> > + */
> > + if (!cc->is_khugepaged && result == SCAN_PAGE_DIRTY_OR_WRITEBACK &&
> > + !triggered_wb && mapping_can_writeback(file->f_mapping)) {
> > + const loff_t lstart = (loff_t)pgoff << PAGE_SHIFT;
> > + const loff_t lend = lstart + HPAGE_PMD_SIZE - 1;
> > +
> > + filemap_write_and_wait_range(file->f_mapping, lstart, lend);
> > + triggered_wb = true;
> > + goto retry;
> > + }
> > + fput(file);
> > +
> > + if (result == SCAN_PTE_MAPPED_HUGEPAGE) {
> > + mmap_read_lock(mm);
> > + if (collapse_test_exit_or_disable(mm))
> > + result = SCAN_ANY_PROCESS;
> > + else
> > + result = try_collapse_pte_mapped_thp(mm, addr,
> > + !cc->is_khugepaged);
> > + if (result == SCAN_PMD_MAPPED)
> > + result = SCAN_SUCCEED;
> > + mmap_read_unlock(mm);
> > + }
> > +end:
> > + if (cc->is_khugepaged && result == SCAN_SUCCEED)
> > + ++khugepaged_pages_collapsed;
> > + return result;
> > +}
> > +
> > static void collapse_scan_mm_slot(unsigned int progress_max,
> > enum scan_result *result, struct collapse_control *cc)
> > __releases(&khugepaged_mm_lock)
> > @@ -2485,46 +2546,21 @@ static void collapse_scan_mm_slot(unsigned int progress_max,
> > VM_BUG_ON(khugepaged_scan.address & ~HPAGE_PMD_MASK);
> >
> > while (khugepaged_scan.address < hend) {
> > - bool mmap_locked = true;
> > + bool lock_dropped = false;
> >
> > cond_resched();
> > if (unlikely(collapse_test_exit_or_disable(mm)))
> > goto breakouterloop;
> >
> > - VM_BUG_ON(khugepaged_scan.address < hstart ||
> > + VM_WARN_ON_ONCE(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, &lock_dropped, cc);
> > /* move to next address */
> > khugepaged_scan.address += HPAGE_PMD_SIZE;
> > - if (!mmap_locked)
> > + if (lock_dropped)
> > /*
> > * We released mmap_lock so break loop. Note
> > * that we drop mmap_lock before all hugepage
> > @@ -2799,7 +2835,6 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
> > unsigned long hstart, hend, addr;
> > enum scan_result last_fail = SCAN_FAIL;
> > int thps = 0;
> > - bool mmap_locked = true;
> >
> > BUG_ON(vma->vm_start > start);
> > BUG_ON(vma->vm_end < end);
> > @@ -2821,13 +2856,11 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
> >
> > for (addr = hstart; addr < hend; addr += HPAGE_PMD_SIZE) {
> > enum scan_result result = SCAN_FAIL;
> > - bool triggered_wb = false;
> >
> > -retry:
> > - if (!mmap_locked) {
> > + if (*lock_dropped) {
> > cond_resched();
> > mmap_read_lock(mm);
> > - mmap_locked = true;
> > + *lock_dropped = false;
>
> So this is the bug. 'lock_dropped' needs to record if the lock was _ever_
> dropped, not if it is _currently_ dropped.
>
> This is probably a mea culpa on my part on review, so apologies.

All good! That code is rather confusing.

>
> See below for a fix-patch.
>
> > result = hugepage_vma_revalidate(mm, addr, false, &vma,
> > cc);
> > if (result != SCAN_SUCCEED) {
> > @@ -2837,45 +2870,14 @@ 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;
> > - *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)) {
> > - 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);
> > - } else {
> > - result = collapse_scan_pmd(mm, vma, addr, &mmap_locked, cc);
> > - }
> > - if (!mmap_locked)
> > - *lock_dropped = true;
> > + result = collapse_single_pmd(addr, vma, lock_dropped, cc);
> >
> > -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_NON_PRESENT:
> > @@ -2898,7 +2900,7 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
> >
> > out_maybelock:
> > /* Caller expects us to hold mmap_lock on return */
> > - if (!mmap_locked)
> > + if (*lock_dropped)
> > mmap_read_lock(mm);
> > out_nolock:
> > mmap_assert_locked(mm);
> > --
> > 2.53.0
> >
>
> Fix patch follows:
>
> ----8<----
> From a4dfc7718a15035449f344a0bc7f58e449366405 Mon Sep 17 00:00:00 2001
> From: "Lorenzo Stoakes (Oracle)" <ljs@kernel.org>
> Date: Tue, 31 Mar 2026 13:11:18 +0100
> Subject: [PATCH] mm/khugepaged: fix issue with tracking lock
>
> We are incorrectly treating lock_dropped to track both whether the lock is
> currently held and whether or not the lock was ever dropped.
>
> Update this change to account for this.
>
> Signed-off-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
> ---

Thanks for fixing this so quickly :) Sysbot didn't send this to me.
Sadly it looked like we indeed needed that doubled "locked" semantics.
Thank you for the very good explanation in-reply-to the sysbot; that
really cleared up some confusion for me.

Reviewed-by: Nico Pache <npache@redhat.com>

> mm/khugepaged.c | 12 ++++++++----
> 1 file changed, 8 insertions(+), 4 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index d21348b85a59..b8452dbdb043 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -2828,6 +2828,7 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
> unsigned long hstart, hend, addr;
> enum scan_result last_fail = SCAN_FAIL;
> int thps = 0;
> + bool mmap_unlocked = false;
>
> BUG_ON(vma->vm_start > start);
> BUG_ON(vma->vm_end < end);
> @@ -2850,10 +2851,11 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
> for (addr = hstart; addr < hend; addr += HPAGE_PMD_SIZE) {
> enum scan_result result = SCAN_FAIL;
>
> - if (*lock_dropped) {
> + if (mmap_unlocked) {
> cond_resched();
> mmap_read_lock(mm);
> - *lock_dropped = false;
> + mmap_unlocked = false;
> + *lock_dropped = true;
> result = hugepage_vma_revalidate(mm, addr, false, &vma,
> cc);
> if (result != SCAN_SUCCEED) {
> @@ -2864,7 +2866,7 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
> hend = min(hend, vma->vm_end & HPAGE_PMD_MASK);
> }
>
> - result = collapse_single_pmd(addr, vma, lock_dropped, cc);
> + result = collapse_single_pmd(addr, vma, &mmap_unlocked, cc);
>
> switch (result) {
> case SCAN_SUCCEED:
> @@ -2893,8 +2895,10 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
>
> out_maybelock:
> /* Caller expects us to hold mmap_lock on return */
> - if (*lock_dropped)
> + if (mmap_unlocked) {
> + *lock_dropped = true;
> mmap_read_lock(mm);
> + }
> out_nolock:
> mmap_assert_locked(mm);
> mmdrop(mm);
> --
> 2.53.0
>