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

Lorenzo Stoakes (Oracle)
2026-03-31T14:01:32Z

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 process is no longer valid to operate on.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.

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>
---
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