Re: [PATCH mm-unstable v18 05/14] mm/khugepaged: require collapse_huge_page to enter/exit with the lock dropped

On Mon, Jun 1, 2026 at 8:13 AM Lorenzo Stoakes [off-list ref] wrote:

On Fri, May 22, 2026 at 09:00:00AM -0600, Nico Pache wrote:

quoted

Currently the collapse_huge_page function requires the mmap_read_lock to
enter with it held, and exit with it dropped. This function moves the
unlock into its parent caller, and changes this semantic to requiring it
to enter/exit with it always unlocked.

In future patches, we need this expectation, as for in mTHP collapse, we
may have already have dropped the lock, and do not want to conditionally
check for this by passing through the lock_dropped variable.

No functional change is expected as one of the first things the
collapse_huge_page function does is drop this lock before allocating the
hugepage.

Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Signed-off-by: Nico Pache <redacted>

One small nit below, otherwise LGTM, so:

Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>

Thank you for reviewing!

quoted

---
 mm/khugepaged.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index e98ba5b15163..fab35d318641 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c

@@ -1208,6 +1208,12 @@ static enum scan_result alloc_charge_folio(struct folio **foliop, struct mm_stru
      return SCAN_SUCCEED;
 }

+/*
+ * collapse_huge_page expects the mmap_lock to be unlocked before entering and
+ * will always return with the lock unlocked, to avoid holding the mmap_lock
+ * while allocating a THP, as that could trigger direct reclaim/compaction.
+ * Note that the VMA must be rechecked after grabbing the mmap_lock again.
+ */
 static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long address,
              int referenced, int unmapped, struct collapse_control *cc)
 {

@@ -1223,14 +1229,6 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a

      VM_BUG_ON(address & ~HPAGE_PMD_MASK);

-     /*
-      * Before allocating the hugepage, release the mmap_lock read lock.
-      * The allocation can take potentially a long time if it involves
-      * sync compaction, and we do not need to hold the mmap_lock during
-      * that. We will recheck the vma after taking it again in write mode.
-      */
-     mmap_read_unlock(mm);
-

NIT: Maybe worth an mmap_assert_locked()?

But it will already be unlocked here. The contract is that we enter
unlocked and exit unlocked.

Cheers,
-- Nico

quoted

      result = alloc_charge_folio(&folio, mm, cc, HPAGE_PMD_ORDER);
      if (result != SCAN_SUCCEED)
              goto out_nolock;

@@ -1535,6 +1533,8 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
 out_unmap:
      pte_unmap_unlock(pte, ptl);
      if (result == SCAN_SUCCEED) {
+             /* collapse_huge_page expects the lock to be dropped before calling */
+             mmap_read_unlock(mm);
              result = collapse_huge_page(mm, start_addr, referenced,
                                          unmapped, cc);
              /* collapse_huge_page will return with the mmap_lock released */
--

2.54.0

Cheers, Lorenzo