[PATCH mm-unstable v17 05/14] mm/khugepaged: require collapse_huge_page to enter/exit with the lock dropped

[Nico Pache <npache@redhat.com>]

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.

Signed-off-by: Nico Pache <npache@redhat.com>
---
 mm/khugepaged.c | 18 ++++++++++--------
 1 file changed, 10 insertions(+), 8 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 27465161fa6d..37a5f6791816 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1199,6 +1199,14 @@ static enum scan_result alloc_charge_folio(struct folio **foliop, struct mm_stru
 	return SCAN_SUCCEED;
 }
 
+/*
+ * collapse_huge_page expects the mmap_read_lock to be dropped before
+ * entering this function. The function will also always return with the lock
+ * dropped. The function starts by allocation a folio, which can potentially
+ * take a long time if it involves sync compaction, and we do not need to hold
+ * the mmap_lock during that. We must recheck the vma after taking it again in
+ * write mode.
+ */
 static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long address,
 		int referenced, int unmapped, struct collapse_control *cc)
 {
@@ -1214,14 +1222,6 @@ 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);
-
 	result = alloc_charge_folio(&folio, mm, cc, HPAGE_PMD_ORDER);
 	if (result != SCAN_SUCCEED)
 		goto out_nolock;
@@ -1526,6 +1526,8 @@ 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