mm/khugepaged: keep mm in mm_slot without MMF_DISABLE_THP check

Previously, we removed the mm from mm_slot and dropped mm_count
if the MMF_THP_DISABLE flag was set. However, we didn't re-add
the mm back after clearing the MMF_THP_DISABLE flag. Additionally,
We add a check for the MMF_THP_DISABLE flag in hugepage_vma_revalidate().

Link: https://lkml.kernel.org/r/20240227035135.54593-1-ioworker0@gmail.com
Fixes: 879c6000 ("mm/khugepaged: bypassing unnecessary scans with MMF_DISABLE_THP check")
Signed-off-by: Lance Yang <ioworker0@gmail.com>
Suggested-by: Yang Shi <shy828301@gmail.com>
Reviewed-by: Yang Shi <shy828301@gmail.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Zach O'Keefe <zokeefe@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/khugepaged.c

@@ -920,7 +920,7 @@ static int hugepage_vma_revalidate(struct mm_struct *mm, unsigned long address,
 {
 	struct vm_area_struct *vma;
 
-	if (unlikely(hpage_collapse_test_exit(mm)))
+	if (unlikely(hpage_collapse_test_exit_or_disable(mm)))
 		return SCAN_ANY_PROCESS;
 
 	*vmap = vma = find_vma(mm, address);
@@ -1428,7 +1428,7 @@ static void collect_mm_slot(struct khugepaged_mm_slot *mm_slot)
 
 	lockdep_assert_held(&khugepaged_mm_lock);
 
-	if (hpage_collapse_test_exit_or_disable(mm)) {
+	if (hpage_collapse_test_exit(mm)) {
 		/* free mm_slot */
 		hash_del(&slot->hash);
 		list_del(&slot->mm_node);
@@ -2456,7 +2456,7 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
 	 * Release the current mm_slot if this mm is about to die, or
 	 * if we scanned all vmas of this mm.
 	 */
-	if (hpage_collapse_test_exit_or_disable(mm) || !vma) {
+	if (hpage_collapse_test_exit(mm) || !vma) {
 		/*
 		 * Make sure that if mm_users is reaching zero while
 		 * khugepaged runs here, khugepaged_exit will find