[PATCH v2 01/12] mm/khugepaged: drop redundant mm_struct pin in madvise_collapse()

[Kiryl Shutsemau]

From: "Kiryl Shutsemau (Meta)" <kas@xxxxxxxxxx>

madvise_collapse() holds an mmgrab() reference across its work. It is
redundant. Every caller already holds mm_users:

 - madvise(2) works on current->mm, which lives as long as the task is in
   the syscall;
 - process_madvise(2) reaches a remote mm through mm_access(), which takes
   an mm_users reference and holds it until the syscall returns;
 - io_uring passes current->mm;
 - DAMON takes one with get_task_mm() and drops it after the call.

Drop the mmgrab()/mmdrop() pair.

Assisted-by: LLM
Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>
Reviewed-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
Signed-off-by: Kiryl Shutsemau (Meta) <kas@xxxxxxxxxx>
---
 mm/khugepaged.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index e13d233b9967..75c876acf527 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -3229,7 +3229,6 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
 	cc->is_khugepaged = false;
 	cc->progress = 0;
 
-	mmgrab(mm);
 	lru_add_drain_all();
 
 	for (addr = hstart; addr < hend; addr += HPAGE_PMD_SIZE) {
@@ -3285,7 +3284,6 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
 	}
 out_nolock:
 	mmap_assert_locked(mm);
-	mmdrop(mm);
 	kfree(cc);
 
 	return thps == ((hend - hstart) >> HPAGE_PMD_SHIFT) ? 0
-- 
2.54.0