[PATCH mm-new v3] mm: mincore: use per-vma lock during page table walk

[Kefeng Wang <wangkefeng.wang@huawei.com>]

do_mincore() performs a read-only, per-VMA residency query,
making it a good candidate for per-VMA locking. Convert it
to acquire the per-VMA lock, thereby reducing contention on
the per-MM mmap_lock.

Reviewed-by: Pedro Falcato <pfalcato@suse.de>
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
---
v3:
- add vma_assert_locked, update changelog/comment, per David
- Add RB 
v2, (RESEND):
- using new vma_start_read_unlocked() API, suggestted by Pedro Falcato
v1:
- https://lore.kernel.org/linux-mm/20260701144047.3786939-2-wangkefeng.wang@huawei.com/

 mm/mincore.c | 29 +++++++++++++++++------------
 1 file changed, 17 insertions(+), 12 deletions(-)

diff --git a/mm/mincore.c b/mm/mincore.c
index c086836bc4bc..0fe50f8a7e62 100644
--- a/mm/mincore.c
+++ b/mm/mincore.c
@@ -235,24 +235,22 @@ static const struct mm_walk_ops mincore_walk_ops = {
 	.pmd_entry		= mincore_pte_range,
 	.pte_hole		= mincore_unmapped_range,
 	.hugetlb_entry		= mincore_hugetlb,
-	.walk_lock		= PGWALK_RDLOCK,
+	.walk_lock		= PGWALK_VMA_RDLOCK_VERIFY,
 };
 
 /*
  * Do a chunk of "sys_mincore()". We've already checked
- * all the arguments, we hold the mmap semaphore: we should
+ * all the arguments, we hold the VMA read lock: we should
  * just return the amount of info we're asked for.
  */
-static long do_mincore(unsigned long addr, unsigned long pages, unsigned char *vec)
+static long do_mincore(struct vm_area_struct *vma, unsigned long addr,
+		unsigned long pages, unsigned char *vec)
 {
-	struct vm_area_struct *vma;
-	unsigned long end;
+	unsigned long end = min(vma->vm_end, addr + (pages << PAGE_SHIFT));
 	int err;
 
-	vma = vma_lookup(current->mm, addr);
-	if (!vma)
-		return -ENOMEM;
-	end = min(vma->vm_end, addr + (pages << PAGE_SHIFT));
+	vma_assert_locked(vma);
+
 	if (!can_do_mincore(vma)) {
 		unsigned long pages = DIV_ROUND_UP(end - addr, PAGE_SIZE);
 		memset(vec, 1, pages);
@@ -319,13 +317,20 @@ SYSCALL_DEFINE3(mincore, unsigned long, start, size_t, len,
 
 	retval = 0;
 	while (pages) {
+		struct vm_area_struct *vma;
+
+		vma = vma_start_read_unlocked(current->mm, start);
+		if (!vma) {
+			retval = -ENOMEM;
+			break;
+		}
+
 		/*
 		 * Do at most PAGE_SIZE entries per iteration, due to
 		 * the temporary buffer size.
 		 */
-		mmap_read_lock(current->mm);
-		retval = do_mincore(start, min(pages, PAGE_SIZE), tmp);
-		mmap_read_unlock(current->mm);
+		retval = do_mincore(vma, start, min(pages, PAGE_SIZE), tmp);
+		vma_end_read(vma);
 
 		if (retval <= 0)
 			break;
-- 
2.55.0