[RFC v9 PATCH 4/4] mm: unmap VM_PFNMAP mappings with optimized path

[Yang Shi <yang.shi@linux.alibaba.com>]

When unmapping VM_PFNMAP mappings, vm flags need to be updated. Since
the vmas have been detached, so it sounds safe to update vm flags with
read mmap_sem.

Cc: Michal Hocko <mhocko@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
---
 mm/mmap.c | 15 +--------------
 1 file changed, 1 insertion(+), 14 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index 086f8b5..0b6b231 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2778,7 +2778,7 @@ static int do_munmap_zap_rlock(struct mm_struct *mm, unsigned long start,
 			       size_t len, struct list_head *uf)
 {
 	unsigned long end;
-	struct vm_area_struct *start_vma, *prev, *vma;
+	struct vm_area_struct *start_vma, *prev;
 	int ret = 0;
 
 	if (!addr_ok(start, len))
@@ -2811,16 +2811,6 @@ static int do_munmap_zap_rlock(struct mm_struct *mm, unsigned long start,
 			goto out;
 	}
 
-	/*
-	 * Unmapping vmas, which have VM_PFNMAP
-	 * need get done with write mmap_sem held since they may update
-	 * vm_flags. Deal with such mappings with regular do_munmap() call.
-	 */
-	for (vma = start_vma; vma && vma->vm_start < end; vma = vma->vm_next) {
-		if (vma->vm_flags & VM_PFNMAP)
-			goto regular_path;
-	}
-
 	/* Handle mlocked vmas */
 	if (mm->locked_vm)
 		munlock_vmas(start_vma, end);
@@ -2844,9 +2834,6 @@ static int do_munmap_zap_rlock(struct mm_struct *mm, unsigned long start,
 
 	return 0;
 
-regular_path:
-	ret = do_munmap(mm, start, len, uf);
-
 out:
 	up_write(&mm->mmap_sem);
 	return ret;
-- 
1.8.3.1