[PATCH v3 3/7] proc/task_mmu: clarify shmem mapping walk conditions in smap_gather_stats()

[Suren Baghdasaryan <surenb@google.com>]

smap_gather_stats() optimizes stats gathering by skipping the walk for
shmem mappings in certain conditions. Update the comment to clarify
these conditions and use vma_is_cow_mapping() for COW identification
instead of open-coding it.
Instead of using (start != 0) condition to identify partial walks, use
more semantically correct (start > vma->vm_start) check.

No functional change intended.

Suggested by: David Hildenbrand (Arm) <david@kernel.org>
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
 fs/proc/task_mmu.c | 24 ++++++++++--------------
 1 file changed, 10 insertions(+), 14 deletions(-)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index cfc7af1b551d..3c40c9cbb9c9 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -1257,6 +1257,7 @@ static void smap_gather_stats(struct proc_maps_private *priv,
 			      struct mem_size_stats *mss, unsigned long start)
 {
 	const struct mm_walk_ops *ops = get_smaps_walk_ops(priv);
+	const bool is_partial = start > vma->vm_start;
 
 	/* Invalid start */
 	if (start >= vma->vm_end)
@@ -1270,23 +1271,18 @@ static void smap_gather_stats(struct proc_maps_private *priv,
 
 	if (vma->vm_file && shmem_mapping(vma->vm_file->f_mapping)) {
 		/*
-		 * For shared or readonly shmem mappings we know that all
-		 * swapped out pages belong to the shmem object, and we can
-		 * obtain the swap value much more efficiently. For private
-		 * writable mappings, we might have COW pages that are
-		 * not affected by the parent swapped out pages of the shmem
-		 * object, so we have to distinguish them during the page walk.
-		 * Unless we know that the shmem object (or the part mapped by
-		 * our VMA) has no swapped out pages at all.
+		 * CoW mappings might map anon folios that do not belong to
+		 * shmem. Perform a less efficient page table walk in this
+		 * situation, unless we know that the shmem object (or the
+		 * part mapped by our VMA) has no swapped out pages at all.
 		 */
-		unsigned long shmem_swapped = shmem_swap_usage(vma);
+		const unsigned long shmem_swapped = shmem_swap_usage(vma);
+		const bool is_cow = vma_is_cow_mapping(vma);
 
-		if (!start && (!shmem_swapped || (vma->vm_flags & VM_SHARED) ||
-					!(vma->vm_flags & VM_WRITE))) {
-			mss->swap += shmem_swapped;
-		} else {
+		if (is_partial || (shmem_swapped && is_cow))
 			ops = get_smaps_shmem_walk_ops(priv);
-		}
+		else
+			mss->swap += shmem_swapped;
 	}
 
 	if (!start)
-- 
2.55.0.1007.g17ff1f9808-goog