Re: [PATCH v2 0/6] mm: make MAP_PRIVATE-/dev/zero mappings truly anonymous

[Andrew Morton]

On Tue, 08 Sep 2026 12:23:37 +0100 "Lorenzo Stoakes (ARM)" <ljs@xxxxxxxxxx> wrote:

> Historically anonymous memory was obtained in linux by MAP_PRIVATE-mapping
> /dev/zero.
> 
> The canonical way of doing these now is mmap() specifying MAP_PRIVATE |
> MAP_ANON, but we must continue to support the legacy means of obtaining these
> mappings.
> 
> As-is these mappings are an unusual edge-case - they satisfy
> vma_is_anonymous() but have non-NULL vma->vm_file, and their page offset is
> the offset into the /dev/zero file.
> 
> Commit 93c0c8dc87f6 ("mm/rmap: use anon pgoff to track MAP_PRIVATE
> file-backed anon folios") causes all other anonymous folios to be tracked
> by their anon index (vma->vm_start >> PAGE_SHIFT at the point of first
> fault), leaving MAP_PRIVATE-/dev/zero as the outlier.
> 
> This series remedies the situation by making MAP_PRIVATE-/dev/zero mappings
> truly anonymous with !vma->vm_file and correct anonymous page offset.
> 
> It starts by bringing the memory character driver into mm/ - this file
> implements /dev/zero, /dev/mem among other things and is already (as
> clearly indicated by its name) within the remit of memory management.
> 
> By doing this, the file_is_dev_zero() function can be provided, internal to
> mm, which allows for positive identification of these mappings.
> 
> Using this, first prevent any other mappings from mapping memory
> anonymously, then make these mappings truly anonymous and eliminate all
> code in the kernel that previously had to account for these strange beasts.
> 
> Finally, it adds userland VMA tests to assert the behaviour and selftests
> to assert expected merge behaviour.

Thanks, I've updated mm-unstable to this version.

> v2:
> * Added tags (thanks everybody!)
> * Reworked comment about mappings not setting themselves anon as per David.
> * Fixed up test typo as per David.
> * Fixed up test close() as per David.

Here's how v2 altered mm.git:

--- a/mm/vma.c~b
+++ a/mm/vma.c
@@ -2801,7 +2801,7 @@ static int call_mmap_prepare(struct mmap
 	if (err)
 		return err;
 
-	/* Hooks cannot mark themselves anonymous. */
+	/* It's invalid for mmap_preprare hooks to clear vm_ops. */
 	if (!desc->vm_ops)
 		return -EINVAL;
 
--- a/tools/testing/selftests/mm/merge.c~b
+++ a/tools/testing/selftests/mm/merge.c
@@ -1381,7 +1381,7 @@ TEST_F(merge, merge_map_private_dev_zero
 	 *
 	 * With these being made truly anonymous upon mapping, they will
 	 * merge. If they were file-backed VMAs the page offsets would prevent
-	 * merge:
+	 * the merge:
 	 *
 	 * |-----||------|    |-------------|
 	 * | ptr || ptr2 | -> |     ptr     |
@@ -1389,17 +1389,11 @@ TEST_F(merge, merge_map_private_dev_zero
 	 */
 	ptr = mmap(carveout, 5 * page_size, PROT_READ | PROT_WRITE,
 		   MAP_FIXED | MAP_PRIVATE, fd_zero, 0);
-	if (ptr == MAP_FAILED) {
-		close(fd_zero);
-		ASSERT_TRUE(false);
-	}
 	ptr2 = mmap(&carveout[5 * page_size], 5 * page_size,
 		   PROT_READ | PROT_WRITE, MAP_FIXED | MAP_PRIVATE, fd_zero, 0);
-	if (ptr2 == MAP_FAILED) {
-		close(fd_zero);
-		ASSERT_TRUE(false);
-	}
 	close(fd_zero);
+	ASSERT_NE(ptr, MAP_FAILED);
+	ASSERT_NE(ptr2, MAP_FAILED);
 
 	/* Assert that they merged. */
 	ASSERT_TRUE(find_vma_procmap(procmap, ptr));
@@ -1430,10 +1424,7 @@ TEST_F(merge, merge_map_private_dev_zero
 	 */
 	ptr = mmap(carveout, 15 * page_size, PROT_READ | PROT_WRITE,
 		   MAP_FIXED | MAP_PRIVATE, fd_zero, 0);
-	if (ptr == MAP_FAILED) {
-		close(fd_zero);
-		ASSERT_TRUE(false);
-	}
+	ASSERT_NE(ptr, MAP_FAILED);
 	memset(ptr, 'x', 15 * page_size);
 
 	/*
_