[PATCH v6 0/6] mm/vmalloc: free unused pages on vrealloc() shrink

[PATCH v6 0/6] mm/vmalloc: free unused pages on vrealloc() shrink

[Shivam Kalra via B4 Relay]

This series implements the TODO in vrealloc() to unmap and free unused
pages when shrinking across a page boundary.

Problem:
When vrealloc() shrinks an allocation, it updates bookkeeping
(requested_size, KASAN shadow) but does not free the underlying physical
pages. This wastes memory for the lifetime of the allocation.

Solution:
- Patch 1: Extracts a vm_area_free_pages(vm, start_idx, end_idx) helper
  from vfree() that frees a range of pages with memcg and nr_vmalloc_pages
  accounting. Freed page pointers are set to NULL to prevent stale
  references.
- Patch 2: Fixes the grow-in-place path to check vm->nr_pages instead
  of get_vm_area_size(), which reflects the virtual reservation and does
  not change on shrink. This is a prerequisite for shrinking.
- Patch 3: Zeros newly exposed memory on vrealloc() grow if __GFP_ZERO
  is requested, preventing stale data leaks from previously shrunk regions.
- Patch 4: Protects /proc/vmallocinfo readers with READ_ONCE() to safely
  handle concurrent decreases to vm->nr_pages and NULL page pointers.
- Patch 5: Uses the helper to free tail pages when vrealloc() shrinks
  across a page boundary. Skips huge page allocations, VM_FLUSH_RESET_PERMS,
  and VM_USERMAP. Updates Kmemleak tracking of the allocation.
- Patch 6: Adds a vrealloc test case to lib/test_vmalloc that exercises
  grow-realloc, shrink-across-boundary, shrink-within-page, and
  grow-in-place paths.

The virtual address reservation is kept intact to preserve the range
for potential future grow-in-place support.
A concrete user is the Rust binder driver's KVVec::shrink_to [1], which
performs explicit vrealloc() shrinks for memory reclamation.

Tested:
- KASAN KUnit (vmalloc_oob passes)
- lib/test_vmalloc stress tests (3/3, 1M iterations each)
- checkpatch, sparse, W=1, allmodconfig, coccicheck clean

[1] https://lore.kernel.org/all/20260216-binder-shrink-vec-v3-v6-0-ece8e8593e53@zohomail.in/

Signed-off-by: Shivam Kalra <shivamkalra98@zohomail.in>
---
Changes in v6:
- Fix VM_USERMAP crash by explicitly bypassing early in the shrink path if the flag is set.(Sashiko)
- Fix Kmemleak scanner panic by calling kmemleak_free_part() to update tracking on shrink.(Sashiko)
- Fix /proc/vmallocinfo race condition by protecting vm->nr_pages access with 
  READ_ONCE()/WRITE_ONCE() for concurrent readers.(Sashiko)
- Fix stale data leak on grow-after-shrink by enforcing mandatory zeroing of the newly exposed memory.(Sashiko)
- Fix memory leaks in vrealloc_test() by using a temporary pointer to preserve and 
  free the original allocation upon failure.(Sashiko)
- Rename vmalloc_free_pages parameters from start/end to start_idx/end_idx for better clarity.(Uladzislau Rezki)
- Link to v5: https://lore.kernel.org/r/20260317-vmalloc-shrink-v5-0-bbfbf54c5265@zohomail.in
- Link to Sashiko: https://sashiko.dev/#/patchset/20260317-vmalloc-shrink-v5-0-bbfbf54c5265%40zohomail.in

Changes in v5:
- Skip vrealloc shrink for VM_FLUSH_RESET_PERMS (Uladzislau Rezki)
- Link to v4: https://lore.kernel.org/r/20260314-vmalloc-shrink-v4-0-c1e2e0bb5455@zohomail.in

Changes in v4:
- Rename vmalloc_free_pages() to vm_area_free_pages() to align with
  vm_area_alloc_pages() (Uladzislau Rezki)
- NULL out freed vm->pages[] entries to prevent stale pointers (Alice Ryhl)
- Remove redundant if (vm->nr_pages) guard in vfree() (Uladzislau Rezki)
- Add vrealloc test case to lib/test_vmalloc (new patch 3/3)
- Link to v3: https://lore.kernel.org/r/20260309-vmalloc-shrink-v3-0-5590fd8de2eb@zohomail.in

Changes in v3:
- Restore the comment.
- Rebase to the latest mm-new 
- Link to v2: https://lore.kernel.org/r/20260304-vmalloc-shrink-v2-0-28c291d60100@zohomail.in

Changes in v2:
- Updated the base-commit to mm-new
- Fix conflicts after rebase
- Ran `clang-format` on the changes made
- Use a single `kasan_vrealloc` (Alice Ryhl)
- Link to v1: https://lore.kernel.org/r/20260302-vmalloc-shrink-v1-0-46deff465b7e@zohomail.in

---
Shivam Kalra (6):
      mm/vmalloc: extract vm_area_free_pages() helper from vfree()
      mm/vmalloc: fix vrealloc() grow-in-place check
      mm/vmalloc: zero newly exposed memory on vrealloc() grow
      mm/vmalloc: protect /proc/vmallocinfo readers with READ_ONCE()
      mm/vmalloc: free unused pages on vrealloc() shrink
      lib/test_vmalloc: add vrealloc test case

 lib/test_vmalloc.c |  62 +++++++++++++++++++++++++++
 mm/vmalloc.c       | 121 ++++++++++++++++++++++++++++++++++++++++-------------
 2 files changed, 154 insertions(+), 29 deletions(-)
---
base-commit: 3b9bd243689fa0ab95062ab53f6c5aff90272056
change-id: 20260302-vmalloc-shrink-04b2fa688a14

Best regards,
-- 
Shivam Kalra <shivamkalra98@zohomail.in>

[PATCH v6 1/6] mm/vmalloc: extract vm_area_free_pages() helper from vfree()

[Shivam Kalra via B4 Relay]

From: Shivam Kalra <shivamkalra98@zohomail.in>

Extract the page-freeing loop and NR_VMALLOC stat accounting from
vfree() into a reusable vm_area_free_pages() helper. The helper operates
on a range [start_idx, end_idx) of pages from a vm_struct, making it
suitable for both full free (vfree) and partial free (upcoming vrealloc
shrink).

Freed page pointers in vm->pages[] are set to NULL to prevent stale
references when the vm_struct outlives the free (as in vrealloc shrink).

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Shivam Kalra <shivamkalra98@zohomail.in>
---
 mm/vmalloc.c | 47 +++++++++++++++++++++++++++++++++--------------
 1 file changed, 33 insertions(+), 14 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index c607307c657a..3977398e56e4 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -3416,6 +3416,38 @@ void vfree_atomic(const void *addr)
 		schedule_work(&p->wq);
 }
 
+/*
+ * vm_area_free_pages - free a range of pages from a vmalloc allocation
+ * @vm: the vm_struct containing the pages
+ * @start_idx: first page index to free (inclusive)
+ * @end_idx: last page index to free (exclusive)
+ *
+ * Free pages [start_idx, end_idx) updating NR_VMALLOC stat accounting.
+ * Freed vm->pages[] entries are set to NULL.
+ * Caller is responsible for unmapping (vunmap_range) and KASAN
+ * poisoning before calling this.
+ */
+static void vm_area_free_pages(struct vm_struct *vm, unsigned int start_idx,
+			       unsigned int end_idx)
+{
+	unsigned int i;
+
+	for (i = start_idx; i < end_idx; i++) {
+		struct page *page = vm->pages[i];
+
+		BUG_ON(!page);
+		/*
+		 * High-order allocs for huge vmallocs are split, so
+		 * can be freed as an array of order-0 allocations
+		 */
+		if (!(vm->flags & VM_MAP_PUT_PAGES))
+			mod_lruvec_page_state(page, NR_VMALLOC, -1);
+		__free_page(page);
+		vm->pages[i] = NULL;
+		cond_resched();
+	}
+}
+
 /**
  * vfree - Release memory allocated by vmalloc()
  * @addr:  Memory base address
@@ -3436,7 +3468,6 @@ void vfree_atomic(const void *addr)
 void vfree(const void *addr)
 {
 	struct vm_struct *vm;
-	int i;
 
 	if (unlikely(in_interrupt())) {
 		vfree_atomic(addr);
@@ -3459,19 +3490,7 @@ void vfree(const void *addr)
 
 	if (unlikely(vm->flags & VM_FLUSH_RESET_PERMS))
 		vm_reset_perms(vm);
-	for (i = 0; i < vm->nr_pages; i++) {
-		struct page *page = vm->pages[i];
-
-		BUG_ON(!page);
-		/*
-		 * High-order allocs for huge vmallocs are split, so
-		 * can be freed as an array of order-0 allocations
-		 */
-		if (!(vm->flags & VM_MAP_PUT_PAGES))
-			mod_lruvec_page_state(page, NR_VMALLOC, -1);
-		__free_page(page);
-		cond_resched();
-	}
+	vm_area_free_pages(vm, 0, vm->nr_pages);
 	kvfree(vm->pages);
 	kfree(vm);
 }

-- 
2.43.0

[PATCH v6 2/6] mm/vmalloc: fix vrealloc() grow-in-place check

[Shivam Kalra via B4 Relay]

From: Shivam Kalra <shivamkalra98@zohomail.in>

Fix the grow-in-place check in vrealloc() to compare the requested size
against the actual physical page count (vm->nr_pages) rather than the
virtual area size (alloced_size, derived from get_vm_area_size()).

The virtual reservation size (get_vm_area_size()) does not decrease when
pages are freed during a shrink operation. Consequently, without this fix,
a subsequent grow-in-place operation after a shrink would incorrectly
succeed and attempt to access freed pages. Correcting this check is a
prerequisite for the upcoming vrealloc() shrink functionality.

Signed-off-by: Shivam Kalra <shivamkalra98@zohomail.in>
---
 mm/vmalloc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 3977398e56e4..7bc7a6892c1a 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -4361,7 +4361,7 @@ void *vrealloc_node_align_noprof(const void *p, size_t size, unsigned long align
 	/*
 	 * We already have the bytes available in the allocation; use them.
 	 */
-	if (size <= alloced_size) {
+	if (size <= (size_t)vm->nr_pages << PAGE_SHIFT) {
 		/*
 		 * No need to zero memory here, as unused memory will have
 		 * already been zeroed at initial allocation time or during

-- 
2.43.0

[PATCH v6 3/6] mm/vmalloc: zero newly exposed memory on vrealloc() grow

[Shivam Kalra via B4 Relay]

From: Shivam Kalra <shivamkalra98@zohomail.in>

When growing an existing vmalloc allocation in-place, zero the newly
exposed memory region [old_size, size) if the caller requested it via
__GFP_ZERO (checked via want_init_on_alloc(flags)).

Previously, the code assumed that the unused capacity in the vm_struct
was already zeroed either at initial allocation time or during a prior
shrink. However, if an intermediate shrink operation occurred without
__GFP_ZERO and without init_on_free enabled, the "freed" portion of the
allocation would retain its old data.

If a subsequent grow-in-place operation then explicitly requests
__GFP_ZERO, failing to zero the memory here would violate the allocation
flags and leak the previously discarded, potentially sensitive data.

Signed-off-by: Shivam Kalra <shivamkalra98@zohomail.in>
---
 mm/vmalloc.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 7bc7a6892c1a..64f5d1088281 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -4362,13 +4362,16 @@ void *vrealloc_node_align_noprof(const void *p, size_t size, unsigned long align
 	 * We already have the bytes available in the allocation; use them.
 	 */
 	if (size <= (size_t)vm->nr_pages << PAGE_SHIFT) {
-		/*
-		 * No need to zero memory here, as unused memory will have
-		 * already been zeroed at initial allocation time or during
-		 * realloc shrink time.
-		 */
 		vm->requested_size = size;
 		kasan_vrealloc(p, old_size, size);
+
+		/*
+		 * Zero the newly exposed bytes if requested.
+		 * The region [old_size, size) may contain stale data from
+		 * a previous shrink that did not use __GFP_ZERO.
+		 */
+		if (want_init_on_alloc(flags))
+			memset((void *)p + old_size, 0, size - old_size);
 		return (void *)p;
 	}
 

-- 
2.43.0

[PATCH v6 4/6] mm/vmalloc: protect /proc/vmallocinfo readers with READ_ONCE()

[Shivam Kalra via B4 Relay]

From: Shivam Kalra <shivamkalra98@zohomail.in>

The /proc/vmallocinfo readers, specifically show_numa_info() and
vmalloc_info_show(), currently read v->nr_pages and the v->pages
array without any concurrent protection.

In preparation for vrealloc() shrink support, where v->nr_pages can
be decreased and entries in the v->pages array can be nulled out
concurrently, these readers must be protected to prevent use-after-free
or NULL pointer dereferences.

Update show_numa_info() to use READ_ONCE(v->nr_pages) and
READ_ONCE(v->pages[nr]), explicitly checking for NULL before
dereferencing the page. Similarly, update vmalloc_info_show() to
read nr_pages safely to avoid parsing a torn or inconsistent value.

Signed-off-by: Shivam Kalra <shivamkalra98@zohomail.in>
---
 mm/vmalloc.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 64f5d1088281..7658fdc087d2 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -5204,7 +5204,7 @@ bool vmalloc_dump_obj(void *object)
 static void show_numa_info(struct seq_file *m, struct vm_struct *v,
 				 unsigned int *counters)
 {
-	unsigned int nr;
+	unsigned int nr, nr_pages;
 	unsigned int step = 1U << vm_area_page_order(v);
 
 	if (!counters)
@@ -5212,8 +5212,13 @@ static void show_numa_info(struct seq_file *m, struct vm_struct *v,
 
 	memset(counters, 0, nr_node_ids * sizeof(unsigned int));
 
-	for (nr = 0; nr < v->nr_pages; nr += step)
-		counters[page_to_nid(v->pages[nr])] += step;
+	nr_pages = READ_ONCE(v->nr_pages);
+	for (nr = 0; nr < nr_pages; nr += step) {
+		struct page *page = READ_ONCE(v->pages[nr]);
+
+		if (page)
+			counters[page_to_nid(page)] += step;
+	}
 	for_each_node_state(nr, N_HIGH_MEMORY)
 		if (counters[nr])
 			seq_printf(m, " N%u=%u", nr, counters[nr]);
@@ -5241,6 +5246,7 @@ static int vmalloc_info_show(struct seq_file *m, void *p)
 	struct vmap_area *va;
 	struct vm_struct *v;
 	unsigned int *counters;
+	unsigned int nr_pages;
 
 	if (IS_ENABLED(CONFIG_NUMA))
 		counters = kmalloc_array(nr_node_ids, sizeof(unsigned int), GFP_KERNEL);
@@ -5270,8 +5276,9 @@ static int vmalloc_info_show(struct seq_file *m, void *p)
 			if (v->caller)
 				seq_printf(m, " %pS", v->caller);
 
-			if (v->nr_pages)
-				seq_printf(m, " pages=%d", v->nr_pages);
+			nr_pages = READ_ONCE(v->nr_pages);
+			if (nr_pages)
+				seq_printf(m, " pages=%d", nr_pages);
 
 			if (v->phys_addr)
 				seq_printf(m, " phys=%pa", &v->phys_addr);

-- 
2.43.0

[PATCH v6 5/6] mm/vmalloc: free unused pages on vrealloc() shrink

[Shivam Kalra via B4 Relay]

From: Shivam Kalra <shivamkalra98@zohomail.in>

When vrealloc() shrinks an allocation and the new size crosses a page
boundary, unmap and free the tail pages that are no longer needed. This
reclaims physical memory that was previously wasted for the lifetime
of the allocation.

The heuristic is simple: always free when at least one full page becomes
unused. Huge page allocations (page_order > 0) are skipped, as partial
freeing would require splitting. Allocations with VM_FLUSH_RESET_PERMS
are also skipped, as their direct-map permissions must be reset before
pages are returned to the page allocator, which is handled by
vm_reset_perms() during vfree().

Additionally, allocations with VM_USERMAP are skipped because
remap_vmalloc_range_partial() validates mapping requests against the
unchanged vm->size; freeing tail pages would cause vmalloc_to_page()
to return NULL for the unmapped range.

To protect concurrent /proc/vmallocinfo readers, the shrink path
updates vm->nr_pages with WRITE_ONCE() before freeing the pages.

Finally, we notify kmemleak of the reduced allocation size using
kmemleak_free_part() to prevent the kmemleak scanner from faulting on
the newly unmapped virtual addresses.

The virtual address reservation (vm->size / vmap_area) is intentionally
kept unchanged, preserving the address for potential future grow-in-place
support.

Suggested-by: Danilo Krummrich <dakr@kernel.org>
Signed-off-by: Shivam Kalra <shivamkalra98@zohomail.in>
---
 mm/vmalloc.c | 42 ++++++++++++++++++++++++++++++++++++++----
 1 file changed, 38 insertions(+), 4 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 7658fdc087d2..8b8752fc10e9 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -4345,14 +4345,48 @@ void *vrealloc_node_align_noprof(const void *p, size_t size, unsigned long align
 			goto need_realloc;
 	}
 
-	/*
-	 * TODO: Shrink the vm_area, i.e. unmap and free unused pages. What
-	 * would be a good heuristic for when to shrink the vm_area?
-	 */
 	if (size <= old_size) {
+		unsigned int new_nr_pages = PAGE_ALIGN(size) >> PAGE_SHIFT;
+
 		/* Zero out "freed" memory, potentially for future realloc. */
 		if (want_init_on_free() || want_init_on_alloc(flags))
 			memset((void *)p + size, 0, old_size - size);
+
+		/*
+		 * Free tail pages when shrink crosses a page boundary.
+		 *
+		 * Skip huge page allocations (page_order > 0) as partial
+		 * freeing would require splitting.
+		 *
+		 * Skip VM_FLUSH_RESET_PERMS, as direct-map permissions must
+		 * be reset before pages are returned to the allocator.
+		 *
+		 * Skip VM_USERMAP, as remap_vmalloc_range_partial() validates
+		 * mapping requests against the unchanged vm->size; freeing
+		 * tail pages would cause vmalloc_to_page() to return NULL for
+		 * the unmapped range.
+		 */
+		if (new_nr_pages < vm->nr_pages && !vm_area_page_order(vm) &&
+		    !(vm->flags & (VM_FLUSH_RESET_PERMS | VM_USERMAP))) {
+			unsigned long addr = (unsigned long)p;
+			unsigned int old_nr_pages = vm->nr_pages;
+
+			/* Notify kmemleak of the reduced allocation size before unmapping. */
+			kmemleak_free_part(
+				(void *)addr + (new_nr_pages << PAGE_SHIFT),
+				(old_nr_pages - new_nr_pages) << PAGE_SHIFT);
+
+			vunmap_range(addr + (new_nr_pages << PAGE_SHIFT),
+				     addr + (old_nr_pages << PAGE_SHIFT));
+
+			/*
+			 * Update nr_pages before freeing pages to prevent
+			 * a concurrent reader (e.g. show_numa_info via
+			 * /proc/vmallocinfo) from accessing NULL entries.
+			 */
+			WRITE_ONCE(vm->nr_pages, new_nr_pages);
+			vm_area_free_pages(vm, new_nr_pages, old_nr_pages);
+		}
 		vm->requested_size = size;
 		kasan_vrealloc(p, old_size, size);
 		return (void *)p;

-- 
2.43.0

[PATCH v6 6/6] lib/test_vmalloc: add vrealloc test case

[Shivam Kalra via B4 Relay]

From: Shivam Kalra <shivamkalra98@zohomail.in>

Introduce a new test case "vrealloc_test" that exercises the vrealloc()
shrink and in-place grow paths:

  - Grow beyond allocated pages (triggers full reallocation).
  - Shrink crossing a page boundary (frees tail pages).
  - Shrink within the same page (no page freeing).
  - Grow within the already allocated page count (in-place).

Data integrity is validated after each realloc step by checking that
the first byte of the original allocation is preserved.

The test is gated behind run_test_mask bit 12 (id 4096).

Signed-off-by: Shivam Kalra <shivamkalra98@zohomail.in>
---
 lib/test_vmalloc.c | 62 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 62 insertions(+)

diff --git a/lib/test_vmalloc.c b/lib/test_vmalloc.c
index 876c72c18a0c..e822830f0887 100644
--- a/lib/test_vmalloc.c
+++ b/lib/test_vmalloc.c
@@ -55,6 +55,7 @@ __param(int, run_test_mask, 7,
 		"\t\tid: 512,  name: kvfree_rcu_2_arg_vmalloc_test\n"
 		"\t\tid: 1024, name: vm_map_ram_test\n"
 		"\t\tid: 2048, name: no_block_alloc_test\n"
+		"\t\tid: 4096, name: vrealloc_test\n"
 		/* Add a new test case description here. */
 );
 
@@ -421,6 +422,66 @@ vm_map_ram_test(void)
 	return nr_allocated != map_nr_pages;
 }
 
+static int vrealloc_test(void)
+{
+	void *ptr, *tmp;
+	int i;
+
+	for (i = 0; i < test_loop_count; i++) {
+		int err = -1;
+
+		ptr = vmalloc(PAGE_SIZE);
+		if (!ptr)
+			return -1;
+
+		*((__u8 *)ptr) = 'a';
+
+		/* Grow: beyond allocated pages, triggers full realloc. */
+		tmp = vrealloc(ptr, 4 * PAGE_SIZE, GFP_KERNEL);
+		if (!tmp)
+			goto error;
+		ptr = tmp;
+
+		if (*((__u8 *)ptr) != 'a')
+			goto error;
+
+		/* Shrink: crosses page boundary, frees tail pages. */
+		tmp = vrealloc(ptr, PAGE_SIZE, GFP_KERNEL);
+		if (!tmp)
+			goto error;
+		ptr = tmp;
+
+		if (*((__u8 *)ptr) != 'a')
+			goto error;
+
+		/* Shrink: within same page, no page freeing. */
+		tmp = vrealloc(ptr, PAGE_SIZE / 2, GFP_KERNEL);
+		if (!tmp)
+			goto error;
+		ptr = tmp;
+
+		if (*((__u8 *)ptr) != 'a')
+			goto error;
+
+		/* Grow: within allocated page, in-place, no realloc. */
+		tmp = vrealloc(ptr, PAGE_SIZE, GFP_KERNEL);
+		if (!tmp)
+			goto error;
+		ptr = tmp;
+
+		if (*((__u8 *)ptr) != 'a')
+			goto error;
+
+		err = 0;
+error:
+		vfree(ptr);
+		if (err)
+			return err;
+	}
+
+	return 0;
+}
+
 struct test_case_desc {
 	const char *test_name;
 	int (*test_func)(void);
@@ -440,6 +501,7 @@ static struct test_case_desc test_case_array[] = {
 	{ "kvfree_rcu_2_arg_vmalloc_test", kvfree_rcu_2_arg_vmalloc_test, },
 	{ "vm_map_ram_test", vm_map_ram_test, },
 	{ "no_block_alloc_test", no_block_alloc_test, true },
+	{ "vrealloc_test", vrealloc_test, },
 	/* Add a new test case here. */
 };
 

-- 
2.43.0

Re: [PATCH v6 0/6] mm/vmalloc: free unused pages on vrealloc() shrink

[Andrew Morton]

On Sat, 21 Mar 2026 23:35:45 +0530 Shivam Kalra via B4 Relay <devnull+shivamkalra98.zohomail.in@kernel.org> wrote:

> This series implements the TODO in vrealloc() to unmap and free unused
> pages when shrinking across a page boundary.

AI review claims to have found several issues:
	https://sashiko.dev/#/patchset/20260321-vmalloc-shrink-v6-0-062ca7b7ceb2@zohomail.in

Re: [PATCH v6 4/6] mm/vmalloc: protect /proc/vmallocinfo readers with READ_ONCE()

[Uladzislau Rezki]

On Sat, Mar 21, 2026 at 11:35:49PM +0530, Shivam Kalra via B4 Relay wrote:
> From: Shivam Kalra <shivamkalra98@zohomail.in>
> 
> The /proc/vmallocinfo readers, specifically show_numa_info() and
> vmalloc_info_show(), currently read v->nr_pages and the v->pages
> array without any concurrent protection.
> 
> In preparation for vrealloc() shrink support, where v->nr_pages can
> be decreased and entries in the v->pages array can be nulled out
> concurrently, these readers must be protected to prevent use-after-free
> or NULL pointer dereferences.
> 
> Update show_numa_info() to use READ_ONCE(v->nr_pages) and
> READ_ONCE(v->pages[nr]), explicitly checking for NULL before
> dereferencing the page. Similarly, update vmalloc_info_show() to
> read nr_pages safely to avoid parsing a torn or inconsistent value.
> 
> Signed-off-by: Shivam Kalra <shivamkalra98@zohomail.in>
> ---
>  mm/vmalloc.c | 17 ++++++++++++-----
>  1 file changed, 12 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 64f5d1088281..7658fdc087d2 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -5204,7 +5204,7 @@ bool vmalloc_dump_obj(void *object)
>  static void show_numa_info(struct seq_file *m, struct vm_struct *v,
>  				 unsigned int *counters)
>  {
> -	unsigned int nr;
> +	unsigned int nr, nr_pages;
>  	unsigned int step = 1U << vm_area_page_order(v);
>  
>  	if (!counters)
> @@ -5212,8 +5212,13 @@ static void show_numa_info(struct seq_file *m, struct vm_struct *v,
>  
>  	memset(counters, 0, nr_node_ids * sizeof(unsigned int));
>  
> -	for (nr = 0; nr < v->nr_pages; nr += step)
> -		counters[page_to_nid(v->pages[nr])] += step;
> +	nr_pages = READ_ONCE(v->nr_pages);
> +	for (nr = 0; nr < nr_pages; nr += step) {
> +		struct page *page = READ_ONCE(v->pages[nr]);
> +
> +		if (page)
> +			counters[page_to_nid(page)] += step;
> +	}
>  	for_each_node_state(nr, N_HIGH_MEMORY)
>  		if (counters[nr])
>  			seq_printf(m, " N%u=%u", nr, counters[nr]);
> @@ -5241,6 +5246,7 @@ static int vmalloc_info_show(struct seq_file *m, void *p)
>  	struct vmap_area *va;
>  	struct vm_struct *v;
>  	unsigned int *counters;
> +	unsigned int nr_pages;
>  
>  	if (IS_ENABLED(CONFIG_NUMA))
>  		counters = kmalloc_array(nr_node_ids, sizeof(unsigned int), GFP_KERNEL);
> @@ -5270,8 +5276,9 @@ static int vmalloc_info_show(struct seq_file *m, void *p)
>  			if (v->caller)
>  				seq_printf(m, " %pS", v->caller);
>  
> -			if (v->nr_pages)
> -				seq_printf(m, " pages=%d", v->nr_pages);
> +			nr_pages = READ_ONCE(v->nr_pages);
> +			if (nr_pages)
> +				seq_printf(m, " pages=%d", nr_pages);
>  
>  			if (v->phys_addr)
>  				seq_printf(m, " phys=%pa", &v->phys_addr);
> 
It is protected by the vn->busy.lock. When you update the page counter
in the vrealloc, we should do it under the lock if i do not miss anything.

--
Uladzislau Rezki