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: Update 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 sizes. This is a prerequisite for shrinking.
- Patch 3: Update vread_iter() to derive the vm area size from
  vm->nr_pages rather than get_vm_area_size(), which would overestimate
  the mapped range after a shrink.
- Patch 4: Uses the helper to free tail pages when vrealloc() shrinks
  across a page boundary. 
- Patch 5: 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/

Suggested-by: Danilo Krummrich <dakr@kernel.org>
Signed-off-by: Shivam Kalra <shivamkalra98@zohomail.in>
---
Changes in v13:
- Collect all r-b tags
- Resubmit for Review. 
- Link to v12: https://lore.kernel.org/r/20260428-vmalloc-shrink-v12-0-3c18c9172eb1@zohomail.in

Changes in v12:
- Rewrite vm_area_free_pages() to use free_pages_bulk() 
  following the upstream vfree() refactoring.(Andrew)
- Drop Reviewed-by tags from Patch 1 due to the rewrite
- Rebase to latest mm-new
- Link to v11: https://lore.kernel.org/r/20260420-vmalloc-shrink-v11-0-cad80b00853a@zohomail.in

Changes in v11:
- Prepare vread_iter() to use vm->nr_pages instead of
  get_vm_area_size() (Uladzislau Rezki, Sashiko)
- Drop (size_t) cast from nr_pages << PAGE_SHIFT (Uladzislau Rezki)
- Link to v10: https://lore.kernel.org/r/20260404-vmalloc-shrink-v10-0-335759165dfa@zohomail.in

Changes in v10:
- Reorder vm->nr_pages to the beginning (Alice Ryhl)
- Link to v9: https://lore.kernel.org/r/20260401-vmalloc-shrink-v9-0-bf58dfb997d8@zohomail.in

Changes in v9:
- Remove READ_ONCE, WRITE_ONCE and drop commit
  about show_numa_info. (Uladzislau Rezki)
- Update the commit message in Patch 2. (Alice Ryhl)
- Remove zero newly exposed memory commit.
- Link to v8: https://lore.kernel.org/r/20260327-vmalloc-shrink-v8-0-cc6b57059ed7@zohomail.in

Changes in v8:
- Strip the KASAN tag from the pointer before addr_to_node()
  to avoid acquiring the wrong node lock (Sashiko).
- Rebase to latest mm-new.
- Link to v7: https://lore.kernel.org/r/20260324-vmalloc-shrink-v7-0-c0e62b8e5d83@zohomail.in

Changes in v7:
- Fix NULL pointer dereference in shrink path (Sashiko)
- Acquire vn->busy.lock when updating vm->nr_pages to synchronize 
  with concurrent readers (Uladzislau Rezki)
- Use READ_ONCE in vmalloc_dump_obj (Sashiko)
- Skip shrink path on GFP_NIO or GFP_NOFS. (Sashiko)
- Fix Overflow issue for large allocations. (Sashiko)
- Use vrealloc instead of vmalloc in vrealloc test.  
- Link to v6: https://lore.kernel.org/r/20260321-vmalloc-shrink-v6-0-062ca7b7ceb2@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

To: Andrew Morton <akpm@linux-foundation.org>
To: Uladzislau Rezki <urezki@gmail.com>
Cc: linux-mm@kvack.org
Cc: linux-kernel@vger.kernel.org

---
Shivam Kalra (5):
      mm/vmalloc: extract vm_area_free_pages() helper from vfree()
      mm/vmalloc: use physical page count for vrealloc() grow-in-place check
      mm/vmalloc: use physical page count in vread_iter()
      mm/vmalloc: free unused pages on vrealloc() shrink
      lib/test_vmalloc: add vrealloc test case

 lib/test_vmalloc.c |  62 +++++++++++++++++++++++++++++++
 mm/vmalloc.c       | 107 ++++++++++++++++++++++++++++++++++++++++++++++-------
 2 files changed, 156 insertions(+), 13 deletions(-)
---
base-commit: 7505bb13a9bb1f214310915ccc06643119fdafc9
change-id: 20260302-vmalloc-shrink-04b2fa688a14

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

On Mon, 11 May 2026 13:47:28 +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.

Thanks.  AI review asked a few questions.  Does anything here look
legitimate?

	https://sashiko.dev/#/patchset/20260511-vmalloc-shrink-v13-0-643b7ec277a9@zohomail.in

On 13/05/26 03:31, Andrew Morton wrote:
> On Mon, 11 May 2026 13:47:28 +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.
> 
> Thanks.  AI review asked a few questions.  Does anything here look
> legitimate?
> 
> 	https://sashiko.dev/#/patchset/20260511-vmalloc-shrink-v13-0-643b7ec277a9@zohomail.in

Thanks for flagging this. I went through the Sashiko review, and,
one of the findings is legitimate and needs a fix.

1) Patch 3 - vmap() mappings with nr_pages == 0 (LEGITIMATE)
This is correct. For vmap() areas created without VM_MAP_PUT_PAGES,
nr_pages is never set and stays 0 (vm_struct is kzalloc'd). With the
current patch, vread_iter() would compute size = 0 for these areas,
skip them entirely, and zero-fill the region in /proc/kcore.

Ulad, this is the approach you suggested during v10 review [1]; we
both checked whether nr_pages could be 0 and concluded it couldn't,
but we only considered the VM_ALLOC (vmalloc) path. vmap() areas also
have vm != NULL but never set nr_pages unless VM_MAP_PUT_PAGES is
passed, which most callers don't use.

Since vrealloc() only operates on VM_ALLOC areas, I think we can
scope the nr_pages path to just that:
	if (vm)
		/*
		 * For VM_ALLOC areas, use nr_pages rather than
		 * get_vm_area_size() because vrealloc() may shrink
		 * the mapping without updating area->size. Other
		 * mapping types (vmap, ioremap) don't set nr_pages.
		 */
		size = (vm->flags & VM_ALLOC) ?
			(vm->nr_pages << PAGE_SHIFT) :
			get_vm_area_size(vm);
	else
		size = va_size(va);
Ulad, does this approach look right to you, or would you prefer to
initialize nr_pages unconditionally in vmap() instead?

[1] https://lore.kernel.org/lkml/aeYBYqPSEJRC8mjh@milan/

On Mon, May 18, 2026 at 04:29:11AM +0530, Shivam Kalra wrote:
> On 13/05/26 03:31, Andrew Morton wrote:
> > On Mon, 11 May 2026 13:47:28 +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.
> > 
> > Thanks.  AI review asked a few questions.  Does anything here look
> > legitimate?
> > 
> > 	https://sashiko.dev/#/patchset/20260511-vmalloc-shrink-v13-0-643b7ec277a9@zohomail.in
> 
> Thanks for flagging this. I went through the Sashiko review, and,
> one of the findings is legitimate and needs a fix.
> 
> 1) Patch 3 - vmap() mappings with nr_pages == 0 (LEGITIMATE)
> This is correct. For vmap() areas created without VM_MAP_PUT_PAGES,
> nr_pages is never set and stays 0 (vm_struct is kzalloc'd). With the
> current patch, vread_iter() would compute size = 0 for these areas,
> skip them entirely, and zero-fill the region in /proc/kcore.
> 
> Ulad, this is the approach you suggested during v10 review [1]; we
> both checked whether nr_pages could be 0 and concluded it couldn't,
> but we only considered the VM_ALLOC (vmalloc) path. vmap() areas also
> have vm != NULL but never set nr_pages unless VM_MAP_PUT_PAGES is
> passed, which most callers don't use.
> 
> Since vrealloc() only operates on VM_ALLOC areas, I think we can
> scope the nr_pages path to just that:
> 	if (vm)
> 		/*
> 		 * For VM_ALLOC areas, use nr_pages rather than
> 		 * get_vm_area_size() because vrealloc() may shrink
> 		 * the mapping without updating area->size. Other
> 		 * mapping types (vmap, ioremap) don't set nr_pages.
> 		 */
> 		size = (vm->flags & VM_ALLOC) ?
> 			(vm->nr_pages << PAGE_SHIFT) :
> 			get_vm_area_size(vm);
> 	else
> 		size = va_size(va);
> Ulad, does this approach look right to you, or would you prefer to
> initialize nr_pages unconditionally in vmap() instead?
> 
> [1] https://lore.kernel.org/lkml/aeYBYqPSEJRC8mjh@milan/
>
Indeed, we missed that point regarding vmap() code. Checking VM_ALLOC
sounds correct to me.

--
Uladzislau Rezki