[PATCH v8 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@xxxxxxxxxxx/

Suggested-by: Danilo Krummrich <dakr@xxxxxxxxxx>
Signed-off-by: Shivam Kalra <shivamkalra98@xxxxxxxxxxx>
---
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@xxxxxxxxxxx

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@xxxxxxxxxxx

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@xxxxxxxxxxx
- 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@xxxxxxxxxxx

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@xxxxxxxxxxx

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@xxxxxxxxxxx

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@xxxxxxxxxxx

---
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: use READ_ONCE() for vmalloc nr_pages status readers
      mm/vmalloc: free unused pages on vrealloc() shrink
      lib/test_vmalloc: add vrealloc test case

 lib/test_vmalloc.c |  62 +++++++++++++++++++++++
 mm/vmalloc.c       | 143 ++++++++++++++++++++++++++++++++++++++++++-----------
 2 files changed, 175 insertions(+), 30 deletions(-)
---
base-commit: f46991f1780ef97efff3b668627b763581032067
change-id: 20260302-vmalloc-shrink-04b2fa688a14

Best regards,
-- 
Shivam Kalra <shivamkalra98@xxxxxxxxxxx>