[PATCH v10 0/4] mm/vmalloc: free unused pages on vrealloc() shrink

Shivam Kalra via B4 Relay
2026-04-04T08:37:15Z

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: Uses the helper to free tail pages when vrealloc() shrinks
across a page boundary.
- Patch 4: 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 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

---
Shivam Kalra (4):
mm/vmalloc: extract vm_area_free_pages() helper from vfree()
mm/vmalloc: use physical page count for vrealloc() grow-in-place check
mm/vmalloc: free unused pages on vrealloc() shrink
lib/test_vmalloc: add vrealloc test case

lib/test_vmalloc.c | 62 ++++++++++++++++++++++++++++++
mm/vmalloc.c | 111 ++++++++++++++++++++++++++++++++++++++++++++---------
2 files changed, 154 insertions(+), 19 deletions(-)
---
base-commit: b47b4fa4c232ee36aae58630e9d6520e35d33f3a
change-id: 20260302-vmalloc-shrink-04b2fa688a14

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