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

[Alice Ryhl <aliceryhl@google.com>]

On Tue, Apr 14, 2026 at 02:09:09PM +0530, Shivam Kalra wrote:
> On 04/04/26 14:06, Shivam Kalra via B4 Relay wrote:
> > 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,
> Hey Ulad, Andrew
> A gentle thread bump. Will we include this in this merge window?
> Let me know if you have any suggestions, I will resolve them
> asap.

If you mean the merge window that opened yesterday, then it's too late
for that, and I believe we agreed to not include it there anyway in [1].

I can try to look at it again. I guess the main difficult thing is
interactions with other parts of the kernel, of which sashiko claims to
have found another one [2]. I can't help but wonder if it would be
better to reduce the vma size to avoid issues with other codepaths using
get_vm_area_size(vm) and assuming that all pages in that range are
mapped.

[1]: https://lore.kernel.org/lkml/20260327113758.75f04588310a707b4d4b1aac@linux-foundation.org/
[2]: https://sashiko.dev/#/patchset/20260404-vmalloc-shrink-v10-0-335759165dfa%40zohomail.in