Re: [PATCH 20/30] mm/vma: introduce vma_assert_can_modify()
From: Pedro Falcato <pfalcato@suse.de>
Date: 2026-07-02 11:16:42
Also in:
damon, dri-devel, kvm, linux-arm-msm, linux-fsdevel, linux-iommu, linux-perf-users, linux-tegra, lkml, nvdimm
On Mon, Jun 29, 2026 at 01:23:31PM +0100, Lorenzo Stoakes wrote:
vma_assert_write_locked() and vma_assert_attached() are useful for their own purposes, however VMA code absolutely does allow the modification of non-write locked VMAs if they are at that point detached (i.e. unreachable from anywhere). It's therefore useful to be able to assert that a VMA is either detached (modification doesn't matter) or write locked (you're explicitly locked for modification).
Hmm, I was wondering why detached does not imply write_locked, and then realized that new VMAs aren't write-locked. Could we do it by default? Like a simple: vma->vm_lock_seq = __vma_raw_mm_seqnum(vma); might do the trick. I don't see why it wouldn't work? Is there some other case I am not considering?
quoted hunk ↗ jump to hunk
Therefore introduce vma_assert_can_modify() for this purpose. While we're here, make vma_is_attached() available generally - if !CONFIG_PER_VMA_LOCKS, then there's no sense in which a VMA is detached (vma_mark_detached() is a noop), so have this default to true in this case. Signed-off-by: Lorenzo Stoakes <ljs@kernel.org> --- include/linux/mmap_lock.h | 8 ++++++++ 1 file changed, 8 insertions(+)diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h index 04b8f61ece5d..d513286d8160 100644 --- a/include/linux/mmap_lock.h +++ b/include/linux/mmap_lock.h@@ -506,6 +506,8 @@ static inline __must_check int vma_start_write_killable(struct vm_area_struct *vma) { return 0; } static inline void vma_assert_write_locked(struct vm_area_struct *vma) { mmap_assert_write_locked(vma->vm_mm); } +static inline bool vma_is_attached(struct vm_area_struct *vma) + { return true; } static inline void vma_assert_attached(struct vm_area_struct *vma) {} static inline void vma_assert_detached(struct vm_area_struct *vma) {} static inline void vma_mark_attached(struct vm_area_struct *vma) {}@@ -530,6 +532,12 @@ static inline void vma_assert_stabilised(struct vm_area_struct *vma) #endif /* CONFIG_PER_VMA_LOCK */ +static inline void vma_assert_can_modify(struct vm_area_struct *vma) +{ + if (vma_is_attached(vma)) + vma_assert_write_locked(vma); +} + static inline void mmap_write_lock(struct mm_struct *mm) { __mmap_lock_trace_start_locking(mm, true);-- 2.54.0
-- Pedro