Re: [PATCH v6 0/6] Use killable vma write locking in most places

Andrew Morton
2026-03-27T23:12:33Z

On Fri, 27 Mar 2026 13:54:51 -0700 Suren Baghdasaryan <surenb@google.com> wrote:

> Now that we have vma_start_write_killable() we can replace most of the
> vma_start_write() calls with it, improving reaction time to the kill
> signal.
>
> There are several places which are left untouched by this patchset:
>
> 1. free_pgtables() because function should free page tables even if a
> fatal signal is pending.
>
> 2. userfaultd code, where some paths calling vma_start_write() can
> handle EINTR and some can't without a deeper code refactoring.
>
> 3. mpol_rebind_mm() which is used by cpusset controller for migrations
> and operates on a remote mm. Incomplete operations here would result
> in an inconsistent cgroup state.
>
> 4. vm_flags_{set|mod|clear} require refactoring that involves moving
> vma_start_write() out of these functions and replacing it with
> vma_assert_write_locked(), then callers of these functions should
> lock the vma themselves using vma_start_write_killable() whenever
> possible.

Updated, thanks.

> Changes since v5 [1]:
> - Added Reviewed-by for unchanged patches, per Lorenzo Stoakes
>
> Patch#2:
> - Fixed locked_vm counter if mlock_vma_pages_range() fails in
> mlock_fixup(), per Sashiko
> - Avoid VMA re-locking in madvise_update_vma(), mprotect_fixup() and
> mseal_apply() when vma_modify_XXX creates a new VMA as it will already be
> locked. This prevents the possibility of incomplete operation if signal
> happens after a successful vma_modify_XXX modified the vma tree,
> per Sashiko
> - Removed obsolete comment in madvise_update_vma() and mprotect_fixup()
>
> Patch#4:
> - Added clarifying comment for vma_start_write_killable() when locking a
> detached VMA
> - Override VMA_MERGE_NOMERGE in vma_expand() to prevent callers from
> falling back to a new VMA allocation, per Sashiko
> - Added a note in the changelog about temporary workaround of using
> ENOMEM to propagate the error in vma_merge_existing_range() and
> vma_expand()
>
> Patch#5:
> - Added fatal_signal_pending() check in do_mbind() to detect
> queue_pages_range() failures due to a pendig fatal signal, per Sashiko

Changes since v5:

mm/madvise.c | 15 ++++++++++-----
mm/mempolicy.c | 9 ++++++++-
mm/mlock.c | 2 ++
mm/mprotect.c | 26 ++++++++++++++++----------
mm/mseal.c | 27 +++++++++++++++++++--------
mm/vma.c | 20 ++++++++++++++++++--
6 files changed, 73 insertions(+), 26 deletions(-)

--- a/mm/madvise.c~b
+++ a/mm/madvise.c
@@ -172,11 +172,16 @@ static int madvise_update_vma(vm_flags_t
if (IS_ERR(vma))
return PTR_ERR(vma);

- madv_behavior->vma = vma;
-
- /* vm_flags is protected by the mmap_lock held in write mode. */
- if (vma_start_write_killable(vma))
- return -EINTR;
+ /*
+ * If a new vma was created during vma_modify_XXX, the resulting
+ * vma is already locked. Skip re-locking new vma in this case.
+ */
+ if (vma == madv_behavior->vma) {
+ if (vma_start_write_killable(vma))
+ return -EINTR;
+ } else {
+ madv_behavior->vma = vma;
+ }

vma->flags = new_vma_flags;
if (set_new_anon_name)
--- a/mm/mempolicy.c~b
+++ a/mm/mempolicy.c
@@ -1546,7 +1546,14 @@ static long do_mbind(unsigned long start
flags | MPOL_MF_INVERT | MPOL_MF_WRLOCK, &pagelist);

if (nr_failed < 0) {
- err = nr_failed;
+ /*
+ * queue_pages_range() might override the original error with -EFAULT.
+ * Confirm that fatal signals are still treated correctly.
+ */
+ if (fatal_signal_pending(current))
+ err = -EINTR;
+ else
+ err = nr_failed;
nr_failed = 0;
} else {
vma_iter_init(&vmi, mm, start);
--- a/mm/mlock.c~b
+++ a/mm/mlock.c
@@ -518,6 +518,8 @@ static int mlock_fixup(struct vma_iterat
vma->flags = new_vma_flags;
} else {
ret = mlock_vma_pages_range(vma, start, end, &new_vma_flags);
+ if (ret)
+ mm->locked_vm -= nr_pages;
}
out:
*prev = vma;
--- a/mm/mprotect.c~b
+++ a/mm/mprotect.c
@@ -716,6 +716,7 @@ mprotect_fixup(struct vma_iterator *vmi,
const vma_flags_t old_vma_flags = READ_ONCE(vma->flags);
vma_flags_t new_vma_flags = legacy_to_vma_flags(newflags);
long nrpages = (end - start) >> PAGE_SHIFT;
+ struct vm_area_struct *new_vma;
unsigned int mm_cp_flags = 0;
unsigned long charged = 0;
int error;
@@ -772,21 +773,26 @@ mprotect_fixup(struct vma_iterator *vmi,
vma_flags_clear(&new_vma_flags, VMA_ACCOUNT_BIT);
}

- vma = vma_modify_flags(vmi, *pprev, vma, start, end, &new_vma_flags);
- if (IS_ERR(vma)) {
- error = PTR_ERR(vma);
+ new_vma = vma_modify_flags(vmi, *pprev, vma, start, end,
+ &new_vma_flags);
+ if (IS_ERR(new_vma)) {
+ error = PTR_ERR(new_vma);
goto fail;
}

- *pprev = vma;
-
/*
- * vm_flags and vm_page_prot are protected by the mmap_lock
- * held in write mode.
+ * If a new vma was created during vma_modify_flags, the resulting
+ * vma is already locked. Skip re-locking new vma in this case.
*/
- error = vma_start_write_killable(vma);
- if (error)
- goto fail;
+ if (new_vma == vma) {
+ error = vma_start_write_killable(vma);
+ if (error)
+ goto fail;
+ } else {
+ vma = new_vma;
+ }
+
+ *pprev = vma;

vma_flags_reset_once(vma, &new_vma_flags);
if (vma_wants_manual_pte_write_upgrade(vma))
--- a/mm/mseal.c~b
+++ a/mm/mseal.c
@@ -70,17 +70,28 @@ static int mseal_apply(struct mm_struct

if (!vma_test(vma, VMA_SEALED_BIT)) {
vma_flags_t vma_flags = vma->flags;
- int err;
+ struct vm_area_struct *new_vma;

vma_flags_set(&vma_flags, VMA_SEALED_BIT);

- vma = vma_modify_flags(&vmi, prev, vma, curr_start,
- curr_end, &vma_flags);
- if (IS_ERR(vma))
- return PTR_ERR(vma);
- err = vma_start_write_killable(vma);
- if (err)
- return err;
+ new_vma = vma_modify_flags(&vmi, prev, vma, curr_start,
+ curr_end, &vma_flags);
+ if (IS_ERR(new_vma))
+ return PTR_ERR(new_vma);
+
+ /*
+ * If a new vma was created during vma_modify_flags,
+ * the resulting vma is already locked.
+ * Skip re-locking new vma in this case.
+ */
+ if (new_vma == vma) {
+ int err = vma_start_write_killable(vma);
+ if (err)
+ return err;
+ } else {
+ vma = new_vma;
+ }
+
vma_set_flags(vma, VMA_SEALED_BIT);
}

--- a/mm/vma.c~b
+++ a/mm/vma.c
@@ -531,6 +531,10 @@ __split_vma(struct vma_iterator *vmi, st
err = vma_start_write_killable(vma);
if (err)
goto out_free_vma;
+ /*
+ * Locking a new detached VMA will always succeed but it's just a
+ * detail of the current implementation, so handle it all the same.
+ */
err = vma_start_write_killable(new);
if (err)
goto out_free_vma;
@@ -1197,8 +1201,14 @@ int vma_expand(struct vma_merge_struct *

mmap_assert_write_locked(vmg->mm);
err = vma_start_write_killable(target);
- if (err)
+ if (err) {
+ /*
+ * Override VMA_MERGE_NOMERGE to prevent callers from
+ * falling back to a new VMA allocation.
+ */
+ vmg->state = VMA_MERGE_ERROR_NOMEM;
return err;
+ }

target_sticky = vma_flags_and_mask(&target->flags, VMA_STICKY_FLAGS);

@@ -1231,8 +1241,14 @@ int vma_expand(struct vma_merge_struct *
* is pending.
*/
err = vma_start_write_killable(next);
- if (err)
+ if (err) {
+ /*
+ * Override VMA_MERGE_NOMERGE to prevent callers from
+ * falling back to a new VMA allocation.
+ */
+ vmg->state = VMA_MERGE_ERROR_NOMEM;
return err;
+ }
err = dup_anon_vma(target, next, &anon_dup);
if (err)
return err;
_