Re: [PATCH v3 2/3] mm: replace vma_start_write() with vma_start_write_killable()

["Lorenzo Stoakes (Oracle)" <ljs@kernel.org>]

On Tue, Mar 03, 2026 at 02:11:31PM -0800, Suren Baghdasaryan wrote:
> On Mon, Mar 2, 2026 at 6:53 AM Lorenzo Stoakes
> <lorenzo.stoakes@oracle.com> wrote:
> >
> > On Wed, Feb 25, 2026 at 11:06:08PM -0800, Suren Baghdasaryan 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 patch:
> > >
> > > 1. free_pgtables() because function should free page tables even if a
> > > fatal signal is pending.
> > >
> > > 2. process_vma_walk_lock(), which requires changes in its callers and
> > > will be handled in the next patch.
> > >
> > > 3. userfaultd code, where some paths calling vma_start_write() can
> > > handle EINTR and some can't without a deeper code refactoring.
> >
> > Surprise surprise :))
> >
> > >
> > > 4. mpol_rebind_mm() which is used by cpusset controller for migrations
> >
> > Incredibly nitty but cpusset -> cpuset?
>
> Ack.
>
> >
> > > and operates on a remote mm. Incomplete operations here would result
> > > in an inconsistent cgroup state.
> > >
> > > 5. 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.
> >
> > This should be dealt with by my ongoing mmap_prepare, vma flags work.
>
> That would be great! It makes it much simpler once you are done with
> that refactoring.

Yeah will be a big series ;) stats++;

>
> >
> > >
> > > Suggested-by: Matthew Wilcox <willy@infradead.org>
> > > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > > Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com> # powerpc
> >
> > Overall I'm a little concerned about whether callers can handle -EINTR in all
> > cases, have you checked? Might we cause some weirdness in userspace if a syscall
> > suddenly returns -EINTR when before it didn't?
>
> I did check the kernel users and put the patchset through AI reviews.
> I haven't checked if any of the affected syscalls do not advertise
> -EINTR as a possible error. Adding that to my todo list for the next
> respin.

Ah yeah as per Matthew's, fair enough. And yes it's only fatal signals.

But then is there any point in doing special handling/not filtering? We may as
well not :)

I also don't want us to run into an issue where something other than a fatal
error results in an -EINTR.

>
> >
> > Also maybe we should update the manpages to reflect this, as semi-usless as the
> > 'possible error codes' sections are...
>
> Ok, I'll check which syscalls need to be updated and will note that in
> cover letter. Once the patchset is accepted I'll update the manpages
> for them.

Yeah obv nop as per Matthew.

>
> >
> > > ---
> > >  arch/powerpc/kvm/book3s_hv_uvmem.c |  5 +-
> > >  mm/khugepaged.c                    |  5 +-
> > >  mm/madvise.c                       |  4 +-
> > >  mm/memory.c                        |  2 +
> > >  mm/mempolicy.c                     |  8 ++-
> > >  mm/mlock.c                         | 21 +++++--
> > >  mm/mprotect.c                      |  4 +-
> > >  mm/mremap.c                        |  4 +-
> > >  mm/vma.c                           | 93 +++++++++++++++++++++---------
> > >  mm/vma_exec.c                      |  6 +-
> > >  10 files changed, 109 insertions(+), 43 deletions(-)
> > >
> > > diff --git a/arch/powerpc/kvm/book3s_hv_uvmem.c b/arch/powerpc/kvm/book3s_hv_uvmem.c
> > > index 5fbb95d90e99..0a28b48a46b8 100644
> > > --- a/arch/powerpc/kvm/book3s_hv_uvmem.c
> > > +++ b/arch/powerpc/kvm/book3s_hv_uvmem.c
> > > @@ -410,7 +410,10 @@ static int kvmppc_memslot_page_merge(struct kvm *kvm,
> > >                       ret = H_STATE;
> > >                       break;
> > >               }
> > > -             vma_start_write(vma);
> > > +             if (vma_start_write_killable(vma)) {
> > > +                     ret = H_STATE;
> > > +                     break;
> > > +             }
> > >               /* Copy vm_flags to avoid partial modifications in ksm_madvise */
> > >               vm_flags = vma->vm_flags;
> > >               ret = ksm_madvise(vma, vma->vm_start, vma->vm_end,
> > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > > index 1dd3cfca610d..6c92e31ee5fb 100644
> > > --- a/mm/khugepaged.c
> > > +++ b/mm/khugepaged.c
> > > @@ -1141,7 +1141,10 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
> > >       if (result != SCAN_SUCCEED)
> > >               goto out_up_write;
> > >       /* check if the pmd is still valid */
> > > -     vma_start_write(vma);
> > > +     if (vma_start_write_killable(vma)) {
> > > +             result = SCAN_FAIL;
> > > +             goto out_up_write;
> > > +     }
> > >       result = check_pmd_still_valid(mm, address, pmd);
> > >       if (result != SCAN_SUCCEED)
> > >               goto out_up_write;
> > > diff --git a/mm/madvise.c b/mm/madvise.c
> > > index c0370d9b4e23..ccdaea6b3b15 100644
> > > --- a/mm/madvise.c
> > > +++ b/mm/madvise.c
> > > @@ -173,7 +173,9 @@ static int madvise_update_vma(vm_flags_t new_flags,
> > >       madv_behavior->vma = vma;
> > >
> > >       /* vm_flags is protected by the mmap_lock held in write mode. */
> > > -     vma_start_write(vma);
> > > +     if (vma_start_write_killable(vma))
> > > +             return -EINTR;
> > > +
> > >       vm_flags_reset(vma, new_flags);
> > >       if (set_new_anon_name)
> > >               return replace_anon_vma_name(vma, anon_name);
> > > diff --git a/mm/memory.c b/mm/memory.c
> > > index 07778814b4a8..691062154cf5 100644
> > > --- a/mm/memory.c
> > > +++ b/mm/memory.c
> > > @@ -379,6 +379,8 @@ void free_pgd_range(struct mmu_gather *tlb,
> > >   * page tables that should be removed.  This can differ from the vma mappings on
> > >   * some archs that may have mappings that need to be removed outside the vmas.
> > >   * Note that the prev->vm_end and next->vm_start are often used.
> > > + * We don't use vma_start_write_killable() because page tables should be freed
> > > + * even if the task is being killed.
> > >   *
> > >   * The vma_end differs from the pg_end when a dup_mmap() failed and the tree has
> > >   * unrelated data to the mm_struct being torn down.
> > > diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> > > index 0e5175f1c767..90939f5bde02 100644
> > > --- a/mm/mempolicy.c
> > > +++ b/mm/mempolicy.c
> > > @@ -1784,7 +1784,8 @@ SYSCALL_DEFINE4(set_mempolicy_home_node, unsigned long, start, unsigned long, le
> > >               return -EINVAL;
> > >       if (end == start)
> > >               return 0;
> > > -     mmap_write_lock(mm);
> > > +     if (mmap_write_lock_killable(mm))
> > > +             return -EINTR;
> >
> > Hmm mmap write lock as well now :) I guess it makes sense here, esp given mmap
> > write lock part of VMA write lock.
>
> Yeah, I thought while we are at it we can make this part a bit better too...

Yup that's fair.

>
> >
> >
> > >       prev = vma_prev(&vmi);
> > >       for_each_vma_range(vmi, vma, end) {
> > >               /*
> > > @@ -1801,13 +1802,16 @@ SYSCALL_DEFINE4(set_mempolicy_home_node, unsigned long, start, unsigned long, le
> > >                       err = -EOPNOTSUPP;
> > >                       break;
> > >               }
> > > +             if (vma_start_write_killable(vma)) {
> > > +                     err = -EINTR;
> > > +                     break;
> > > +             }
> > >               new = mpol_dup(old);
> > >               if (IS_ERR(new)) {
> > >                       err = PTR_ERR(new);
> > >                       break;
> > >               }
> > >
> > > -             vma_start_write(vma);
> >
> > Are we ok with moving this to before mpol_dup()? Does this matter? Confused as
> > to why you moved this up?
>
> I thought if locking fails, it would be better to check that earlier
> before allocating a new mempolicy. That seems to be safe, but thinking
> about this now, if allocation goes into direct reclaim and causes the
> lock to be held for longer then that might not be such a hot idea...

Yeah :) it's stuff like this that makes me worry. But moreso some horrendous
lock ordering issue...

>
> >
> > >               new->home_node = home_node;
> > >               err = mbind_range(&vmi, vma, &prev, start, end, new);
> > >               mpol_put(new);
> > > diff --git a/mm/mlock.c b/mm/mlock.c
> > > index 2f699c3497a5..c562c77c3ee0 100644
> > > --- a/mm/mlock.c
> > > +++ b/mm/mlock.c
> > > @@ -420,7 +420,7 @@ static int mlock_pte_range(pmd_t *pmd, unsigned long addr,
> > >   * Called for mlock(), mlock2() and mlockall(), to set @vma VM_LOCKED;
> > >   * called for munlock() and munlockall(), to clear VM_LOCKED from @vma.
> > >   */
> >
> > You should update the comment to reflect this possible return value.
>
> Ack.

Thanks!

>
> >
> > > -static void mlock_vma_pages_range(struct vm_area_struct *vma,
> > > +static int mlock_vma_pages_range(struct vm_area_struct *vma,
> > >       unsigned long start, unsigned long end, vm_flags_t newflags)
> > >  {
> > >       static const struct mm_walk_ops mlock_walk_ops = {
> > > @@ -441,7 +441,9 @@ static void mlock_vma_pages_range(struct vm_area_struct *vma,
> > >        */
> > >       if (newflags & VM_LOCKED)
> > >               newflags |= VM_IO;
> > > -     vma_start_write(vma);
> > > +     if (vma_start_write_killable(vma))
> > > +             return -EINTR;
> > > +
> > >       vm_flags_reset_once(vma, newflags);
> > >
> > >       lru_add_drain();
> > > @@ -452,6 +454,7 @@ static void mlock_vma_pages_range(struct vm_area_struct *vma,
> > >               newflags &= ~VM_IO;
> > >               vm_flags_reset_once(vma, newflags);
> > >       }
> > > +     return 0;
> > >  }
> > >
> > >  /*
> > > @@ -501,10 +504,12 @@ static int mlock_fixup(struct vma_iterator *vmi, struct vm_area_struct *vma,
> > >        */
> > >       if ((newflags & VM_LOCKED) && (oldflags & VM_LOCKED)) {
> > >               /* No work to do, and mlocking twice would be wrong */
> >
> > I'd move this comment down to the vm_flags_reset() line as it's not applicable
> > if we fail to get the lock.
>
> Ack.

Thanks!

>
> >
> >
> > > -             vma_start_write(vma);
> > > +             ret = vma_start_write_killable(vma);
> > > +             if (ret)
> > > +                     goto out;
> > >               vm_flags_reset(vma, newflags);
> > >       } else {
> > > -             mlock_vma_pages_range(vma, start, end, newflags);
> > > +             ret = mlock_vma_pages_range(vma, start, end, newflags);
> > >       }
> > >  out:
> > >       *prev = vma;
> > > @@ -733,9 +738,13 @@ static int apply_mlockall_flags(int flags)
> > >
> > >               error = mlock_fixup(&vmi, vma, &prev, vma->vm_start, vma->vm_end,
> > >                                   newflags);
> > > -             /* Ignore errors, but prev needs fixing up. */
> > > -             if (error)
> > > +             /* Ignore errors except EINTR, but prev needs fixing up. */
> >
> > Well, except you're not fixing it up on -EINTR? This comment should be redone.
>
> Hmm, should we fixup if the process is terminating? Does it matter at
> this point?

We may as well keep the comment accurate though? I mean this isn't a big deal :)

>
> >
> > But I wonder if this is correct? We are ignoring all other errors that
> > interrupted the operation, so why are we special casing -EINTR?
>
> Well, -EINTR means all the work you are doing here is useless because
> the process is about to go away. So, in that respect I think it's
> different from other errors.

Well -EINTR doesn't _just_ mean fatal signal. We happen to know here that what
we're calling will only return -EINTR in that case, but if somebody changes
downstream code to return -EINTR for _another_ reason there's an issue.

Can we just replace these horrid -EINTR checks with
is_fatal_signal_pending(current)?

>
> >
> > > +             if (error) {
> > > +                     if (error == -EINTR)
> > > +                             return error;
> > > +
> > >                       prev = vma;
> > > +             }
> > >               cond_resched();
> > >       }
> > >  out:
> > > diff --git a/mm/mprotect.c b/mm/mprotect.c
> > > index c0571445bef7..49dbb7156936 100644
> > > --- a/mm/mprotect.c
> > > +++ b/mm/mprotect.c
> > > @@ -765,7 +765,9 @@ mprotect_fixup(struct vma_iterator *vmi, struct mmu_gather *tlb,
> > >        * vm_flags and vm_page_prot are protected by the mmap_lock
> > >        * held in write mode.
> > >        */
> > > -     vma_start_write(vma);
> > > +     error = vma_start_write_killable(vma);
> > > +     if (error < 0)
> >
> > Weird inconstency here, this should be if (error).
>
> Ack.

Thanks!

>
> >
> > > +             goto fail;
> > >       vm_flags_reset_once(vma, newflags);
> > >       if (vma_wants_manual_pte_write_upgrade(vma))
> > >               mm_cp_flags |= MM_CP_TRY_CHANGE_WRITABLE;
> > > diff --git a/mm/mremap.c b/mm/mremap.c
> > > index 2be876a70cc0..aef1e5f373c7 100644
> > > --- a/mm/mremap.c
> > > +++ b/mm/mremap.c
> > > @@ -1286,7 +1286,9 @@ static unsigned long move_vma(struct vma_remap_struct *vrm)
> > >               return -ENOMEM;
> > >
> > >       /* We don't want racing faults. */
> > > -     vma_start_write(vrm->vma);
> > > +     err = vma_start_write_killable(vrm->vma);
> > > +     if (err)
> > > +             return err;
> > >
> > >       /* Perform copy step. */
> > >       err = copy_vma_and_data(vrm, &new_vma);
> > > diff --git a/mm/vma.c b/mm/vma.c
> > > index bb4d0326fecb..9f2664f1d078 100644
> > > --- a/mm/vma.c
> > > +++ b/mm/vma.c
> > > @@ -530,6 +530,13 @@ __split_vma(struct vma_iterator *vmi, struct vm_area_struct *vma,
> > >       if (err)
> > >               goto out_free_vmi;
> > >
> > > +     err = vma_start_write_killable(vma);
> > > +     if (err)
> > > +             goto out_free_mpol;
> > > +     err = vma_start_write_killable(new);
> > > +     if (err)
> > > +             goto out_free_mpol;
> > > +
> > >       err = anon_vma_clone(new, vma, VMA_OP_SPLIT);
> > >       if (err)
> > >               goto out_free_mpol;
> > > @@ -540,9 +547,6 @@ __split_vma(struct vma_iterator *vmi, struct vm_area_struct *vma,
> > >       if (new->vm_ops && new->vm_ops->open)
> > >               new->vm_ops->open(new);
> > >
> > > -     vma_start_write(vma);
> > > -     vma_start_write(new);
> > > -
> >
> > Again you're changing ordering seemingly arbitrarily.
>
> I moved it up to avoid undoing all the work above (vm_ops->open,
> vma_dup_policy, vma_iter_prealloc, vm_area_dup,..)
>
> > I think this is actually a
> > more problematic case as you're now invoking vm_ops->open() with a VMA write
> > lock held.
>
> Are you concerned about potential increased duration of the vma lock
> being held or that open() might try to take that lock itself (which is
> not a problem because vma write locks are reentrant)? Maybe some other
> concern I'm missing?

General concern about reordering, but fair point re: undoing.

rmap.c says it's ok so we're good:

 *           vma_start_write
 *             mapping->i_mmap_rwsem
 *               anon_vma->rwsem

>
> >
> > So I think you should keep the existing position.
>
> If we do that then we would have to undo a bunch of operations. I'm
> fine adding that if there are reasons to avoid this move.

OK fair enough in this case.

>
> >
> > >       init_vma_prep(&vp, vma);
> > >       vp.insert = new;
> > >       vma_prepare(&vp);
> > > @@ -895,16 +899,22 @@ static __must_check struct vm_area_struct *vma_merge_existing_range(
> > >       }
> > >
> > >       /* No matter what happens, we will be adjusting middle. */
> > > -     vma_start_write(middle);
> > > +     err = vma_start_write_killable(middle);
> > > +     if (err)
> > > +             goto abort;
> > >
> > >       if (merge_right) {
> > > -             vma_start_write(next);
> > > +             err = vma_start_write_killable(next);
> > > +             if (err)
> > > +                     goto abort;
> > >               vmg->target = next;
> > >               sticky_flags |= (next->vm_flags & VM_STICKY);
> > >       }
> > >
> > >       if (merge_left) {
> > > -             vma_start_write(prev);
> > > +             err = vma_start_write_killable(prev);
> > > +             if (err)
> > > +                     goto abort;
> > >               vmg->target = prev;
> > >               sticky_flags |= (prev->vm_flags & VM_STICKY);
> > >       }
> > > @@ -1155,10 +1165,12 @@ int vma_expand(struct vma_merge_struct *vmg)
> > >       struct vm_area_struct *next = vmg->next;
> > >       bool remove_next = false;
> > >       vm_flags_t sticky_flags;
> > > -     int ret = 0;
> > > +     int ret;
> > >
> > >       mmap_assert_write_locked(vmg->mm);
> > > -     vma_start_write(target);
> > > +     ret = vma_start_write_killable(target);
> > > +     if (ret)
> > > +             return ret;
> > >
> > >       if (next && target != next && vmg->end == next->vm_end)
> > >               remove_next = true;
> > > @@ -1187,6 +1199,9 @@ int vma_expand(struct vma_merge_struct *vmg)
> > >        * we don't need to account for vmg->give_up_on_mm here.
> > >        */
> > >       if (remove_next) {
> > > +             ret = vma_start_write_killable(next);
> > > +             if (ret)
> > > +                     return ret;
> > >               ret = dup_anon_vma(target, next, &anon_dup);
> > >               if (ret)
> > >                       return ret;
> > > @@ -1197,10 +1212,8 @@ int vma_expand(struct vma_merge_struct *vmg)
> > >                       return ret;
> > >       }
> > >
> > > -     if (remove_next) {
> > > -             vma_start_write(next);
> > > +     if (remove_next)
> > >               vmg->__remove_next = true;
> > > -     }
> >
> > Hmm you're moving the ordering of things around again :) You should have made
> > this change as part of patch 1 anyway first so this patch wouldn't have a
> > refactoring in it too.
> >
> > Top of rmap.c suggests you _can_ take the VMA write lock prior to trying the dup
> > but I'm just not sure why you'd want to switch these around in this patch?
> >
> > Can we try to keep original ordering unless there's a really good reason not to?
>
> Again, I'm trying to avoid undoing things if locking fails but this
> function already has rollback code, so I can reuse it. I'll keep the
> old placement here.

Ack yeah... :)

We're safe with vma -> anon rmap lock anyway.

>
> >
> > >       if (commit_merge(vmg))
> > >               goto nomem;
> > >
> > > @@ -1233,6 +1246,7 @@ int vma_shrink(struct vma_iterator *vmi, struct vm_area_struct *vma,
> > >              unsigned long start, unsigned long end, pgoff_t pgoff)
> > >  {
> > >       struct vma_prepare vp;
> > > +     int err;
> > >
> > >       WARN_ON((vma->vm_start != start) && (vma->vm_end != end));
> > >
> > > @@ -1244,7 +1258,11 @@ int vma_shrink(struct vma_iterator *vmi, struct vm_area_struct *vma,
> > >       if (vma_iter_prealloc(vmi, NULL))
> > >               return -ENOMEM;
> > >
> > > -     vma_start_write(vma);
> > > +     err = vma_start_write_killable(vma);
> > > +     if (err) {
> > > +             vma_iter_free(vmi);
> > > +             return err;
> > > +     }
> > >
> > >       init_vma_prep(&vp, vma);
> > >       vma_prepare(&vp);
> > > @@ -1434,7 +1452,9 @@ static int vms_gather_munmap_vmas(struct vma_munmap_struct *vms,
> > >                       if (error)
> > >                               goto end_split_failed;
> > >               }
> > > -             vma_start_write(next);
> > > +             error = vma_start_write_killable(next);
> > > +             if (error)
> > > +                     goto munmap_gather_failed;
> > >               mas_set(mas_detach, vms->vma_count++);
> > >               error = mas_store_gfp(mas_detach, next, GFP_KERNEL);
> > >               if (error)
> > > @@ -1828,12 +1848,17 @@ static void vma_link_file(struct vm_area_struct *vma, bool hold_rmap_lock)
> > >  static int vma_link(struct mm_struct *mm, struct vm_area_struct *vma)
> > >  {
> > >       VMA_ITERATOR(vmi, mm, 0);
> > > +     int err;
> > >
> > >       vma_iter_config(&vmi, vma->vm_start, vma->vm_end);
> > >       if (vma_iter_prealloc(&vmi, vma))
> > >               return -ENOMEM;
> > >
> > > -     vma_start_write(vma);
> > > +     err = vma_start_write_killable(vma);
> > > +     if (err) {
> > > +             vma_iter_free(&vmi);
> > > +             return err;
> > > +     }
> > >       vma_iter_store_new(&vmi, vma);
> > >       vma_link_file(vma, /* hold_rmap_lock= */false);
> > >       mm->map_count++;
> > > @@ -2215,9 +2240,8 @@ int mm_take_all_locks(struct mm_struct *mm)
> > >        * is reached.
> > >        */
> > >       for_each_vma(vmi, vma) {
> > > -             if (signal_pending(current))
> > > +             if (signal_pending(current) || vma_start_write_killable(vma))
> > >                       goto out_unlock;
> > > -             vma_start_write(vma);
> > >       }
> > >
> > >       vma_iter_init(&vmi, mm, 0);
> > > @@ -2522,6 +2546,11 @@ static int __mmap_new_vma(struct mmap_state *map, struct vm_area_struct **vmap)
> > >       if (!vma)
> > >               return -ENOMEM;
> > >
> > > +     /* Lock the VMA since it is modified after insertion into VMA tree */
> > > +     error = vma_start_write_killable(vma);
> > > +     if (error)
> > > +             goto free_vma;
> > > +
> >
> > You're doing it again :)
> >
> > Can we please keep the lock acquisition at the point it is in unless there's a
> > really good reason not to.
> >
> > And if there is a good reason, please do it in another commit prior to the
> > massive 'change everything' one so it's more easily reviewable :)
>
> The reason for this one is that I want to avoid undoing
> __mmap_new_file_vma() if we fail to lock the VMA later. Undoing that
> one would be messy, so I would prefer locking it earlier. These
> operations are already performed under the mmap write lock. Is that
> really a problem if we write-lock the VMA as well?

This is a fair point, I obviously glanced over this without thinking about that
:)

I think you should include in the commit message why you're reordering though.

>
> >
> > >       vma_iter_config(vmi, map->addr, map->end);
> > >       vma_set_range(vma, map->addr, map->end, map->pgoff);
> > >       vm_flags_init(vma, map->vm_flags);
> > > @@ -2552,8 +2581,6 @@ static int __mmap_new_vma(struct mmap_state *map, struct vm_area_struct **vmap)
> > >       WARN_ON_ONCE(!arch_validate_flags(map->vm_flags));
> > >  #endif
> > >
> > > -     /* Lock the VMA since it is modified after insertion into VMA tree */
> > > -     vma_start_write(vma);
> > >       vma_iter_store_new(vmi, vma);
> > >       map->mm->map_count++;
> > >       vma_link_file(vma, map->hold_file_rmap_lock);
> > > @@ -2864,6 +2891,7 @@ int do_brk_flags(struct vma_iterator *vmi, struct vm_area_struct *vma,
> > >                unsigned long addr, unsigned long len, vm_flags_t vm_flags)
> > >  {
> > >       struct mm_struct *mm = current->mm;
> > > +     int err = -ENOMEM;
> >
> > I hate this 'default error code' pattern, it's a code smell. Please update
> > everything that jumps to the failure case to set err, and leave this
> > uninitialised.
> >
> > We've had real bugs come out of this before!
>
> Ack.

Thanks!

>
> >
> > >
> > >       /*
> > >        * Check against address space limits by the changed size
> > > @@ -2908,7 +2936,10 @@ int do_brk_flags(struct vma_iterator *vmi, struct vm_area_struct *vma,
> > >       vma_set_range(vma, addr, addr + len, addr >> PAGE_SHIFT);
> > >       vm_flags_init(vma, vm_flags);
> > >       vma->vm_page_prot = vm_get_page_prot(vm_flags);
> > > -     vma_start_write(vma);
> > > +     if (vma_start_write_killable(vma)) {
> > > +             err = -EINTR;
> > > +             goto mas_store_fail;
> > > +     }
> > >       if (vma_iter_store_gfp(vmi, vma, GFP_KERNEL))
> > >               goto mas_store_fail;
> > >
> > > @@ -2928,7 +2959,7 @@ int do_brk_flags(struct vma_iterator *vmi, struct vm_area_struct *vma,
> > >       vm_area_free(vma);
> > >  unacct_fail:
> > >       vm_unacct_memory(len >> PAGE_SHIFT);
> > > -     return -ENOMEM;
> > > +     return err;
> > >  }
> > >
> > >  /**
> > > @@ -3089,7 +3120,7 @@ int expand_upwards(struct vm_area_struct *vma, unsigned long address)
> > >       struct mm_struct *mm = vma->vm_mm;
> > >       struct vm_area_struct *next;
> > >       unsigned long gap_addr;
> > > -     int error = 0;
> > > +     int error;
> > >       VMA_ITERATOR(vmi, mm, vma->vm_start);
> > >
> > >       if (!(vma->vm_flags & VM_GROWSUP))
> > > @@ -3126,12 +3157,14 @@ int expand_upwards(struct vm_area_struct *vma, unsigned long address)
> > >
> > >       /* We must make sure the anon_vma is allocated. */
> > >       if (unlikely(anon_vma_prepare(vma))) {
> > > -             vma_iter_free(&vmi);
> > > -             return -ENOMEM;
> > > +             error = -ENOMEM;
> > > +             goto free;
> > >       }
> > >
> > >       /* Lock the VMA before expanding to prevent concurrent page faults */
> > > -     vma_start_write(vma);
> > > +     error = vma_start_write_killable(vma);
> > > +     if (error)
> > > +             goto free;
> > >       /* We update the anon VMA tree. */
> > >       anon_vma_lock_write(vma->anon_vma);
> > >
> > > @@ -3160,6 +3193,7 @@ int expand_upwards(struct vm_area_struct *vma, unsigned long address)
> > >               }
> > >       }
> > >       anon_vma_unlock_write(vma->anon_vma);
> > > +free:
> >
> > Nitty, but this kinda sucks as a label name, generally when the error label
> > contains 'free' it is free_xxx where 'xxx' is some specific thing.
> >
> > So somethiing like 'fail' would be good.
>
> Ack. Will change to something more appropriate.

Thanks!

>
> >
> > >       vma_iter_free(&vmi);
> > >       validate_mm(mm);
> > >       return error;
> > > @@ -3174,7 +3208,7 @@ int expand_downwards(struct vm_area_struct *vma, unsigned long address)
> > >  {
> > >       struct mm_struct *mm = vma->vm_mm;
> > >       struct vm_area_struct *prev;
> > > -     int error = 0;
> > > +     int error;
> > >       VMA_ITERATOR(vmi, mm, vma->vm_start);
> > >
> > >       if (!(vma->vm_flags & VM_GROWSDOWN))
> > > @@ -3205,12 +3239,14 @@ int expand_downwards(struct vm_area_struct *vma, unsigned long address)
> > >
> > >       /* We must make sure the anon_vma is allocated. */
> > >       if (unlikely(anon_vma_prepare(vma))) {
> > > -             vma_iter_free(&vmi);
> > > -             return -ENOMEM;
> > > +             error = -ENOMEM;
> > > +             goto free;
> > >       }
> > >
> > >       /* Lock the VMA before expanding to prevent concurrent page faults */
> > > -     vma_start_write(vma);
> > > +     error = vma_start_write_killable(vma);
> > > +     if (error)
> > > +             goto free;
> > >       /* We update the anon VMA tree. */
> > >       anon_vma_lock_write(vma->anon_vma);
> > >
> > > @@ -3240,6 +3276,7 @@ int expand_downwards(struct vm_area_struct *vma, unsigned long address)
> > >               }
> > >       }
> > >       anon_vma_unlock_write(vma->anon_vma);
> > > +free:
> >
> > Obviously same comment her :)
>
> Ack.

Thanks!

>
> >
> > >       vma_iter_free(&vmi);
> > >       validate_mm(mm);
> > >       return error;
> > > diff --git a/mm/vma_exec.c b/mm/vma_exec.c
> > > index 8134e1afca68..a4addc2a8480 100644
> > > --- a/mm/vma_exec.c
> > > +++ b/mm/vma_exec.c
> > > @@ -40,6 +40,7 @@ int relocate_vma_down(struct vm_area_struct *vma, unsigned long shift)
> > >       struct vm_area_struct *next;
> > >       struct mmu_gather tlb;
> > >       PAGETABLE_MOVE(pmc, vma, vma, old_start, new_start, length);
> > > +     int err;
> > >
> > >       BUG_ON(new_start > new_end);
> > >
> > > @@ -55,8 +56,9 @@ int relocate_vma_down(struct vm_area_struct *vma, unsigned long shift)
> > >        * cover the whole range: [new_start, old_end)
> > >        */
> > >       vmg.target = vma;
> > > -     if (vma_expand(&vmg))
> > > -             return -ENOMEM;
> > > +     err = vma_expand(&vmg);
> > > +     if (err)
> > > +             return err;
> >
> > Hmm. But before we were filtering the errors and now we're not... I guess not an
> > issue as before it could _only_ return -ENOMEM, but again, are we sure all
> > callers are fine with -EINTR I guess :)
>
> This function is called only from setup_arg_pages() and all its
> callers end up being linux_binfmt.load_binary handlers. The returned
> error propagates all the way to execve() and its friends. And with my
> extreme "luck" the execve syscall lists probably every single possible
> error code except EINTR :) This is depressing...

Well as per Matthew's point this is probably fine.

>
> Thanks for the detailed review, Lorenzo! I guess we need to discuss
> these lock moves a bit more before I start on the new version.

Nah I think we're good after this!

>
> >
> > >
> > >       /*
> > >        * move the page tables downwards, on failure we rely on
> > > --
> > > 2.53.0.414.gf7e9f6c205-goog
> > >
> >
> > Cheers, Lorenzo
> >

Cheers, Lorenzo