Re: [PATCH v3 04/16] mm/mmap: Extract the gathering of vmas from do_vmi_align_munmap()

["Liam R. Howlett" <Liam.Howlett@oracle.com>]

* Suren Baghdasaryan <surenb@google.com> [240710 12:07]:
> On Thu, Jul 4, 2024 at 11:27 AM Liam R. Howlett <Liam.Howlett@oracle.com> wrote:
> >
> > Create vmi_gather_munmap_vmas() to handle the gathering of vmas into a
> > detached maple tree for removal later.  Part of the gathering is the
> > splitting of vmas that span the boundary.
> >
> > Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
> > ---
> >  mm/mmap.c | 82 +++++++++++++++++++++++++++++++++++++++----------------
> >  1 file changed, 58 insertions(+), 24 deletions(-)
> >
> > diff --git a/mm/mmap.c b/mm/mmap.c
> > index 411798f46932..8dc8ffbf9d8d 100644
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -2656,32 +2656,29 @@ vmi_complete_munmap_vmas(struct vma_iterator *vmi, struct vm_area_struct *vma,
> >  }
> >
> >  /*
> > - * do_vmi_align_munmap() - munmap the aligned region from @start to @end.
> > + * vmi_gather_munmap_vmas() - Put all VMAs within a range into a maple tree
> > + * for removal at a later date.  Handles splitting first and last if necessary
> > + * and marking the vmas as isolated.
> > + *
> >   * @vmi: The vma iterator
> >   * @vma: The starting vm_area_struct
> >   * @mm: The mm_struct
> >   * @start: The aligned start address to munmap.
> >   * @end: The aligned end address to munmap.
> >   * @uf: The userfaultfd list_head
> > - * @unlock: Set to true to drop the mmap_lock.  unlocking only happens on
> > - * success.
> > + * @mas_detach: The maple state tracking the detached tree
> >   *
> > - * Return: 0 on success and drops the lock if so directed, error and leaves the
> > - * lock held otherwise.
> > + * Return: 0 on success
> >   */
> >  static int
> > -do_vmi_align_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
> > +vmi_gather_munmap_vmas(struct vma_iterator *vmi, struct vm_area_struct *vma,
> >                     struct mm_struct *mm, unsigned long start,
> > -                   unsigned long end, struct list_head *uf, bool unlock)
> > +                   unsigned long end, struct list_head *uf,
> > +                   struct ma_state *mas_detach, unsigned long *locked_vm)
> >  {
> >         struct vm_area_struct *next = NULL;
> > -       struct maple_tree mt_detach;
> >         int count = 0;
> >         int error = -ENOMEM;
> > -       unsigned long locked_vm = 0;
> > -       MA_STATE(mas_detach, &mt_detach, 0, 0);
> > -       mt_init_flags(&mt_detach, vmi->mas.tree->ma_flags & MT_FLAGS_LOCK_MASK);
> > -       mt_on_stack(mt_detach);
> >
> >         /*
> >          * If we need to split any vma, do it now to save pain later.
> > @@ -2720,15 +2717,14 @@ do_vmi_align_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
> >                                 goto end_split_failed;
> >                 }
> >                 vma_start_write(next);
> > -               mas_set(&mas_detach, count);
> > -               error = mas_store_gfp(&mas_detach, next, GFP_KERNEL);
> > +               mas_set(mas_detach, count++);
> > +               if (next->vm_flags & VM_LOCKED)
> > +                       *locked_vm += vma_pages(next);
> 
> Uh, this was confusing. You moved locked_vm/count accounting before
> mas_store_gfp(), so if mas_store_gfp() fails, they will be one-off
> because we incremented them but failed to insert the element. Only
> later I realized that if mas_store_gfp() fails then we never use these
> counters. The code is still correct but I'm wondering if this movement
> was necessary. We wouldn't use wrong values but why make them wrong in
> the first place?
> In later patches you account for more things in here and all that is
> also done before mas_store_gfp(). Would moving all that after
> mas_store_gfp() and keeping them always correct be an issue?

The accounting is only ever used in the even of a successful munmap()
operation, but I can make this change.  I didn't see this as the author
so thanks for pointing it out.

> 
> 
> 
> 
> > +
> > +               error = mas_store_gfp(mas_detach, next, GFP_KERNEL);
> >                 if (error)
> >                         goto munmap_gather_failed;
> >                 vma_mark_detached(next, true);
> > -               if (next->vm_flags & VM_LOCKED)
> > -                       locked_vm += vma_pages(next);
> > -
> > -               count++;
> >                 if (unlikely(uf)) {
> >                         /*
> >                          * If userfaultfd_unmap_prep returns an error the vmas
> > @@ -2753,7 +2749,7 @@ do_vmi_align_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
> >  #if defined(CONFIG_DEBUG_VM_MAPLE_TREE)
> >         /* Make sure no VMAs are about to be lost. */
> >         {
> > -               MA_STATE(test, &mt_detach, 0, 0);
> > +               MA_STATE(test, mas_detach->tree, 0, 0);
> >                 struct vm_area_struct *vma_mas, *vma_test;
> >                 int test_count = 0;
> >
> > @@ -2773,6 +2769,48 @@ do_vmi_align_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
> >         while (vma_iter_addr(vmi) > start)
> >                 vma_iter_prev_range(vmi);
> >
> > +       return 0;
> > +
> > +userfaultfd_error:
> > +munmap_gather_failed:
> > +end_split_failed:
> > +       abort_munmap_vmas(mas_detach);
> > +start_split_failed:
> > +map_count_exceeded:
> > +       return error;
> > +}
> > +
> > +/*
> > + * do_vmi_align_munmap() - munmap the aligned region from @start to @end.
> > + * @vmi: The vma iterator
> > + * @vma: The starting vm_area_struct
> > + * @mm: The mm_struct
> > + * @start: The aligned start address to munmap.
> > + * @end: The aligned end address to munmap.
> > + * @uf: The userfaultfd list_head
> > + * @unlock: Set to true to drop the mmap_lock.  unlocking only happens on
> > + * success.
> > + *
> > + * Return: 0 on success and drops the lock if so directed, error and leaves the
> > + * lock held otherwise.
> > + */
> > +static int
> > +do_vmi_align_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
> > +                   struct mm_struct *mm, unsigned long start,
> > +                   unsigned long end, struct list_head *uf, bool unlock)
> > +{
> > +       struct maple_tree mt_detach;
> > +       MA_STATE(mas_detach, &mt_detach, 0, 0);
> > +       mt_init_flags(&mt_detach, vmi->mas.tree->ma_flags & MT_FLAGS_LOCK_MASK);
> > +       mt_on_stack(mt_detach);
> > +       int error;
> > +       unsigned long locked_vm = 0;
> > +
> > +       error = vmi_gather_munmap_vmas(vmi, vma, mm, start, end, uf,
> > +                                      &mas_detach, &locked_vm);
> > +       if (error)
> > +               goto gather_failed;
> > +
> >         error = vma_iter_clear_gfp(vmi, start, end, GFP_KERNEL);
> >         if (error)
> >                 goto clear_tree_failed;
> > @@ -2783,12 +2821,8 @@ do_vmi_align_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
> >         return 0;
> >
> >  clear_tree_failed:
> > -userfaultfd_error:
> > -munmap_gather_failed:
> > -end_split_failed:
> >         abort_munmap_vmas(&mas_detach);
> > -start_split_failed:
> > -map_count_exceeded:
> > +gather_failed:
> >         validate_mm(mm);
> >         return error;
> >  }
> > --
> > 2.43.0
> >