Re: [PATCH v3 8/8] mm: handle userfaults under VMA lock

[Suren Baghdasaryan <surenb@google.com>]

On Tue, Jun 27, 2023 at 8:54 AM Peter Xu <peterx@redhat.com> wrote:
>
> On Mon, Jun 26, 2023 at 09:23:21PM -0700, Suren Baghdasaryan wrote:
> > Enable handle_userfault to operate under VMA lock by releasing VMA lock
> > instead of mmap_lock and retrying.
>
> This mostly good to me (besides the new DROP flag.. of course), thanks.
> Still some nitpicks below.
>
> >
> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > ---
> >  fs/userfaultfd.c | 42 ++++++++++++++++++++++--------------------
> >  mm/memory.c      |  9 ---------
> >  2 files changed, 22 insertions(+), 29 deletions(-)
> >
> > diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> > index 4e800bb7d2ab..b88632c404b6 100644
> > --- a/fs/userfaultfd.c
> > +++ b/fs/userfaultfd.c
> > @@ -277,17 +277,17 @@ static inline struct uffd_msg userfault_msg(unsigned long address,
> >   * hugepmd ranges.
> >   */
> >  static inline bool userfaultfd_huge_must_wait(struct userfaultfd_ctx *ctx,
> > -                                      struct vm_area_struct *vma,
> > -                                      unsigned long address,
> > -                                      unsigned long flags,
> > -                                      unsigned long reason)
> > +                                           struct vm_fault *vmf,
> > +                                           unsigned long reason)
> >  {
> > +     struct vm_area_struct *vma = vmf->vma;
> >       pte_t *ptep, pte;
> >       bool ret = true;
> >
> > -     mmap_assert_locked(ctx->mm);
> > +     if (!(vmf->flags & FAULT_FLAG_VMA_LOCK))
> > +             mmap_assert_locked(ctx->mm);
>
> Maybe we can have a helper asserting proper vma protector locks (mmap for
> !VMA_LOCK and vma read lock for VMA_LOCK)?  It basically tells the context
> the vma is still safe to access.
>
> >
> > -     ptep = hugetlb_walk(vma, address, vma_mmu_pagesize(vma));
> > +     ptep = hugetlb_walk(vma, vmf->address, vma_mmu_pagesize(vma));
> >       if (!ptep)
> >               goto out;
> >
> > @@ -308,10 +308,8 @@ static inline bool userfaultfd_huge_must_wait(struct userfaultfd_ctx *ctx,
> >  }
> >  #else
> >  static inline bool userfaultfd_huge_must_wait(struct userfaultfd_ctx *ctx,
> > -                                      struct vm_area_struct *vma,
> > -                                      unsigned long address,
> > -                                      unsigned long flags,
> > -                                      unsigned long reason)
> > +                                           struct vm_fault *vmf,
> > +                                           unsigned long reason)
> >  {
> >       return false;   /* should never get here */
> >  }
> > @@ -325,11 +323,11 @@ static inline bool userfaultfd_huge_must_wait(struct userfaultfd_ctx *ctx,
> >   * threads.
> >   */
> >  static inline bool userfaultfd_must_wait(struct userfaultfd_ctx *ctx,
> > -                                      unsigned long address,
> > -                                      unsigned long flags,
> > +                                      struct vm_fault *vmf,
> >                                        unsigned long reason)
> >  {
> >       struct mm_struct *mm = ctx->mm;
> > +     unsigned long address = vmf->address;
> >       pgd_t *pgd;
> >       p4d_t *p4d;
> >       pud_t *pud;
> > @@ -337,7 +335,8 @@ static inline bool userfaultfd_must_wait(struct userfaultfd_ctx *ctx,
> >       pte_t *pte;
> >       bool ret = true;
> >
> > -     mmap_assert_locked(mm);
> > +     if (!(vmf->flags & FAULT_FLAG_VMA_LOCK))
> > +             mmap_assert_locked(mm);
>
> (the assert helper can also be used here)
>
> >
> >       pgd = pgd_offset(mm, address);
> >       if (!pgd_present(*pgd))
> > @@ -445,7 +444,8 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
> >        * Coredumping runs without mmap_lock so we can only check that
> >        * the mmap_lock is held, if PF_DUMPCORE was not set.
> >        */
> > -     mmap_assert_locked(mm);
> > +     if (!(vmf->flags & FAULT_FLAG_VMA_LOCK))
> > +             mmap_assert_locked(mm);
> >
> >       ctx = vma->vm_userfaultfd_ctx.ctx;
> >       if (!ctx)
> > @@ -561,15 +561,17 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
> >       spin_unlock_irq(&ctx->fault_pending_wqh.lock);
> >
> >       if (!is_vm_hugetlb_page(vma))
> > -             must_wait = userfaultfd_must_wait(ctx, vmf->address, vmf->flags,
> > -                                               reason);
> > +             must_wait = userfaultfd_must_wait(ctx, vmf, reason);
> >       else
> > -             must_wait = userfaultfd_huge_must_wait(ctx, vma,
> > -                                                    vmf->address,
> > -                                                    vmf->flags, reason);
> > +             must_wait = userfaultfd_huge_must_wait(ctx, vmf, reason);
> >       if (is_vm_hugetlb_page(vma))
> >               hugetlb_vma_unlock_read(vma);
> > -     mmap_read_unlock(mm);
> > +     if (vmf->flags & FAULT_FLAG_VMA_LOCK) {
> > +             /* WARNING: VMA can't be used after this */
> > +             vma_end_read(vma);
> > +     } else
> > +             mmap_read_unlock(mm);
>
> I also think maybe we should have a helper mm_release_fault_lock() just
> release different locks for with/without VMA_LOCK.  It can also be used in
> the other patch of folio_lock_or_retry().

All seem to be good suggestions. I'll try implementing them in the
next version. Thanks!

>
> > +     vmf->flags |= FAULT_FLAG_LOCK_DROPPED;
> >
> >       if (likely(must_wait && !READ_ONCE(ctx->released))) {
> >               wake_up_poll(&ctx->fd_wqh, EPOLLIN);
> > diff --git a/mm/memory.c b/mm/memory.c
> > index bdf46fdc58d6..923c1576bd14 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -5316,15 +5316,6 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
> >       if (!vma_start_read(vma))
> >               goto inval;
> >
> > -     /*
> > -      * Due to the possibility of userfault handler dropping mmap_lock, avoid
> > -      * it for now and fall back to page fault handling under mmap_lock.
> > -      */
> > -     if (userfaultfd_armed(vma)) {
> > -             vma_end_read(vma);
> > -             goto inval;
> > -     }
> > -
> >       /* Check since vm_start/vm_end might change before we lock the VMA */
> >       if (unlikely(address < vma->vm_start || address >= vma->vm_end)) {
> >               vma_end_read(vma);
> > --
> > 2.41.0.178.g377b9f9a00-goog
> >
>
> --
> Peter Xu
>
> --
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