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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Thu, 29 Jun 2023 18:30:41 -0700
Message-ID: <CAJuCfpEujwtcKUE9cZ-_k3=DJqZ93yPgD+s4rXj88Z3umF_Y2g@mail.gmail.com>
Subject: Re: [PATCH v5 5/6] mm: handle swap page faults under per-VMA lock
To:     Alistair Popple <apopple@nvidia.com>
Cc:     akpm@linux-foundation.org, willy@infradead.org, hannes@cmpxchg.org,
        mhocko@suse.com, josef@toxicpanda.com, jack@suse.cz,
        ldufour@linux.ibm.com, laurent.dufour@fr.ibm.com,
        michel@lespinasse.org, liam.howlett@oracle.com, jglisse@google.com,
        vbabka@suse.cz, minchan@google.com, dave@stgolabs.net,
        punit.agrawal@bytedance.com, lstoakes@gmail.com, hdanton@sina.com,
        peterx@redhat.com, ying.huang@intel.com, david@redhat.com,
        yuzhao@google.com, dhowells@redhat.com, hughd@google.com,
        viro@zeniv.linux.org.uk, brauner@kernel.org,
        pasha.tatashin@soleen.com, linux-mm@kvack.org,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel-team@android.com
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On Wed, Jun 28, 2023 at 11:06=E2=80=AFPM Alistair Popple <apopple@nvidia.co=
m> wrote:
>
>
> Looks good and passed the HMM selftests. So:
>
> Tested-by: Alistair Popple <apopple@nvidia.com>
> Reviewed-by: Alistair Popple <apopple@nvidia.com>

Thanks!

>
> Suren Baghdasaryan <surenb@google.com> writes:
>
> > When page fault is handled under per-VMA lock protection, all swap page
> > faults are retried with mmap_lock because folio_lock_or_retry has to dr=
op
> > and reacquire mmap_lock if folio could not be immediately locked.
> > Follow the same pattern as mmap_lock to drop per-VMA lock when waiting
> > for folio and retrying once folio is available.
> > With this obstacle removed, enable do_swap_page to operate under
> > per-VMA lock protection. Drivers implementing ops->migrate_to_ram might
> > still rely on mmap_lock, therefore we have to fall back to mmap_lock in
> > that particular case.
> > Note that the only time do_swap_page calls synchronous swap_readpage
> > is when SWP_SYNCHRONOUS_IO is set, which is only set for
> > QUEUE_FLAG_SYNCHRONOUS devices: brd, zram and nvdimms (both btt and
> > pmem). Therefore we don't sleep in this path, and there's no need to
> > drop the mmap or per-VMA lock.
> >
> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > Acked-by: Peter Xu <peterx@redhat.com>
> > ---
> >  include/linux/mm.h | 13 +++++++++++++
> >  mm/filemap.c       | 17 ++++++++---------
> >  mm/memory.c        | 16 ++++++++++------
> >  3 files changed, 31 insertions(+), 15 deletions(-)
> >
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index fec149585985..bbaec479bf98 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -723,6 +723,14 @@ static inline void vma_mark_detached(struct vm_are=
a_struct *vma, bool detached)
> >  struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
> >                                         unsigned long address);
> >
> > +static inline void release_fault_lock(struct vm_fault *vmf)
> > +{
> > +     if (vmf->flags & FAULT_FLAG_VMA_LOCK)
> > +             vma_end_read(vmf->vma);
> > +     else
> > +             mmap_read_unlock(vmf->vma->vm_mm);
> > +}
> > +
> >  #else /* CONFIG_PER_VMA_LOCK */
> >
> >  static inline void vma_init_lock(struct vm_area_struct *vma) {}
> > @@ -736,6 +744,11 @@ static inline void vma_assert_write_locked(struct =
vm_area_struct *vma) {}
> >  static inline void vma_mark_detached(struct vm_area_struct *vma,
> >                                    bool detached) {}
> >
> > +static inline void release_fault_lock(struct vm_fault *vmf)
> > +{
> > +     mmap_read_unlock(vmf->vma->vm_mm);
> > +}
> > +
> >  #endif /* CONFIG_PER_VMA_LOCK */
> >
> >  /*
> > diff --git a/mm/filemap.c b/mm/filemap.c
> > index 52bcf12dcdbf..d4d8f474e0c5 100644
> > --- a/mm/filemap.c
> > +++ b/mm/filemap.c
> > @@ -1703,27 +1703,26 @@ static int __folio_lock_async(struct folio *fol=
io, struct wait_page_queue *wait)
> >   * Return values:
> >   * 0 - folio is locked.
> >   * VM_FAULT_RETRY - folio is not locked.
> > - *     mmap_lock has been released (mmap_read_unlock(), unless flags h=
ad both
> > - *     FAULT_FLAG_ALLOW_RETRY and FAULT_FLAG_RETRY_NOWAIT set, in
> > - *     which case mmap_lock is still held.
> > + *     mmap_lock or per-VMA lock has been released (mmap_read_unlock()=
 or
> > + *     vma_end_read()), unless flags had both FAULT_FLAG_ALLOW_RETRY a=
nd
> > + *     FAULT_FLAG_RETRY_NOWAIT set, in which case the lock is still he=
ld.
> >   *
> >   * If neither ALLOW_RETRY nor KILLABLE are set, will always return 0
> > - * with the folio locked and the mmap_lock unperturbed.
> > + * with the folio locked and the mmap_lock/per-VMA lock is left unpert=
urbed.
> >   */
> >  vm_fault_t __folio_lock_or_retry(struct folio *folio, struct vm_fault =
*vmf)
> >  {
> > -     struct mm_struct *mm =3D vmf->vma->vm_mm;
> >       unsigned int flags =3D vmf->flags;
> >
> >       if (fault_flag_allow_retry_first(flags)) {
> >               /*
> > -              * CAUTION! In this case, mmap_lock is not released
> > -              * even though return VM_FAULT_RETRY.
> > +              * CAUTION! In this case, mmap_lock/per-VMA lock is not
> > +              * released even though returning VM_FAULT_RETRY.
> >                */
> >               if (flags & FAULT_FLAG_RETRY_NOWAIT)
> >                       return VM_FAULT_RETRY;
> >
> > -             mmap_read_unlock(mm);
> > +             release_fault_lock(vmf);
> >               if (flags & FAULT_FLAG_KILLABLE)
> >                       folio_wait_locked_killable(folio);
> >               else
> > @@ -1735,7 +1734,7 @@ vm_fault_t __folio_lock_or_retry(struct folio *fo=
lio, struct vm_fault *vmf)
> >
> >               ret =3D __folio_lock_killable(folio);
> >               if (ret) {
> > -                     mmap_read_unlock(mm);
> > +                     release_fault_lock(vmf);
> >                       return VM_FAULT_RETRY;
> >               }
> >       } else {
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 345080052003..4fb8ecfc6d13 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -3712,12 +3712,6 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >       if (!pte_unmap_same(vmf))
> >               goto out;
> >
> > -     if (vmf->flags & FAULT_FLAG_VMA_LOCK) {
> > -             ret =3D VM_FAULT_RETRY;
> > -             vma_end_read(vma);
> > -             goto out;
> > -     }
> > -
> >       entry =3D pte_to_swp_entry(vmf->orig_pte);
> >       if (unlikely(non_swap_entry(entry))) {
> >               if (is_migration_entry(entry)) {
> > @@ -3727,6 +3721,16 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >                       vmf->page =3D pfn_swap_entry_to_page(entry);
> >                       ret =3D remove_device_exclusive_entry(vmf);
> >               } else if (is_device_private_entry(entry)) {
> > +                     if (vmf->flags & FAULT_FLAG_VMA_LOCK) {
> > +                             /*
> > +                              * migrate_to_ram is not yet ready to ope=
rate
> > +                              * under VMA lock.
> > +                              */
> > +                             vma_end_read(vma);
> > +                             ret =3D VM_FAULT_RETRY;
> > +                             goto out;
> > +                     }
> > +
> >                       vmf->page =3D pfn_swap_entry_to_page(entry);
> >                       vmf->pte =3D pte_offset_map_lock(vma->vm_mm, vmf-=
>pmd,
> >                                       vmf->address, &vmf->ptl);
>