Re: [PATCH v3 6/8] mm: handle swap page faults under per-VMA lock

[Suren Baghdasaryan <surenb@google.com>]

On Tue, Jun 27, 2023 at 8:41 AM Peter Xu <peterx@redhat.com> wrote:
>
> On Mon, Jun 26, 2023 at 09:23:19PM -0700, Suren Baghdasaryan wrote:
> > When page fault is handled under per-VMA lock protection, all swap page
> > faults are retried with mmap_lock because folio_lock_fault (formerly
> > known as folio_lock_or_retry) had to drop 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 in folio_lock_fault 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>
> > ---
> >  mm/filemap.c | 24 ++++++++++++++++--------
> >  mm/memory.c  | 21 ++++++++++++++-------
> >  2 files changed, 30 insertions(+), 15 deletions(-)
> >
> > diff --git a/mm/filemap.c b/mm/filemap.c
> > index 8ad06d69895b..683f11f244cd 100644
> > --- a/mm/filemap.c
> > +++ b/mm/filemap.c
> > @@ -1703,12 +1703,14 @@ static int __folio_lock_async(struct folio *folio, 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 had both
> > - *     FAULT_FLAG_ALLOW_RETRY and FAULT_FLAG_RETRY_NOWAIT set, in
> > - *     which case mmap_lock is still held.
> > + *     FAULT_FLAG_LOCK_DROPPED bit in vmf flags will be set if mmap_lock or
>
> This "FAULT_FLAG_LOCK_DROPPED" should belong to that patch when introduced.
> But again I still think this flag as a whole with that patch is not needed
> and should be dropped, unless I miss something important..
>
> > + *     per-VMA lock got dropped. mmap_lock/per-VMA lock is dropped when
> > + *     function fails to lock the folio, unless flags had both
> > + *     FAULT_FLAG_ALLOW_RETRY and FAULT_FLAG_RETRY_NOWAIT set, in which case
> > + *     the lock is still held.
> >   *
> >   * 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 unperturbed.
> >   */
> >  vm_fault_t __folio_lock_fault(struct folio *folio, struct vm_fault *vmf)
> >  {
> > @@ -1716,13 +1718,16 @@ vm_fault_t __folio_lock_fault(struct folio *folio, struct vm_fault *vmf)
> >
> >       if (fault_flag_allow_retry_first(vmf->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 (vmf->flags & FAULT_FLAG_RETRY_NOWAIT)
> >                       return VM_FAULT_RETRY;
> >
> > -             mmap_read_unlock(mm);
> > +             if (vmf->flags & FAULT_FLAG_VMA_LOCK)
> > +                     vma_end_read(vmf->vma);
> > +             else
> > +                     mmap_read_unlock(mm);
> >               vmf->flags |= FAULT_FLAG_LOCK_DROPPED;
> >               if (vmf->flags & FAULT_FLAG_KILLABLE)
> >                       folio_wait_locked_killable(folio);
> > @@ -1735,7 +1740,10 @@ vm_fault_t __folio_lock_fault(struct folio *folio, struct vm_fault *vmf)
> >
> >               ret = __folio_lock_killable(folio);
> >               if (ret) {
> > -                     mmap_read_unlock(mm);
> > +                     if (vmf->flags & FAULT_FLAG_VMA_LOCK)
> > +                             vma_end_read(vmf->vma);
> > +                     else
> > +                             mmap_read_unlock(mm);
> >                       vmf->flags |= FAULT_FLAG_LOCK_DROPPED;
> >                       return VM_FAULT_RETRY;
> >               }
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 3c2acafcd7b6..5caaa4c66ea2 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -3712,11 +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) {
>
> So if with my imagination, here we'll already have the vma_read_end() and
> this patch will remove it, which makes sense.  Then...
>
> > -             ret = VM_FAULT_RETRY;
> > -             goto out;
> > -     }
> > -
> >       entry = pte_to_swp_entry(vmf->orig_pte);
> >       if (unlikely(non_swap_entry(entry))) {
> >               if (is_migration_entry(entry)) {
> > @@ -3726,6 +3721,15 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >                       vmf->page = pfn_swap_entry_to_page(entry);
> >                       ret = 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 operate
> > +                              * under VMA lock.
> > +                              */
>
> ... here we probably just do vma_read_end(), then...
>
> > +                             ret |= VM_FAULT_RETRY;
> > +                             goto out;
> > +                     }
> > +
> >                       vmf->page = pfn_swap_entry_to_page(entry);
> >                       vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
> >                                       vmf->address, &vmf->ptl);
> > @@ -5089,9 +5093,12 @@ static vm_fault_t __handle_mm_fault(struct vm_area_struct *vma,
> >               /*
> >                * In case of VM_FAULT_RETRY or VM_FAULT_COMPLETED we might
> >                * be still holding per-VMA lock to keep the vma stable as long
> > -              * as possible. Drop it before returning.
> > +              * as possible. In this situation vmf.flags has
> > +              * FAULT_FLAG_VMA_LOCK set and FAULT_FLAG_LOCK_DROPPED unset.
> > +              * Drop the lock before returning when this happens.
> >                */
> > -             if (vmf.flags & FAULT_FLAG_VMA_LOCK)
> > +             if ((vmf.flags & (FAULT_FLAG_VMA_LOCK | FAULT_FLAG_LOCK_DROPPED)) ==
> > +                 FAULT_FLAG_VMA_LOCK)
> >                       vma_end_read(vma);
>
> This whole chunk should have been dropped altogether with my comment in
> previous patch, iiuc, and it should be no-op anyway for swap case.  For the
> real "waiting for page lock during swapin" phase we should always 100%
> release the vma lock in folio_lock_or_retry() - just like mmap lock.

Yep, we drop FAULT_FLAG_LOCK_DROPPED, release vma lock when we return
RETRY and that makes all this unnecessary. I just need to make sure we
do not access VMA after we drop its lock since we will be releasing it
now earlier than before.

>
> Thanks,
>
> >       }
> >       return ret;
> > --
> > 2.41.0.178.g377b9f9a00-goog
> >
>
> --
> Peter Xu
>