Re: [PATCH v4 5/6] mm: handle swap page faults under per-VMA lock

[Suren Baghdasaryan]

On Wed, Jun 28, 2023 at 6:43 AM Peter Xu <peterx@xxxxxxxxxx> wrote:
>
> On Wed, Jun 28, 2023 at 12:17:59AM -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_or_retry has 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 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@xxxxxxxxxx>
>
> Acked-by: Peter Xu <peterx@xxxxxxxxxx>
>
> One nit below:
>
> > ---
> >  mm/filemap.c | 25 ++++++++++++++++---------
> >  mm/memory.c  | 16 ++++++++++------
> >  2 files changed, 26 insertions(+), 15 deletions(-)
> >
> > diff --git a/mm/filemap.c b/mm/filemap.c
> > index 52bcf12dcdbf..7ee078e1a0d2 100644
> > --- a/mm/filemap.c
> > +++ b/mm/filemap.c
> > @@ -1699,31 +1699,38 @@ static int __folio_lock_async(struct folio *folio, struct wait_page_queue *wait)
> >       return ret;
> >  }
> >
> > +static 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);
> > +}
> > +
> >  /*
> >   * 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.
> > + *     mmap_lock or per-VMA lock has been released (mmap_read_unlock() or
> > + *     vma_end_read()), 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 is left unperturbed.
> >   */
> >  vm_fault_t __folio_lock_or_retry(struct folio *folio, struct vm_fault *vmf)
> >  {
> > -     struct mm_struct *mm = vmf->vma->vm_mm;
> >       unsigned int flags = 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 +1742,7 @@ vm_fault_t __folio_lock_or_retry(struct folio *folio, struct vm_fault *vmf)
> >
> >               ret = __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..76c7907e7286 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 = VM_FAULT_RETRY;
> > -             vma_end_read(vma);
> > -             goto out;
> > -     }
> > -
> >       entry = 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 = 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.
> > +                              */
> > +                             vma_end_read(vma);
> > +                             ret |= VM_FAULT_RETRY;
>
> Here IIUC ret==0 is guaranteed, so maybe "ret = VM_FAULT_RETRY" is slightly
> clearer.

Ack.

>
> > +                             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);
> > --
> > 2.41.0.162.gfafddb0af9-goog
> >
>
> --
> Peter Xu
>
> --
> To unsubscribe from this group and stop receiving emails from it, send an email to kernel-team+unsubscribe@xxxxxxxxxxx.
>