Re: [PATCH v5 5/6] mm: handle swap page faults under per-VMA lock

Suren Baghdasaryan
2023-06-30T01:31:03Z

On Wed, Jun 28, 2023 at 11:06 PM Alistair Popple <apopple@nvidia.com> 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 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@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_area_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 *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.
> > + * 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 +1734,7 @@ 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..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 = 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;
> > + 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);
>