Re: [PATCH v2 2/6] mm: handle swap page faults under VMA lock if page is uncontended

[Peter Xu <peterx@redhat.com>]

On Thu, Jun 08, 2023 at 05:51:54PM -0700, Suren Baghdasaryan wrote:
> When page fault is handled under VMA lock protection, all swap page
> faults are retried with mmap_lock because folio_lock_or_retry
> implementation has to drop and reacquire mmap_lock if folio could
> not be immediately locked.
> Instead of retrying all swapped page faults, retry only when folio
> locking fails.
> 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.
> Drivers implementing ops->migrate_to_ram might still rely on mmap_lock,
> therefore fall back to mmap_lock in this case.
> 
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> ---
>  mm/filemap.c |  6 ++++++
>  mm/memory.c  | 14 +++++++++-----
>  2 files changed, 15 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/filemap.c b/mm/filemap.c
> index b4c9bd368b7e..7cb0a3776a07 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -1706,6 +1706,8 @@ static int __folio_lock_async(struct folio *folio, struct wait_page_queue *wait)
>   *     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.
> + *     If flags had FAULT_FLAG_VMA_LOCK set, meaning the operation is performed
> + *     with VMA lock only, the VMA lock is still held.
>   *
>   * If neither ALLOW_RETRY nor KILLABLE are set, will always return true
>   * with the folio locked and the mmap_lock unperturbed.
> @@ -1713,6 +1715,10 @@ static int __folio_lock_async(struct folio *folio, struct wait_page_queue *wait)
>  bool __folio_lock_or_retry(struct folio *folio, struct mm_struct *mm,
>  			 unsigned int flags)
>  {
> +	/* Can't do this if not holding mmap_lock */
> +	if (flags & FAULT_FLAG_VMA_LOCK)
> +		return false;

If here what we need is the page lock, can we just conditionally release
either mmap lock or vma lock depending on FAULT_FLAG_VMA_LOCK?

-- 
Peter Xu