On Fri, Apr 14, 2023 at 11:00 AM Suren Baghdasaryan <surenb@xxxxxxxxxx> 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.
I just realized that the title of the patch is misleading. It's about
handling page fault under VMA lock. A better title would be something
like:
"mm: handle swap page faults under vma lock if page is uncontended"
>
> Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> ---
> Patch applies cleanly over linux-next and mm-unstable
>
> mm/filemap.c | 6 ++++++
> mm/memory.c | 5 -----
> 2 files changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 6f3a7e53fccf..67b937b0f436 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 (fault_flag_allow_retry_first(flags)) {
> /*
> * CAUTION! In this case, mmap_lock is not released
> diff --git a/mm/memory.c b/mm/memory.c
> index d88f370eacd1..3301a8d01820 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3715,11 +3715,6 @@ 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;
> - goto out;
> - }
> -
> entry = pte_to_swp_entry(vmf->orig_pte);
> if (unlikely(non_swap_entry(entry))) {
> if (is_migration_entry(entry)) {
> --
> 2.40.0.634.g4ca3ef3211-goog
>
