linux-kernel - Re: [PATCH 1/1] mm: handle swap page faults if the faulting page can be locked

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Date:   Fri, 14 Apr 2023 11:32:08 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     akpm@...ux-foundation.org
Cc:     willy@...radead.org, hannes@...xchg.org, mhocko@...e.com,
        josef@...icpanda.com, jack@...e.cz, ldufour@...ux.ibm.com,
        laurent.dufour@...ibm.com, michel@...pinasse.org,
        liam.howlett@...cle.com, jglisse@...gle.com, vbabka@...e.cz,
        minchan@...gle.com, dave@...olabs.net, punit.agrawal@...edance.com,
        lstoakes@...il.com, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com
Subject: Re: [PATCH 1/1] mm: handle swap page faults if the faulting page can
 be locked

On Fri, Apr 14, 2023 at 11:00 AM Suren Baghdasaryan <surenb@...gle.com> 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@...gle.com>
> ---
> 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
>