linux-kernel - Re: [PATCH v5] mm: shrink skip folio mapped by an exiting process

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Message-ID: <CAGsJ_4zrC6Ma82OcOM+yMH7Tj-9oUO5Lbv7hCejH7dQqqO_Q6w@mail.gmail.com>
Date: Mon, 8 Jul 2024 22:05:37 +1200
From: Barry Song <baohua@...nel.org>
To: David Hildenbrand <david@...hat.com>
Cc: Zhiguo Jiang <justinjiang@...o.com>, Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, opensource.kernel@...o.com
Subject: Re: [PATCH v5] mm: shrink skip folio mapped by an exiting process

On Mon, Jul 8, 2024 at 9:49 PM David Hildenbrand <david@...hat.com> wrote:
>
> On 08.07.24 11:46, Barry Song wrote:
> > On Mon, Jul 8, 2024 at 9:36 PM David Hildenbrand <david@...hat.com> wrote:
> >>
> >> On 08.07.24 11:04, Zhiguo Jiang wrote:
> >>> The releasing process of the non-shared anonymous folio mapped solely by
> >>> an exiting process may go through two flows: 1) the anonymous folio is
> >>> firstly is swaped-out into swapspace and transformed into a swp_entry
> >>> in shrink_folio_list; 2) then the swp_entry is released in the process
> >>> exiting flow. This will increase the cpu load of releasing a non-shared
> >>> anonymous folio mapped solely by an exiting process, because the folio
> >>> go through swap-out and the releasing the swapspace and swp_entry.
> >>>
> >>> When system is low memory, it is more likely to occur, because more
> >>> backend applidatuions will be killed.
> >>>
> >>> The modification is that shrink skips the non-shared anonymous folio
> >>> solely mapped by an exting process and the folio is only released
> >>> directly in the process exiting flow, which will save swap-out time
> >>> and alleviate the load of the process exiting.
> >>>
> >>> Signed-off-by: Zhiguo Jiang <justinjiang@...o.com>
> >>> ---
> >>>
> >>> Change log:
> >>> v4->v5:
> >>> 1.Modify to skip non-shared anonymous folio only.
> >>> 2.Update comments for pra->referenced = -1.
> >>> v3->v4:
> >>> 1.Modify that the unshared folios mapped only in exiting task are skip.
> >>> v2->v3:
> >>> Nothing.
> >>> v1->v2:
> >>> 1.The VM_EXITING added in v1 patch is removed, because it will fail
> >>> to compile in 32-bit system.
> >>>
> >>>    mm/rmap.c   | 13 +++++++++++++
> >>>    mm/vmscan.c |  7 ++++++-
> >>>    2 files changed, 19 insertions(+), 1 deletion(-)
> >>>
> >>> diff --git a/mm/rmap.c b/mm/rmap.c
> >>> index 26806b49a86f..5b5281d71dbb
> >>> --- a/mm/rmap.c
> >>> +++ b/mm/rmap.c
> >>> @@ -843,6 +843,19 @@ static bool folio_referenced_one(struct folio *folio,
> >>>        int referenced = 0;
> >>>        unsigned long start = address, ptes = 0;
> >>>
> >>> +     /*
> >>> +      * Skip the non-shared anonymous folio mapped solely by
> >>> +      * the single exiting process, and release it directly
> >>> +      * in the process exiting.
> >>> +      */
> >>> +     if ((!atomic_read(&vma->vm_mm->mm_users) ||
> >>> +             test_bit(MMF_OOM_SKIP, &vma->vm_mm->flags)) &&
> >>> +             folio_test_anon(folio) && folio_test_swapbacked(folio) &&
> >>> +             !folio_likely_mapped_shared(folio)) {
> >>
> >> I'm currently working on moving all folio_likely_mapped_shared() under
> >> the PTL, where we are then sure that the folio is actually mapped by
> >> this process (e.g., no concurrent unmapping poisslbe).
> >>
> >> Can we do the same here directly?
> >
> > Implementing this is challenging because page_vma_mapped_walk() is
> > responsible for
> > traversing the page table to acquire and release the PTL. This becomes
> > particularly
> > complex with mTHP, as we may need to interrupt the page_vma_mapped_walk
> > loop at the first PTE.
>
> Why can't we perform the check under the PTL and bail out? I'm missing
> something important.

You are right. we might use page_vma_mapped_walk_done() to bail out.

>
> --
> Cheers,
>
> David / dhildenb
>