Message-ID: <CAGsJ_4w4vDu4p-ALrTSKMZhGKDK6TpyDLrAyvY4vkPJEJ3N5Lw@mail.gmail.com>
Date: Tue, 7 May 2024 20:43:57 +1200
From: Barry Song <21cnbao@...il.com>
To: David Hildenbrand <david@...hat.com>
Cc: Ryan Roberts <ryan.roberts@....com>, akpm@...ux-foundation.org, linux-mm@...ck.org, 
	baolin.wang@...ux.alibaba.com, chrisl@...nel.org, hanchuanhua@...o.com, 
	hannes@...xchg.org, hughd@...gle.com, kasong@...cent.com, 
	linux-kernel@...r.kernel.org, surenb@...gle.com, v-songbaohua@...o.com, 
	willy@...radead.org, xiang@...nel.org, ying.huang@...el.com, 
	yosryahmed@...gle.com, yuzhao@...gle.com, ziy@...dia.com
Subject: Re: [PATCH v3 6/6] mm: swap: entirely map large folios found in swapcache

On Tue, May 7, 2024 at 8:24 PM David Hildenbrand <david@...hat.com> wrote:
>
> On 07.05.24 00:58, Barry Song wrote:
> > On Tue, May 7, 2024 at 1:16 AM David Hildenbrand <david@...hat.com> wrote:
> >>
> >> On 06.05.24 14:58, Barry Song wrote:
> >>> On Tue, May 7, 2024 at 12:38 AM Barry Song <21cnbao@...il.com> wrote:
> >>>>
> >>>> On Tue, May 7, 2024 at 12:07 AM David Hildenbrand <david@...hat.com> wrote:
> >>>>>
> >>>>> On 04.05.24 01:23, Barry Song wrote:
> >>>>>> On Fri, May 3, 2024 at 6:50 PM Ryan Roberts <ryan.roberts@....com> wrote:
> >>>>>>>
> >>>>>>> On 03/05/2024 01:50, Barry Song wrote:
> >>>>>>>> From: Chuanhua Han <hanchuanhua@...o.com>
> >>>>>>>>
> >>>>>>>> When a large folio is found in the swapcache, the current implementation
> >>>>>>>> requires calling do_swap_page() nr_pages times, resulting in nr_pages
> >>>>>>>> page faults. This patch opts to map the entire large folio at once to
> >>>>>>>> minimize page faults. Additionally, redundant checks and early exits
> >>>>>>>> for ARM64 MTE restoring are removed.
> >>>>>>>>
> >>>>>>>> Signed-off-by: Chuanhua Han <hanchuanhua@...o.com>
> >>>>>>>> Co-developed-by: Barry Song <v-songbaohua@...o.com>
> >>>>>>>> Signed-off-by: Barry Song <v-songbaohua@...o.com>
> >>>>>>>
> >>>>>>> With the suggested changes below:
> >>>>>>>
> >>>>>>> Reviewed-by: Ryan Roberts <ryan.roberts@....com>
> >>>>>>>
> >>>>>>>> ---
> >>>>>>>>     mm/memory.c | 60 ++++++++++++++++++++++++++++++++++++++++++-----------
> >>>>>>>>     1 file changed, 48 insertions(+), 12 deletions(-)
> >>>>>>>>
> >>>>>>>> diff --git a/mm/memory.c b/mm/memory.c
> >>>>>>>> index 22e7c33cc747..940fdbe69fa1 100644
> >>>>>>>> --- a/mm/memory.c
> >>>>>>>> +++ b/mm/memory.c
> >>>>>>>> @@ -3968,6 +3968,10 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >>>>>>>>          pte_t pte;
> >>>>>>>>          vm_fault_t ret = 0;
> >>>>>>>>          void *shadow = NULL;
> >>>>>>>> +     int nr_pages = 1;
> >>>>>>>> +     unsigned long page_idx = 0;
> >>>>>>>> +     unsigned long address = vmf->address;
> >>>>>>>> +     pte_t *ptep;
> >>>>>>>
> >>>>>>> nit: Personally I'd prefer all these to get initialised just before the "if
> >>>>>>> (folio_test_large()..." block below. That way it is clear they are fresh (incase
> >>>>>>> any logic between here and there makes an adjustment) and its clear that they
> >>>>>>> are only to be used after that block (the compiler will warn if using an
> >>>>>>> uninitialized value).
> >>>>>>
> >>>>>> right. I agree this will make the code more readable.
> >>>>>>
> >>>>>>>
> >>>>>>>>
> >>>>>>>>          if (!pte_unmap_same(vmf))
> >>>>>>>>                  goto out;
> >>>>>>>> @@ -4166,6 +4170,36 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >>>>>>>>                  goto out_nomap;
> >>>>>>>>          }
> >>>>>>>>
> >>>>>>>> +     ptep = vmf->pte;
> >>>>>>>> +     if (folio_test_large(folio) && folio_test_swapcache(folio)) {
> >>>>>>>> +             int nr = folio_nr_pages(folio);
> >>>>>>>> +             unsigned long idx = folio_page_idx(folio, page);
> >>>>>>>> +             unsigned long folio_start = vmf->address - idx * PAGE_SIZE;
> >>>>>>>> +             unsigned long folio_end = folio_start + nr * PAGE_SIZE;
> >>>>>>>> +             pte_t *folio_ptep;
> >>>>>>>> +             pte_t folio_pte;
> >>>>>>>> +
> >>>>>>>> +             if (unlikely(folio_start < max(vmf->address & PMD_MASK, vma->vm_start)))
> >>>>>>>> +                     goto check_folio;
> >>>>>>>> +             if (unlikely(folio_end > pmd_addr_end(vmf->address, vma->vm_end)))
> >>>>>>>> +                     goto check_folio;
> >>>>>>>> +
> >>>>>>>> +             folio_ptep = vmf->pte - idx;
> >>>>>>>> +             folio_pte = ptep_get(folio_ptep);
> >>>>>>>> +             if (!pte_same(folio_pte, pte_move_swp_offset(vmf->orig_pte, -idx)) ||
> >>>>>>>> +                 swap_pte_batch(folio_ptep, nr, folio_pte) != nr)
> >>>>>>>> +                     goto check_folio;
> >>>>>>>> +
> >>>>>>>> +             page_idx = idx;
> >>>>>>>> +             address = folio_start;
> >>>>>>>> +             ptep = folio_ptep;
> >>>>>>>> +             nr_pages = nr;
> >>>>>>>> +             entry = folio->swap;
> >>>>>>>> +             page = &folio->page;
> >>>>>>>> +     }
> >>>>>>>> +
> >>>>>>>> +check_folio:
> >>>>>>>
> >>>>>>> Is this still the correct label name, given the checks are now above the new
> >>>>>>> block? Perhaps "one_page" or something like that?
> >>>>>>
> >>>>>> not quite sure about this, as the code after one_page can be multiple_pages.
> >>>>>> On the other hand, it seems we are really checking folio after "check_folio"
> >>>>>> :-)
> >>>>>>
> >>>>>>
> >>>>>> BUG_ON(!folio_test_anon(folio) && folio_test_mappedtodisk(folio));
> >>>>>> BUG_ON(folio_test_anon(folio) && PageAnonExclusive(page));
> >>>>>>
> >>>>>> /*
> >>>>>> * Check under PT lock (to protect against concurrent fork() sharing
> >>>>>> * the swap entry concurrently) for certainly exclusive pages.
> >>>>>> */
> >>>>>> if (!folio_test_ksm(folio)) {
> >>>>>>
> >>>>>>
> >>>>>>>
> >>>>>>>> +
> >>>>>>>>          /*
> >>>>>>>>           * PG_anon_exclusive reuses PG_mappedtodisk for anon pages. A swap pte
> >>>>>>>>           * must never point at an anonymous page in the swapcache that is
> >>>>>>>> @@ -4225,12 +4259,13 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >>>>>>>>           * We're already holding a reference on the page but haven't mapped it
> >>>>>>>>           * yet.
> >>>>>>>>           */
> >>>>>>>> -     swap_free_nr(entry, 1);
> >>>>>>>> +     swap_free_nr(entry, nr_pages);
> >>>>>>>>          if (should_try_to_free_swap(folio, vma, vmf->flags))
> >>>>>>>>                  folio_free_swap(folio);
> >>>>>>>>
> >>>>>>>> -     inc_mm_counter(vma->vm_mm, MM_ANONPAGES);
> >>>>>>>> -     dec_mm_counter(vma->vm_mm, MM_SWAPENTS);
> >>>>>>>> +     folio_ref_add(folio, nr_pages - 1);
> >>>>>>>> +     add_mm_counter(vma->vm_mm, MM_ANONPAGES, nr_pages);
> >>>>>>>> +     add_mm_counter(vma->vm_mm, MM_SWAPENTS, -nr_pages);
> >>>>>>>>          pte = mk_pte(page, vma->vm_page_prot);
> >>>>>>>>
> >>>>>>>>          /*
> >>>>>>>> @@ -4240,34 +4275,35 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >>>>>>>>           * exclusivity.
> >>>>>>>>           */
> >>>>>>>>          if (!folio_test_ksm(folio) &&
> >>>>>>>> -         (exclusive || folio_ref_count(folio) == 1)) {
> >>>>>>>> +         (exclusive || (folio_ref_count(folio) == nr_pages &&
> >>>>>>>> +                        folio_nr_pages(folio) == nr_pages))) {
> >>>>>>>
> >>>>>>> I think in practice there is no change here? If nr_pages > 1 then the folio is
> >>>>>>> in the swapcache, so there is an extra ref on it? I agree with the change for
> >>>>>>> robustness sake. Just checking my understanding.
> >>>>>>
> >>>>>> This is the code showing we are reusing/(mkwrite) a folio either
> >>>>>> 1. we meet a small folio and we are the only one hitting the small folio
> >>>>>> 2. we meet a large folio and we are the only one hitting the large folio
> >>>>>>
> >>>>>> any corner cases besides the above two seems difficult. for example,
> >>>>>>
> >>>>>> while we hit a large folio in swapcache but if we can't entirely map it
> >>>>>> (nr_pages==1) due to partial unmap, we will have folio_ref_count(folio)
> >>>>>> == nr_pages == 1
> >>>>>
> >>>>> No, there would be other references from the swapcache and
> >>>>> folio_ref_count(folio) > 1. See my other reply.
> >>>>
> >>>> right. can be clearer by:
> >>>
> >>> Wait, do we still need folio_nr_pages(folio) == nr_pages even if we use
> >>> folio_ref_count(folio) == 1 and moving folio_ref_add(folio, nr_pages - 1)?
> >>
> >> I don't think that we will "need" it.
> >>
> >>>
> >>> one case is that we have a large folio with 16 PTEs, and we unmap
> >>> 15 swap PTE entries, thus we have only one swap entry left. Then
> >>> we hit the large folio in swapcache.  but we have only one PTE thus we will
> >>> map only one PTE. lacking folio_nr_pages(folio) == nr_pages, we reuse the
> >>> large folio for one PTE. with it, do_wp_page() will migrate the large
> >>> folio to a small one?
> >>
> >> We will set PAE bit and do_wp_page() will unconditionally reuse that page.
> >>
> >> Note that this is the same as if we had pte_swp_exclusive() set and
> >> would have run into "exclusive=true" here.
> >>
> >> If we'd want a similar "optimization" as we have in
> >> wp_can_reuse_anon_folio(), you'd want something like
> >>
> >> exclusive || (folio_ref_count(folio) == 1 &&
> >>                (!folio_test_large(folio) || nr_pages > 1)
> >
> > I feel like
> >
> > A :   !folio_test_large(folio) || nr_pages > 1
> >
> > equals
> >
> > B:    folio_nr_pages(folio) == nr_pages
> >
> > if folio is small,  folio_test_large(folio) is false, both A and B will be true;
> > if folio is large, and we map the whole large folio, A will be true
> > because of nr_pages > 1;
> > B is also true;
> > if folio is large, and we map single one PTE, A will be false;
> > B is also false, because nr_pages == 1 but  folio_nr_pages(folio) > 1;
> >
> > right?
>
> Let's assume a single subpage of a large folio is no longer mapped.
> Then, we'd have:
>
> nr_pages == folio_nr_pages(folio) - 1.
>
> You could simply map+reuse most of the folio without COWing.

yes. This is good but the pte which is no longer mapped could be
anyone within the nr_pages PTEs. so it could be quite tricky for
set_ptes.

>
> Once we support COW reuse of PTE-mapped THP we'd do the same. Here, it's
> just easy to detect that the folio is exclusive (folio_ref_count(folio)
> == 1 before mapping anything).
>
> If you really want to mimic what do_wp_page() currently does, you should
> have:
>
> exclusive || (folio_ref_count(folio) == 1 && !folio_test_large(folio))

I actually dislike the part that do_wp_page() handles the reuse of a large
folio which is entirely mapped. For example, A forks B, B exit, we write
A's large folio, we get nr_pages CoW of small folios. Ideally, we can
reuse the whole folios for writing.

>
> Personally, I think we should keep it simple here and use:
>
> exclusive || folio_ref_count(folio) == 1

I feel this is still better than
exclusive || (folio_ref_count(folio) == 1 && !folio_test_large(folio))
as we reuse the whole large folio. the do_wp_page() behaviour
doesn't have this.

>
> IMHO, that's as clear as it gets.

I agree this is clear. But I wonder if there is a possibility to optimize.

Using your instance,
"Let's assume a single subpage of a large folio is no longer mapped."

For a large folio with 16 PTEs and in case we have unmapped one of them.
Thus, we have 15 swap entries left.

The first PTE which gets page faults will reuse the whole large folio having
"exclusive || folio_ref_count(folio) == 1" only. The left 14 will
allocate 14 small
folios(swapcache has been dropped), thus, we use 16 + 14 = 30 pages
memory.

with either
A :   !folio_test_large(folio) || nr_pages > 1
or
B:    folio_nr_pages(folio) == nr_pages

We consume 15 pages.

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

Thanks
Barry

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