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Message-ID: <CAGsJ_4z93FwPZx7w2VuCEkHP_JCwkO0whKwymonRJ9bSiKMVyQ@mail.gmail.com>
Date: Tue, 7 May 2024 00:38:22 +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 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:
@@ -4263,7 +4264,6 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
if (should_try_to_free_swap(folio, vma, vmf->flags))
folio_free_swap(folio);
- 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);
@@ -4275,14 +4275,14 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
* exclusivity.
*/
if (!folio_test_ksm(folio) &&
- (exclusive || (folio_ref_count(folio) == nr_pages &&
- folio_nr_pages(folio) == nr_pages))) {
+ (exclusive || folio_ref_count(folio) == 1)) {
if (vmf->flags & FAULT_FLAG_WRITE) {
pte = maybe_mkwrite(pte_mkdirty(pte), vma);
vmf->flags &= ~FAULT_FLAG_WRITE;
}
rmap_flags |= RMAP_EXCLUSIVE;
}
+ folio_ref_add(folio, nr_pages - 1);
flush_icache_pages(vma, page, nr_pages);
if (pte_swp_soft_dirty(vmf->orig_pte))
pte = pte_mksoft_dirty(pte);
>
> --
> Cheers,
>
> David / dhildenb
>
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