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Message-ID: <87y19e115l.fsf@yhuang6-desk2.ccr.corp.intel.com>
Date: Tue, 16 Apr 2024 10:39:34 +0800
From: "Huang, Ying" <ying.huang@...el.com>
To: Barry Song <21cnbao@...il.com>
Cc: Khalid Aziz <khalid.aziz@...cle.com>, akpm@...ux-foundation.org,
linux-mm@...ck.org, baolin.wang@...ux.alibaba.com, chrisl@...nel.org,
david@...hat.com, hanchuanhua@...o.com, hannes@...xchg.org,
hughd@...gle.com, kasong@...cent.com, ryan.roberts@....com,
surenb@...gle.com, v-songbaohua@...o.com, willy@...radead.org,
xiang@...nel.org, yosryahmed@...gle.com, yuzhao@...gle.com,
ziy@...dia.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 4/5] mm: swap: entirely map large folios found in
swapcache
Barry Song <21cnbao@...il.com> writes:
> On Tue, Apr 16, 2024 at 2:27 PM Huang, Ying <ying.huang@...el.com> wrote:
>>
>>
>> Added Khalid for arch_do_swap_page().
>>
>> Barry Song <21cnbao@...il.com> writes:
>>
>> > On Mon, Apr 15, 2024 at 8:39 PM Huang, Ying <ying.huang@...el.com> wrote:
>> >>
>> >> Barry Song <21cnbao@...il.com> writes:
>>
>> [snip]
>>
>> >>
>> >> > + bool any_swap_shared = false;
>> >> >
>> >> > if (!pte_unmap_same(vmf))
>> >> > goto out;
>> >> > @@ -4137,6 +4141,35 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>> >> > */
>> >> > vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd, vmf->address,
>> >> > &vmf->ptl);
>> >>
>> >> We should move pte check here. That is,
>> >>
>> >> if (unlikely(!vmf->pte || !pte_same(ptep_get(vmf->pte), vmf->orig_pte)))
>> >> goto out_nomap;
>> >>
>> >> This will simplify the situation for large folio.
>> >
>> > the plan is moving the whole code block
>> >
>> > if (start_pte && folio_test_large(folio) && folio_test_swapcache(folio))
>> >
>> > after
>> > if (unlikely(!folio_test_uptodate(folio))) {
>> > ret = VM_FAULT_SIGBUS;
>> > goto out_nomap;
>> > }
>> >
>> > though we couldn't be !folio_test_uptodate(folio)) for hitting
>> > swapcache but it seems
>> > logically better for future use.
>>
>> LGTM, Thanks!
>>
>> >>
>> >> > +
>> >> > + /* We hit large folios in swapcache */
>> >>
>> >> The comments seems unnecessary because the code tells that already.
>> >>
>> >> > + if (start_pte && folio_test_large(folio) && folio_test_swapcache(folio)) {
>> >> > + int nr = folio_nr_pages(folio);
>> >> > + int 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_pte;
>> >> > + if (unlikely(folio_end > pmd_addr_end(vmf->address, vma->vm_end)))
>> >> > + goto check_pte;
>> >> > +
>> >> > + folio_ptep = vmf->pte - idx;
>> >> > + folio_pte = ptep_get(folio_ptep);
>> >>
>> >> It's better to construct pte based on fault PTE via generalizing
>> >> pte_next_swp_offset() (may be pte_move_swp_offset()). Then we can find
>> >> inconsistent PTEs quicker.
>> >
>> > it seems your point is getting the pte of page0 by pte_next_swp_offset()
>> > unfortunately pte_next_swp_offset can't go back. on the other hand,
>> > we have to check the real pte value of the 0nd entry right now because
>> > swap_pte_batch() only really reads pte from the 1st entry. it assumes
>> > pte argument is the real value for the 0nd pte entry.
>> >
>> > static inline int swap_pte_batch(pte_t *start_ptep, int max_nr, pte_t pte)
>> > {
>> > pte_t expected_pte = pte_next_swp_offset(pte);
>> > const pte_t *end_ptep = start_ptep + max_nr;
>> > pte_t *ptep = start_ptep + 1;
>> >
>> > VM_WARN_ON(max_nr < 1);
>> > VM_WARN_ON(!is_swap_pte(pte));
>> > VM_WARN_ON(non_swap_entry(pte_to_swp_entry(pte)));
>> >
>> > while (ptep < end_ptep) {
>> > pte = ptep_get(ptep);
>> >
>> > if (!pte_same(pte, expected_pte))
>> > break;
>> >
>> > expected_pte = pte_next_swp_offset(expected_pte);
>> > ptep++;
>> > }
>> >
>> > return ptep - start_ptep;
>> > }
>>
>> Yes. You are right.
>>
>> But we may check whether the pte of page0 is same as "vmf->orig_pte -
>> folio_page_idx()" (fake code).
>
> right, that is why we are reading and checking PTE0 before calling
> swap_pte_batch()
> right now.
>
> folio_ptep = vmf->pte - idx;
> folio_pte = ptep_get(folio_ptep);
> if (!is_swap_pte(folio_pte) || non_swap_entry(pte_to_swp_entry(folio_pte)) ||
> swap_pte_batch(folio_ptep, nr, folio_pte, &any_swap_shared) != nr)
> goto check_pte;
>
> So, if I understand correctly, you're proposing that we should directly check
> PTE0 in swap_pte_batch(). Personally, I don't have any objections to this idea.
> However, I'd also like to hear the feedback from Ryan and David :-)
I mean that we can replace
!is_swap_pte(folio_pte) || non_swap_entry(pte_to_swp_entry(folio_pte))
in above code with pte_same() with constructed expected first pte.
>>
>> You need to check the pte of page 0 anyway.
>>
>> >>
>> >> > + if (!is_swap_pte(folio_pte) || non_swap_entry(pte_to_swp_entry(folio_pte)) ||
>> >> > + swap_pte_batch(folio_ptep, nr, folio_pte, &any_swap_shared) != nr)
>> >> > + goto check_pte;
>> >> > +
>> >> > + start_address = folio_start;
>> >> > + start_pte = folio_ptep;
>> >> > + nr_pages = nr;
>> >> > + entry = folio->swap;
>> >> > + page = &folio->page;
>> >> > + }
>> >> > +
>> >> > +check_pte:
>> >> > if (unlikely(!vmf->pte || !pte_same(ptep_get(vmf->pte), vmf->orig_pte)))
>> >> > goto out_nomap;
>> >> >
>> >> > @@ -4190,6 +4223,10 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>> >> > */
>> >> > exclusive = false;
>> >> > }
>> >> > +
>> >> > + /* Reuse the whole large folio iff all entries are exclusive */
>> >> > + if (nr_pages > 1 && any_swap_shared)
>> >> > + exclusive = false;
>> >> > }
>> >> >
[snip]
--
Best Regards,
Huang, Ying
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