Re: [PATCH v3 6/6] mm: swap: entirely map large folios found in swapcache

[David Hildenbrand <david@redhat.com>]

On 04.05.24 01:23, Barry Song wrote:
> On Fri, May 3, 2024 at 6:50 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
>>
>> On 03/05/2024 01:50, Barry Song wrote:
>>> From: Chuanhua Han <hanchuanhua@oppo.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@oppo.com>
>>> Co-developed-by: Barry Song <v-songbaohua@oppo.com>
>>> Signed-off-by: Barry Song <v-songbaohua@oppo.com>
>>
>> With the suggested changes below:
>>
>> Reviewed-by: Ryan Roberts <ryan.roberts@arm.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.

-- 
Cheers,

David / dhildenb