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Date: Tue, 7 May 2024 09:17:23 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Barry Song <21cnbao@...il.com>
Cc: 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, 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 04/05/2024 00:23, 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"
> :-)
Yeah fair enough. Ignore my comment.
>
>
> 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, in this case, lacking folio_nr_pages(folio) == nr_pages
> might lead to mkwrite() on a single pte within a partially unmapped large
> folio. not quite sure this is wrong, but seems buggy and arduous.
>
>>
>>> if (vmf->flags & FAULT_FLAG_WRITE) {
>>> pte = maybe_mkwrite(pte_mkdirty(pte), vma);
>>> vmf->flags &= ~FAULT_FLAG_WRITE;
>>> }
>>> rmap_flags |= RMAP_EXCLUSIVE;
>>> }
>>> - flush_icache_page(vma, page);
>>> + flush_icache_pages(vma, page, nr_pages);
>>> if (pte_swp_soft_dirty(vmf->orig_pte))
>>> pte = pte_mksoft_dirty(pte);
>>> if (pte_swp_uffd_wp(vmf->orig_pte))
>>> pte = pte_mkuffd_wp(pte);
>>> - vmf->orig_pte = pte;
>>> + vmf->orig_pte = pte_advance_pfn(pte, page_idx);
>>>
>>> /* ksm created a completely new copy */
>>> if (unlikely(folio != swapcache && swapcache)) {
>>> - folio_add_new_anon_rmap(folio, vma, vmf->address);
>>> + folio_add_new_anon_rmap(folio, vma, address);
>>> folio_add_lru_vma(folio, vma);
>>> } else {
>>> - folio_add_anon_rmap_pte(folio, page, vma, vmf->address,
>>> + folio_add_anon_rmap_ptes(folio, page, nr_pages, vma, address,
>>> rmap_flags);
>>> }
>>>
>>> VM_BUG_ON(!folio_test_anon(folio) ||
>>> (pte_write(pte) && !PageAnonExclusive(page)));
>>> - set_pte_at(vma->vm_mm, vmf->address, vmf->pte, pte);
>>> - arch_do_swap_page_nr(vma->vm_mm, vma, vmf->address,
>>> - pte, vmf->orig_pte, 1);
>>> + set_ptes(vma->vm_mm, address, ptep, pte, nr_pages);
>>> + arch_do_swap_page_nr(vma->vm_mm, vma, address,
>>> + pte, pte, nr_pages);
>>>
>>> folio_unlock(folio);
>>> if (folio != swapcache && swapcache) {
>>> @@ -4291,7 +4327,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>>> }
>>>
>>> /* No need to invalidate - it was non-present before */
>>> - update_mmu_cache_range(vmf, vma, vmf->address, vmf->pte, 1);
>>> + update_mmu_cache_range(vmf, vma, address, ptep, nr_pages);
>>> unlock:
>>> if (vmf->pte)
>>> pte_unmap_unlock(vmf->pte, vmf->ptl);
>>
>
> Thanks
> Barry
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