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Date: Fri, 31 May 2024 14:10:33 +0200
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Subject: Re: [RFC PATCH] mm: swap: reuse exclusive folio directly instead of
 wp page faults
To: Barry Song <21cnbao@gmail.com>
Cc: akpm@linux-foundation.org, linux-mm@kvack.org, chrisl@kernel.org,
 surenb@google.com, kasong@tencent.com, minchan@kernel.org,
 willy@infradead.org, ryan.roberts@arm.com, linux-kernel@vger.kernel.org,
 Barry Song <v-songbaohua@oppo.com>
References: <20240531104819.140218-1-21cnbao@gmail.com>
 <87ac9610-5650-451f-aa54-e634a6310af4@redhat.com>
 <CAGsJ_4zgf8j0oRnVBhe-__=K2RFDHTCo-JnMak95HTvxtMUwnQ@mail.gmail.com>
From: David Hildenbrand <david@redhat.com>
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In-Reply-To: <CAGsJ_4zgf8j0oRnVBhe-__=K2RFDHTCo-JnMak95HTvxtMUwnQ@mail.gmail.com>
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On 31.05.24 13:55, Barry Song wrote:
> On Fri, May 31, 2024 at 11:08 PM David Hildenbrand <david@redhat.com> wrote:
>>
>> On 31.05.24 12:48, Barry Song wrote:
>>> From: Barry Song <v-songbaohua@oppo.com>
>>>
>>> After swapping out, we perform a swap-in operation. If we first read
>>> and then write, we encounter a major fault in do_swap_page for reading,
>>> along with additional minor faults in do_wp_page for writing. However,
>>> the latter appears to be unnecessary and inefficient. Instead, we can
>>> directly reuse in do_swap_page and completely eliminate the need for
>>> do_wp_page.
>>>
>>> This patch achieves that optimization specifically for exclusive folios.
>>> The following microbenchmark demonstrates the significant reduction in
>>> minor faults.
>>>
>>>    #define DATA_SIZE (2UL * 1024 * 1024)
>>>    #define PAGE_SIZE (4UL * 1024)
>>>
>>>    static void *read_write_data(char *addr)
>>>    {
>>>            char tmp;
>>>
>>>            for (int i = 0; i < DATA_SIZE; i += PAGE_SIZE) {
>>>                    tmp = *(volatile char *)(addr + i);
>>>                    *(volatile char *)(addr + i) = tmp;
>>>            }
>>>    }
>>>
>>>    int main(int argc, char **argv)
>>>    {
>>>            struct rusage ru;
>>>
>>>            char *addr = mmap(NULL, DATA_SIZE, PROT_READ | PROT_WRITE,
>>>                            MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
>>>            memset(addr, 0x11, DATA_SIZE);
>>>
>>>            do {
>>>                    long old_ru_minflt, old_ru_majflt;
>>>                    long new_ru_minflt, new_ru_majflt;
>>>
>>>                    madvise(addr, DATA_SIZE, MADV_PAGEOUT);
>>>
>>>                    getrusage(RUSAGE_SELF, &ru);
>>>                    old_ru_minflt = ru.ru_minflt;
>>>                    old_ru_majflt = ru.ru_majflt;
>>>
>>>                    read_write_data(addr);
>>>                    getrusage(RUSAGE_SELF, &ru);
>>>                    new_ru_minflt = ru.ru_minflt;
>>>                    new_ru_majflt = ru.ru_majflt;
>>>
>>>                    printf("minor faults:%ld major faults:%ld\n",
>>>                            new_ru_minflt - old_ru_minflt,
>>>                            new_ru_majflt - old_ru_majflt);
>>>            } while(0);
>>>
>>>            return 0;
>>>    }
>>>
>>> w/o patch,
>>> / # ~/a.out
>>> minor faults:512 major faults:512
>>>
>>> w/ patch,
>>> / # ~/a.out
>>> minor faults:0 major faults:512
>>>
>>> Minor faults decrease to 0!
>>>
>>> Signed-off-by: Barry Song <v-songbaohua@oppo.com>
>>> ---
>>>    mm/memory.c | 7 ++++---
>>>    1 file changed, 4 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/mm/memory.c b/mm/memory.c
>>> index eef4e482c0c2..e1d2e339958e 100644
>>> --- a/mm/memory.c
>>> +++ b/mm/memory.c
>>> @@ -4325,9 +4325,10 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>>>         */
>>>        if (!folio_test_ksm(folio) &&
>>>            (exclusive || folio_ref_count(folio) == 1)) {
>>> -             if (vmf->flags & FAULT_FLAG_WRITE) {
>>> -                     pte = maybe_mkwrite(pte_mkdirty(pte), vma);
>>> -                     vmf->flags &= ~FAULT_FLAG_WRITE;
>>> +             if (vma->vm_flags & VM_WRITE) {
>>> +                     pte = pte_mkwrite(pte_mkdirty(pte), vma);
>>> +                     if (vmf->flags & FAULT_FLAG_WRITE)
>>> +                             vmf->flags &= ~FAULT_FLAG_WRITE;
>>
>> This implies, that even on a read fault, you would mark the pte dirty
>> and it would have to be written back to swap if still in the swap cache
>> and only read.
>>
>> That is controversial.
>>
>> What is less controversial is doing what mprotect() via
>> change_pte_range()/can_change_pte_writable() would do: mark the PTE
>> writable but not dirty.
>>
>> I suggest setting the pte only dirty if FAULT_FLAG_WRITE is set.
> 
> Thanks!
> 
> I assume you mean something as below?

It raises an important point: uffd-wp must be handled accordingly.

> 
> diff --git a/mm/memory.c b/mm/memory.c
> index eef4e482c0c2..dbf1ba8ccfd6 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4317,6 +4317,10 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>          add_mm_counter(vma->vm_mm, MM_SWAPENTS, -nr_pages);
>          pte = mk_pte(page, vma->vm_page_prot);
> 
> +       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);
>          /*
>           * Same logic as in do_wp_page(); however, optimize for pages that are
>           * certainly not shared either because we just allocated them without
> @@ -4325,18 +4329,19 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>           */
>          if (!folio_test_ksm(folio) &&
>              (exclusive || folio_ref_count(folio) == 1)) {
> -               if (vmf->flags & FAULT_FLAG_WRITE) {
> -                       pte = maybe_mkwrite(pte_mkdirty(pte), vma);
> -                       vmf->flags &= ~FAULT_FLAG_WRITE;
> +               if (vma->vm_flags & VM_WRITE) {
> +                       if (vmf->flags & FAULT_FLAG_WRITE) {
> +                               pte = pte_mkwrite(pte_mkdirty(pte), vma);
> +                               vmf->flags &= ~FAULT_FLAG_WRITE;
> +                       } else if ((!vma_soft_dirty_enabled(vma) ||
> pte_soft_dirty(pte))
> +                                   && !userfaultfd_pte_wp(vma, pte)) {
> +                                       pte = pte_mkwrite(pte, vma);

Even with FAULT_FLAG_WRITE we must respect uffd-wp and *not* do a 
pte_mkwrite(pte). So we have to catch and handle that earlier (I could 
have sworn we handle that somehow).

Note that the existing
	pte = pte_mkuffd_wp(pte);

Will fix that up because it does an implicit pte_wrprotect().


So maybe what would work is


if ((vma->vm_flags & VM_WRITE) && !userfaultfd_pte_wp(vma, pte) &&
     !vma_soft_dirty_enabled(vma)) {
	pte = pte_mkwrite(pte);

	/* Only set the PTE dirty on write fault. */
	if (vmf->flags & FAULT_FLAG_WRITE) {
		pte = pte_mkdirty(pte);
		vmf->flags &= ~FAULT_FLAG_WRITE;
	}
}

-- 
Cheers,

David / dhildenb