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Date: Fri, 31 May 2024 14:10:33 +0200
From: David Hildenbrand <david@...hat.com>
To: Barry Song <21cnbao@...il.com>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org, chrisl@...nel.org,
surenb@...gle.com, kasong@...cent.com, minchan@...nel.org,
willy@...radead.org, ryan.roberts@....com, linux-kernel@...r.kernel.org,
Barry Song <v-songbaohua@...o.com>
Subject: Re: [RFC PATCH] mm: swap: reuse exclusive folio directly instead of
wp page faults
On 31.05.24 13:55, Barry Song wrote:
> On Fri, May 31, 2024 at 11:08 PM David Hildenbrand <david@...hat.com> wrote:
>>
>> On 31.05.24 12:48, Barry Song wrote:
>>> From: Barry Song <v-songbaohua@...o.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@...o.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
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