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Message-ID: <3cbda55e-a09f-7745-4634-dcbe7c81da88@arm.com>
Date: Thu, 16 Mar 2023 16:50:47 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: "Yin, Fengwei" <fengwei.yin@...el.com>,
"Matthew Wilcox (Oracle)" <willy@...radead.org>,
linux-arch@...r.kernel.org, will@...nel.org
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 35/36] mm: Convert do_set_pte() to set_pte_range()
On 16/03/2023 16:41, Yin, Fengwei wrote:
>
>
> On 3/17/2023 12:38 AM, Ryan Roberts wrote:
>> On 16/03/2023 16:23, Yin, Fengwei wrote:
>>>
>>>
>>> On 3/15/2023 11:26 PM, Ryan Roberts wrote:
>>>> On 15/03/2023 05:14, Matthew Wilcox (Oracle) wrote:
>>>>> From: Yin Fengwei <fengwei.yin@...el.com>
>>>>>
>>>>> set_pte_range() allows to setup page table entries for a specific
>>>>> range. It takes advantage of batched rmap update for large folio.
>>>>> It now takes care of calling update_mmu_cache_range().
>>>>>
>>>>> Signed-off-by: Yin Fengwei <fengwei.yin@...el.com>
>>>>> Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
>>>>> ---
>>>>> Documentation/filesystems/locking.rst | 2 +-
>>>>> include/linux/mm.h | 3 ++-
>>>>> mm/filemap.c | 3 +--
>>>>> mm/memory.c | 27 +++++++++++++++------------
>>>>> 4 files changed, 19 insertions(+), 16 deletions(-)
>>>>>
>>>>> diff --git a/Documentation/filesystems/locking.rst b/Documentation/filesystems/locking.rst
>>>>> index 7de7a7272a5e..922886fefb7f 100644
>>>>> --- a/Documentation/filesystems/locking.rst
>>>>> +++ b/Documentation/filesystems/locking.rst
>>>>> @@ -663,7 +663,7 @@ locked. The VM will unlock the page.
>>>>> Filesystem should find and map pages associated with offsets from "start_pgoff"
>>>>> till "end_pgoff". ->map_pages() is called with page table locked and must
>>>>> not block. If it's not possible to reach a page without blocking,
>>>>> -filesystem should skip it. Filesystem should use do_set_pte() to setup
>>>>> +filesystem should skip it. Filesystem should use set_pte_range() to setup
>>>>> page table entry. Pointer to entry associated with the page is passed in
>>>>> "pte" field in vm_fault structure. Pointers to entries for other offsets
>>>>> should be calculated relative to "pte".
>>>>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>>>>> index ee755bb4e1c1..81788c985a8c 100644
>>>>> --- a/include/linux/mm.h
>>>>> +++ b/include/linux/mm.h
>>>>> @@ -1299,7 +1299,8 @@ static inline pte_t maybe_mkwrite(pte_t pte, struct vm_area_struct *vma)
>>>>> }
>>>>>
>>>>> vm_fault_t do_set_pmd(struct vm_fault *vmf, struct page *page);
>>>>> -void do_set_pte(struct vm_fault *vmf, struct page *page, unsigned long addr);
>>>>> +void set_pte_range(struct vm_fault *vmf, struct folio *folio,
>>>>> + struct page *page, unsigned int nr, unsigned long addr);
>>>>>
>>>>> vm_fault_t finish_fault(struct vm_fault *vmf);
>>>>> vm_fault_t finish_mkwrite_fault(struct vm_fault *vmf);
>>>>> diff --git a/mm/filemap.c b/mm/filemap.c
>>>>> index 6e2b0778db45..e2317623dcbf 100644
>>>>> --- a/mm/filemap.c
>>>>> +++ b/mm/filemap.c
>>>>> @@ -3504,8 +3504,7 @@ static vm_fault_t filemap_map_folio_range(struct vm_fault *vmf,
>>>>> ret = VM_FAULT_NOPAGE;
>>>>>
>>>>> ref_count++;
>>>>> - do_set_pte(vmf, page, addr);
>>>>> - update_mmu_cache(vma, addr, vmf->pte);
>>>>> + set_pte_range(vmf, folio, page, 1, addr);
>>>>> } while (vmf->pte++, page++, addr += PAGE_SIZE, ++count < nr_pages);
>>>>>
>>>>> /* Restore the vmf->pte */
>>>>> diff --git a/mm/memory.c b/mm/memory.c
>>>>> index 6aa21e8f3753..9a654802f104 100644
>>>>> --- a/mm/memory.c
>>>>> +++ b/mm/memory.c
>>>>> @@ -4274,7 +4274,8 @@ vm_fault_t do_set_pmd(struct vm_fault *vmf, struct page *page)
>>>>> }
>>>>> #endif
>>>>>
>>>>> -void do_set_pte(struct vm_fault *vmf, struct page *page, unsigned long addr)
>>>>> +void set_pte_range(struct vm_fault *vmf, struct folio *folio,
>>>>> + struct page *page, unsigned int nr, unsigned long addr)
>>>>> {
>>>>> struct vm_area_struct *vma = vmf->vma;
>>>>> bool uffd_wp = vmf_orig_pte_uffd_wp(vmf);
>>>>> @@ -4282,7 +4283,7 @@ void do_set_pte(struct vm_fault *vmf, struct page *page, unsigned long addr)
>>>>> bool prefault = vmf->address != addr;
>>>>
>>>> I think you are changing behavior here - is this intentional? Previously this
>>>> would be evaluated per page, now its evaluated once for the whole range. The
>>>> intention below is that directly faulted pages are mapped young and prefaulted
>>>> pages are mapped old. But now a whole range will be mapped the same.
>>>
>>> Yes. You are right here.
>>>
>>> Look at the prefault and cpu_has_hw_af for ARM64, it looks like we
>>> can avoid to handle vmf->address == addr specially. It's OK to
>>> drop prefault and change the logic here a little bit to:
>>> if (arch_wants_old_prefaulted_pte())
>>> entry = pte_mkold(entry);
>>> else
>>> entry = pte_sw_mkyong(entry);
>>>
>>> It's not necessary to use pte_sw_mkyong for vmf->address == addr
>>> because HW will set the ACCESS bit in page table entry.
>>>
>>> Add Will Deacon in case I missed something here. Thanks.
>>
>> I'll defer to Will's response, but not all arm HW supports HW access flag
>> management. In that case it's done by SW, so I would imagine that by setting
>> this to old initially, we will get a second fault to set the access bit, which
>> will slow things down. I wonder if you will need to split this into (up to) 3
>> calls to set_ptes()?
> If no HW access flag, arch_wants_old_prefaulted_pte() will return false. So
> path will goto pte_sw_mkyong(entry). Right?
Oops... yes, I agree with you - disregard my previous comment.
>
>
> Regards
> Yin, Fengwei
>
>>
>>>
>>>
>>> Regards
>>> Yin, Fengwei
>>>
>>>>
>>>> Thanks,
>>>> Ryan
>>>>
>>>>> pte_t entry;
>>>>>
>>>>> - flush_icache_page(vma, page);
>>>>> + flush_icache_pages(vma, page, nr);
>>>>> entry = mk_pte(page, vma->vm_page_prot);
>>>>>
>>>>> if (prefault && arch_wants_old_prefaulted_pte())
>>>>> @@ -4296,14 +4297,18 @@ void do_set_pte(struct vm_fault *vmf, struct page *page, unsigned long addr)
>>>>> entry = pte_mkuffd_wp(entry);
>>>>> /* copy-on-write page */
>>>>> if (write && !(vma->vm_flags & VM_SHARED)) {
>>>>> - inc_mm_counter(vma->vm_mm, MM_ANONPAGES);
>>>>> - page_add_new_anon_rmap(page, vma, addr);
>>>>> - lru_cache_add_inactive_or_unevictable(page, vma);
>>>>> + add_mm_counter(vma->vm_mm, MM_ANONPAGES, nr);
>>>>> + VM_BUG_ON_FOLIO(nr != 1, folio);
>>>>> + folio_add_new_anon_rmap(folio, vma, addr);
>>>>> + folio_add_lru_vma(folio, vma);
>>>>> } else {
>>>>> - inc_mm_counter(vma->vm_mm, mm_counter_file(page));
>>>>> - page_add_file_rmap(page, vma, false);
>>>>> + add_mm_counter(vma->vm_mm, mm_counter_file(page), nr);
>>>>> + folio_add_file_rmap_range(folio, page, nr, vma, false);
>>>>> }
>>>>> - set_pte_at(vma->vm_mm, addr, vmf->pte, entry);
>>>>> + set_ptes(vma->vm_mm, addr, vmf->pte, entry, nr);
>>>>> +
>>>>> + /* no need to invalidate: a not-present page won't be cached */
>>>>> + update_mmu_cache_range(vma, addr, vmf->pte, nr);
>>>>> }
>>>>>
>>>>> static bool vmf_pte_changed(struct vm_fault *vmf)
>>>>> @@ -4376,11 +4381,9 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
>>>>>
>>>>> /* Re-check under ptl */
>>>>> if (likely(!vmf_pte_changed(vmf))) {
>>>>> - do_set_pte(vmf, page, vmf->address);
>>>>> -
>>>>> - /* no need to invalidate: a not-present page won't be cached */
>>>>> - update_mmu_cache(vma, vmf->address, vmf->pte);
>>>>> + struct folio *folio = page_folio(page);
>>>>>
>>>>> + set_pte_range(vmf, folio, page, 1, vmf->address);
>>>>> ret = 0;
>>>>> } else {
>>>>> update_mmu_tlb(vma, vmf->address, vmf->pte);
>>>>
>>
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