Re: [PATCH v4 35/36] mm: Convert do_set_pte() to set_pte_range()

[Ryan Roberts]

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@xxxxxxxxx>
>>>>>
>>>>> 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@xxxxxxxxx>
>>>>> Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
>>>>> ---
>>>>>  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);
>>>>
>>