Re: [RFC PATCH v3 3/4] mm: add do_set_pte_range()

["Yin, Fengwei" <fengwei.yin@intel.com>]

On 2/3/2023 9:25 PM, David Hildenbrand wrote:
> On 03.02.23 14:16, Yin Fengwei wrote:
>> do_set_pte_range() allows to setup page table entries for a
>> specific range. It calls folio_add_file_rmap_range() to take
>> advantage of batched rmap update for large folio.
>>
>> Signed-off-by: Yin Fengwei <fengwei.yin@intel.com>
>> ---
>>   include/linux/mm.h |  3 +++
>>   mm/filemap.c       |  1 -
>>   mm/memory.c        | 59 ++++++++++++++++++++++++++++++----------------
>>   3 files changed, 42 insertions(+), 21 deletions(-)
>>
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index d6f8f41514cc..93192f04b276 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -1162,6 +1162,9 @@ 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 do_set_pte_range(struct vm_fault *vmf, struct folio *folio,
>> +        unsigned long addr, pte_t *pte,
>> +        unsigned long start, unsigned int nr);
>>     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 f444684db9f2..74046a3a0ff5 100644
>> --- a/mm/filemap.c
>> +++ b/mm/filemap.c
>> @@ -3386,7 +3386,6 @@ static vm_fault_t filemap_map_folio_range(struct vm_fault *vmf,
>>             ref_count++;
>>           do_set_pte(vmf, page, addr);
>> -        update_mmu_cache(vma, addr, vmf->pte);
>>       } 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 7a04a1130ec1..3754b2ef166a 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -4257,36 +4257,58 @@ 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 do_set_pte_range(struct vm_fault *vmf, struct folio *folio,
>> +        unsigned long addr, pte_t *pte,
>> +        unsigned long start, unsigned int nr)
>>   {
>>       struct vm_area_struct *vma = vmf->vma;
>>       bool uffd_wp = pte_marker_uffd_wp(vmf->orig_pte);
>>       bool write = vmf->flags & FAULT_FLAG_WRITE;
>> +    bool cow = write && !(vma->vm_flags & VM_SHARED);
>>       bool prefault = vmf->address != addr;
>> +    struct page *page = folio_page(folio, start);
>>       pte_t entry;
>>   -    flush_icache_page(vma, page);
>> -    entry = mk_pte(page, vma->vm_page_prot);
>> +    if (!cow) {
>> +        folio_add_file_rmap_range(folio, start, nr, vma, false);
>> +        add_mm_counter(vma->vm_mm, mm_counter_file(page), nr);
>> +    }
>>   -    if (prefault && arch_wants_old_prefaulted_pte())
>> -        entry = pte_mkold(entry);
>> -    else
>> -        entry = pte_sw_mkyoung(entry);
>> +    do {
>> +        flush_icache_page(vma, page);
>> +        entry = mk_pte(page, vma->vm_page_prot);
>>   -    if (write)
>> -        entry = maybe_mkwrite(pte_mkdirty(entry), vma);
>> -    if (unlikely(uffd_wp))
>> -        entry = pte_mkuffd_wp(entry);
>> -    /* copy-on-write page */
>> -    if (write && !(vma->vm_flags & VM_SHARED)) {
>> +        if (prefault && arch_wants_old_prefaulted_pte())
>> +            entry = pte_mkold(entry);
>> +        else
>> +            entry = pte_sw_mkyoung(entry);
>> +
>> +        if (write)
>> +            entry = maybe_mkwrite(pte_mkdirty(entry), vma);
>> +        if (unlikely(uffd_wp))
>> +            entry = pte_mkuffd_wp(entry);
>> +        set_pte_at(vma->vm_mm, addr, pte, entry);
>> +
>> +        /* no need to invalidate: a not-present page won't be cached */
>> +        update_mmu_cache(vma, addr, pte);
>> +    } while (pte++, page++, addr += PAGE_SIZE, --nr > 0);
>> +}
>> +
>> +void do_set_pte(struct vm_fault *vmf, struct page *page, unsigned long addr)
>> +{
>> +    struct folio *folio = page_folio(page);
>> +    struct vm_area_struct *vma = vmf->vma;
>> +    bool cow = (vmf->flags & FAULT_FLAG_WRITE) &&
>> +            !(vma->vm_flags & VM_SHARED);
>> +
>> +    if (cow) {
>>           inc_mm_counter(vma->vm_mm, MM_ANONPAGES);
>>           page_add_new_anon_rmap(page, vma, addr);
> 
> As raised, we cannot PTE-map a multi-page folio that way.
> 
> This function only supports single-page anon folios.
> 
> page_add_new_anon_rmap() -> folio_add_new_anon_rmap(). As that documents:
> 
> "If the folio is large, it is accounted as a THP" -- for example, we would only increment the "entire mapcount" and set the PageAnonExclusive bit only on the head page.
> 
> So this really doesn't work for multi-page folios and if the function would be used for that, we'd be in trouble.
> 
> We'd want some fence here to detect that and bail out if we'd be instructed to do that. At least a WARN_ON_ONCE() I guess.
> update_mmu_tlb(vma, vmf->address, vmf->pte);
> 
> Right now the function looks like it might just handle that.
You are right. I thought moving cow case out of it can make it explicit.
But looks like it doesn't. I will add WARN_ON_ONCE(). Thanks.


Regards
Yin, Fengwei

>