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

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

On 2/3/2023 9:34 PM, David Hildenbrand wrote:
> On 03.02.23 14:30, Yin, Fengwei wrote:
>>
>>
>> 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.
> 
> I guess I would move the cow check into  do_set_pte_range() as well, and verify in there that we are really only dealing with a single-page folio, commenting that rmap code would need serious adjustment to make it work and that current code never passes a multi-page folio.

Yes. There is !cow in do_set_pte_range() already. I planned to add
WARN_ON_ONCE() on else case there. Thanks.


Regards
Yin, Fengwei

> 
> Thanks!
>