Re: [PATCH 2/8] mm: memory: extend finish_fault() to support large folio

[Ryan Roberts <ryan.roberts@arm.com>]

On 08/05/2024 04:44, Baolin Wang wrote:
> 
> 
> On 2024/5/7 18:37, Ryan Roberts wrote:
>> On 06/05/2024 09:46, Baolin Wang wrote:
>>> Add large folio mapping establishment support for finish_fault() as a
>>> preparation,
>>> to support multi-size THP allocation of anonymous shmem pages in the following
>>> patches.
>>>
>>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>>> ---
>>>   mm/memory.c | 43 +++++++++++++++++++++++++++++++++----------
>>>   1 file changed, 33 insertions(+), 10 deletions(-)
>>>
>>> diff --git a/mm/memory.c b/mm/memory.c
>>> index eea6e4984eae..936377220b77 100644
>>> --- a/mm/memory.c
>>> +++ b/mm/memory.c
>>> @@ -4747,9 +4747,12 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
>>>   {
>>>       struct vm_area_struct *vma = vmf->vma;
>>>       struct page *page;
>>> +    struct folio *folio;
>>>       vm_fault_t ret;
>>>       bool is_cow = (vmf->flags & FAULT_FLAG_WRITE) &&
>>>                 !(vma->vm_flags & VM_SHARED);
>>> +    int type, nr_pages, i;
>>> +    unsigned long addr = vmf->address;
>>>         /* Did we COW the page? */
>>>       if (is_cow)
>>> @@ -4780,24 +4783,44 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
>>>               return VM_FAULT_OOM;
>>>       }
>>>   +    folio = page_folio(page);
>>> +    nr_pages = folio_nr_pages(folio);
>>> +
>>> +    if (unlikely(userfaultfd_armed(vma))) {
>>> +        nr_pages = 1;
>>> +    } else if (nr_pages > 1) {
>>> +        unsigned long start = ALIGN_DOWN(vmf->address, nr_pages * PAGE_SIZE);
>>> +        unsigned long end = start + nr_pages * PAGE_SIZE;
>>> +
>>> +        /* In case the folio size in page cache beyond the VMA limits. */
>>> +        addr = max(start, vma->vm_start);
>>> +        nr_pages = (min(end, vma->vm_end) - addr) >> PAGE_SHIFT;
>>> +
>>> +        page = folio_page(folio, (addr - start) >> PAGE_SHIFT);
>>
>> I still don't really follow the logic in this else if block. Isn't it possible
>> that finish_fault() gets called with a page from a folio that isn't aligned with
>> vmf->address?
>>
>> For example, let's say we have a file who's size is 64K and which is cached in a
>> single large folio in the page cache. But the file is mapped into a process at
>> VA 16K to 80K. Let's say we fault on the first page (VA=16K). You will calculate
> 
> For shmem, this doesn't happen because the VA is aligned with the hugepage size
> in the shmem_get_unmapped_area() function. See patch 7.

Certainly agree that shmem can always make sure that it packs a vma in a way
such that its folios are naturally aligned in VA when faulting in memory. If you
mremap it, that alignment will be lost; I don't think that would be a problem
for a single process; mremap will take care of moving the ptes correctly and
this path is not involved.

But what about the case when a process mmaps a shmem region, then forks, then
the child mremaps the shmem region. Then the parent faults in a THP into the
region (nicely aligned). Then the child faults in the same offset in the region
and gets the THP that the parent allocated; that THP will be aligned in the
parent's VM space but not in the child's.

> 
>> start=0 and end=64K I think?
> 
> Yes. Unfortunately, some file systems that support large mappings do not perform
> alignment for multi-size THP (non-PMD sized, for example: 64K). I think this
> requires modification to __get_unmapped_area--->thp_get_unmapped_area_vmflags()
> or file->f_op->get_unmapped_area() to align VA for multi-size THP in future.

By nature of the fact that a file mapping is shared between multiple processes
and each process can map it where ever it wants down to 1 page granularity, its
impossible for any THP containing a part of that file to be VA-aligned in every
process it is mapped in.

> 
> So before adding that VA alignment changes, only allow building the large folio
> mapping for anonymous shmem:
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 936377220b77..9e4d51826d23 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4786,7 +4786,7 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
>         folio = page_folio(page);
>         nr_pages = folio_nr_pages(folio);
> 
> -       if (unlikely(userfaultfd_armed(vma))) {
> +       if (unlikely(userfaultfd_armed(vma)) || !vma_is_anon_shmem(vma)) {

If the above theoretical flow for fork & mremap is valid, then I don't think
this is sufficient.

>                 nr_pages = 1;
>         } else if (nr_pages > 1) {
>                 unsigned long start = ALIGN_DOWN(vmf->address, nr_pages *
> PAGE_SIZE);
> 
>> Additionally, I think this path will end up mapping the entire folio (as long as
>> it fits in the VMA). But this bypasses the fault-around configuration. As I
>> think I mentioned against the RFC, this will inflate the RSS of the process and
>> can cause behavioural changes as a result. I believe the current advice is to
>> disable fault-around to prevent this kind of bloat when needed.
> 
> With above change, I do not think this is a problem? since users already want to
> use mTHP for anonymous shmem.
> 
>> It might be that you need a special variant of finish_fault() for shmem?
>>
>>
>>> +    }
>>>       vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
>>> -                      vmf->address, &vmf->ptl);
>>> +                       addr, &vmf->ptl);
>>>       if (!vmf->pte)
>>>           return VM_FAULT_NOPAGE;
>>>         /* Re-check under ptl */
>>> -    if (likely(!vmf_pte_changed(vmf))) {
>>> -        struct folio *folio = page_folio(page);
>>> -        int type = is_cow ? MM_ANONPAGES : mm_counter_file(folio);
>>> -
>>> -        set_pte_range(vmf, folio, page, 1, vmf->address);
>>> -        add_mm_counter(vma->vm_mm, type, 1);
>>> -        ret = 0;
>>> -    } else {
>>> -        update_mmu_tlb(vma, vmf->address, vmf->pte);
>>> +    if (nr_pages == 1 && unlikely(vmf_pte_changed(vmf))) {
>>> +        update_mmu_tlb(vma, addr, vmf->pte);
>>> +        ret = VM_FAULT_NOPAGE;
>>> +        goto unlock;
>>> +    } else if (nr_pages > 1 && !pte_range_none(vmf->pte, nr_pages)) {
>>> +        for (i = 0; i < nr_pages; i++)
>>> +            update_mmu_tlb(vma, addr + PAGE_SIZE * i, vmf->pte + i);
>>>           ret = VM_FAULT_NOPAGE;
>>> +        goto unlock;
>>>       }
>>>   +    set_pte_range(vmf, folio, page, nr_pages, addr);
>>> +    type = is_cow ? MM_ANONPAGES : mm_counter_file(folio);
>>> +    add_mm_counter(vma->vm_mm, type, nr_pages);
>>> +    ret = 0;
>>> +
>>> +unlock:
>>>       pte_unmap_unlock(vmf->pte, vmf->ptl);
>>>       return ret;
>>>   }