Re: [RFC PATCH 1/5] mm: memory: extend finish_fault() to support large folio

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

On 24/04/2024 10:26, Baolin Wang wrote:
> 
> 
> On 2024/4/24 16:07, Ryan Roberts wrote:
>> On 24/04/2024 04:23, Baolin Wang wrote:
>>>
>>>
>>> On 2024/4/23 19:03, Ryan Roberts wrote:
>>>> On 22/04/2024 08:02, Baolin Wang wrote:
>>>>> Add large folio mapping establishment support for finish_fault() as a
>>>>> preparation,
>>>>> to support multi-size THP allocation of anonymous shared pages in the
>>>>> following
>>>>> patches.
>>>>>
>>>>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>>>>> ---
>>>>>    mm/memory.c | 25 ++++++++++++++++++-------
>>>>>    1 file changed, 18 insertions(+), 7 deletions(-)
>>>>>
>>>>> diff --git a/mm/memory.c b/mm/memory.c
>>>>> index b6fa5146b260..094a76730776 100644
>>>>> --- a/mm/memory.c
>>>>> +++ b/mm/memory.c
>>>>> @@ -4766,7 +4766,10 @@ 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;
>>>>> +    int nr_pages, i;
>>>>> +    unsigned long addr;
>>>>>          /* Did we COW the page? */
>>>>>        if ((vmf->flags & FAULT_FLAG_WRITE) && !(vma->vm_flags & VM_SHARED))
>>>>> @@ -4797,22 +4800,30 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
>>>>>                return VM_FAULT_OOM;
>>>>>        }
>>>>>    +    folio = page_folio(page);
>>>>> +    nr_pages = folio_nr_pages(folio);
>>>>> +    addr = ALIGN_DOWN(vmf->address, nr_pages * PAGE_SIZE);
>>>>
>>>> I'm not sure this is safe. IIUC, finish_fault() is called for any file-backed
>>>> mapping. So you could have a situation where part of a (regular) file is mapped
>>>> in the process, faults and hits in the pagecache. But the folio returned by the
>>>> pagecache is bigger than the portion that the process has mapped. So you now
>>>> end
>>>> up mapping beyond the VMA limits? In the pagecache case, you also can't assume
>>>> that the folio is naturally aligned in virtual address space.
>>>
>>> Good point. Yes, I think you are right, I need consider the VMA limits, and I
>>> should refer to the calculations of the start pte and end pte in
>>> do_fault_around().
>>
>> You might also need to be careful not to increase reported RSS. I have a vague
>> recollection that David once mentioned a problem with fault-around because it
>> causes the reported RSS to increase for the process and this could lead to
>> different decisions in other places. IIRC Redhat had an advisory somewhere with
>> suggested workaround being to disable fault-around. For the anon-shared memory
>> case, it shouldn't be a problem because the user has opted into allocating
>> bigger blocks, but there may be a need to ensure we don't also start eagerly
>> mapping regular files beyond what fault-around is configured for.
> 
> Thanks for reminding. And I also agree with you that this should not be a
> problem since user has selected the larger folio, which is not the same as
> fault-around.
> 
>>>>>        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);
>>>>> -
>>>>> -        set_pte_range(vmf, folio, page, 1, vmf->address);
>>>>> -        ret = 0;
>>>>> -    } else {
>>>>> +    if (nr_pages == 1 && vmf_pte_changed(vmf)) {
>>>>>            update_mmu_tlb(vma, vmf->address, vmf->pte);
>>>>>            ret = VM_FAULT_NOPAGE;
>>>>> +        goto unlock;
>>>>> +    } else if (nr_pages > 1 && !pte_range_none(vmf->pte, nr_pages)) {
>>>>
>>>> I think you have grabbed this from do_anonymous_page()? But I'm not sure it
>>>> works in the same way here as it does there. For the anon case, if userfaultfd
>>>> is armed, alloc_anon_folio() will only ever allocate order-0. So we end up in
>>>
>>> IMO, the userfaultfd validation should do in the vma->vm_ops->fault() callback,
>>> to make sure the nr_pages is always 1 if userfaultfd is armed.
>>
>> OK. Are you saying there is already logic to do that today? Great!
> 
> I mean I should add the userfaultfd validation in shmem_fault(), and may be need
> add a warning in finish_fault() to catch this issue if other
> vma->vm_ops->fault() will support large folio allocation?
> 
> WARN_ON(nr_pages > 1 && userfaultfd_armed(vma));

That adds quite a subtle requirement to vm_ops::fault() though, which I guess is
implemented in a lot of places. It would be better if it could be handled
centrally - i.e. that all the ptes are either none or a uffd marker? I'm sure
there would be corner cases to think about if taking that route.