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

[Baolin Wang <baolin.wang@linux.alibaba.com>]

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));