Re: [PATCH v3 1/6] mm: memory: extend finish_fault() to support large folio

[Baolin Wang]

On 2024/6/3 13:28, Barry Song wrote:
> On Thu, May 30, 2024 at 2:04 PM Baolin Wang
> <baolin.wang@xxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxx>
> > ---
> >   mm/memory.c | 58 ++++++++++++++++++++++++++++++++++++++++++++---------
> >   1 file changed, 48 insertions(+), 10 deletions(-)
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index eef4e482c0c2..435187ff7ea4 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -4831,9 +4831,12 @@ 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)
> > @@ -4864,24 +4867,59 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
> >                          return VM_FAULT_OOM;
> >          }
> >
> > +       folio = page_folio(page);
> > +       nr_pages = folio_nr_pages(folio);
> > +
> > +       /*
> > +        * Using per-page fault to maintain the uffd semantics, and same
> > +        * approach also applies to non-anonymous-shmem faults to avoid
> > +        * inflating the RSS of the process.
>
> I don't feel the comment explains the root cause.
> For non-shmem, anyway we have allocated the memory? Avoiding inflating
> RSS seems not so useful as we have occupied the memory. the memory footprint

This is also to keep the same behavior as before for non-anon-shmem, and 
will be discussed in the future.

> is what we really care about. so we want to rely on read-ahead hints of subpage
> to determine read-ahead size? that is why we don't map nr_pages for non-shmem
> files though we can potentially reduce nr_pages - 1 page faults?

IMHO, there is 2 cases for non-anon-shmem:
(1) read mmap() faults: we can rely on the 'fault_around_bytes' 
interface to determin what size of mapping to build.
(2) writable mmap() faults: I want to keep the same behavior as before 
(per-page fault), but we can talk about this when I send new patches to 
use mTHP to control large folio allocation for writable mmap().

> > +        */
> > +       if (!vma_is_anon_shmem(vma) || unlikely(userfaultfd_armed(vma))) {
> > +               nr_pages = 1;
> > +       } else if (nr_pages > 1) {
> > +               pgoff_t idx = folio_page_idx(folio, page);
> > +               /* The page offset of vmf->address within the VMA. */
> > +               pgoff_t vma_off = vmf->pgoff - vmf->vma->vm_pgoff;
> > +
> > +               /*
> > +                * Fallback to per-page fault in case the folio size in page
> > +                * cache beyond the VMA limits.
> > +                */
> > +               if (unlikely(vma_off < idx ||
> > +                            vma_off + (nr_pages - idx) > vma_pages(vma))) {
> > +                       nr_pages = 1;
> > +               } else {
> > +                       /* Now we can set mappings for the whole large folio. */
> > +                       addr = vmf->address - idx * PAGE_SIZE;
> > +                       page = &folio->page;
> > +               }
> > +       }
> > +
> >          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)) {
>
> In what case we can't use !pte_range_none(vmf->pte, 1) for nr_pages == 1
> then unify the code for nr_pages==1 and nr_pages > 1?
>
> It seems this has been discussed before, but I forget the reason.

IIUC, this is for uffd case, which is not a none pte entry.