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

[Barry Song]

On Mon, Jun 3, 2024 at 8:58 PM Barry Song <21cnbao@xxxxxxxxx> wrote:
>
> On Mon, Jun 3, 2024 at 8:29 PM Baolin Wang
> <baolin.wang@xxxxxxxxxxxxxxxxx> wrote:
> >
> >
> >
> > 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.
>
> OK.
>
> >
> > > 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().
>
> OK.
>
> >
> > >> +        */
> > >> +       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.
>
> Is it possible to have a COW case for shmem? For example, if someone
> maps a shmem
> file as read-only and then writes to it, would that prevent the use of
> pte_range_none?

sorry, i mean PRIVATE but not READ-ONLY.

>
> Furthermore, if we encounter a large folio in shmem while reading,
> does it necessarily
> mean we can map the entire folio? Is it possible for some processes to
> only map part
> of large folios? For instance, if process A allocates large folios and
> process B maps
> only part of this shmem file or partially unmaps a large folio, how
> would that be handled?
>
> Apologies for not debugging this thoroughly, but these two corner
> cases seem worth
> considering. If these scenarios have already been addressed, please disregard my
> comments.
>
> Thanks
> Barry