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

Lance Yang
2024-06-03T04:44:50Z

On Thu, May 30, 2024 at 10:04 AM Baolin Wang
<baolin.wang@linux.alibaba.com> 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 | 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.
> + */
> + 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)) {
> + for (i = 0; i < nr_pages; i++)
> + update_mmu_tlb(vma, addr + PAGE_SIZE * i, vmf->pte + i);

Just a friendly reminder: Bang has added the update_mmu_tlb_range()[1] batch
function to update TLB in batches, so we can use it instead of the
update_mmu_tlb()
loop.

[1] https://lore.kernel.org/linux-mm/20240522061204.117421-1-libang.li@antgroup.com/

Thanks,
Lance

> + ret = VM_FAULT_NOPAGE;
> + goto unlock;
> }
>
> + folio_ref_add(folio, nr_pages - 1);
> + 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;
> }
> --
> 2.39.3
>