Re: [PATCH v6 06/23] mm/shmem: Handle uffd-wp special pte in page fault handler

Alistair Popple
2021-12-16T05:56:54Z

On Monday, 15 November 2021 6:55:05 PM AEDT Peter Xu wrote:

[...]

> diff --git a/mm/memory.c b/mm/memory.c
> index d5966d9e24c3..e8557d43a87d 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3452,6 +3452,43 @@ static vm_fault_t remove_device_exclusive_entry(struct vm_fault *vmf)
> return 0;
> }
>
> +static vm_fault_t pte_marker_clear(struct vm_fault *vmf)
> +{
> + vmf->pte = pte_offset_map_lock(vmf->vma->vm_mm, vmf->pmd,
> + vmf->address, &vmf->ptl);
> + /*
> + * Be careful so that we will only recover a special uffd-wp pte into a
> + * none pte. Otherwise it means the pte could have changed, so retry.
> + */
> + if (is_pte_marker(*vmf->pte))
> + pte_clear(vmf->vma->vm_mm, vmf->address, vmf->pte);
> + pte_unmap_unlock(vmf->pte, vmf->ptl);
> + return 0;
> +}
> +
> +/*
> + * This is actually a page-missing access, but with uffd-wp special pte
> + * installed. It means this pte was wr-protected before being unmapped.
> + */
> +static vm_fault_t pte_marker_handle_uffd_wp(struct vm_fault *vmf)
> +{
> + /* Careful! vmf->pte unmapped after return */
> + if (!pte_unmap_same(vmf))

Hasn't vmf->pte already been unmapped by do_swap_page() by the time we get
here?

> + return 0;
> +
> + /*
> + * Just in case there're leftover special ptes even after the region
> + * got unregistered - we can simply clear them. We can also do that
> + * proactively when e.g. when we do UFFDIO_UNREGISTER upon some uffd-wp
> + * ranges, but it should be more efficient to be done lazily here.
> + */
> + if (unlikely(!userfaultfd_wp(vmf->vma) || vma_is_anonymous(vmf->vma)))
> + return pte_marker_clear(vmf);
> +
> + /* do_fault() can handle pte markers too like none pte */
> + return do_fault(vmf);
> +}
> +
> static vm_fault_t handle_pte_marker(struct vm_fault *vmf)
> {
> swp_entry_t entry = pte_to_swp_entry(vmf->orig_pte);
> @@ -3465,8 +3502,11 @@ static vm_fault_t handle_pte_marker(struct vm_fault *vmf)
> if (WARN_ON_ONCE(vma_is_anonymous(vmf->vma) || !marker))
> return VM_FAULT_SIGBUS;
>
> - /* TODO: handle pte markers */
> - return 0;
> + if (marker & PTE_MARKER_UFFD_WP)

Can we make this check `marker == PTE_MARKER_UFFD_WP`? There is currently only
one user of pte markers, and from what I can tell pte_marker_handle_uffd_wp()
wouldn't do the correct thing if other users were added because it could clear
non-uffd-wp markers. I don't think it's worth making it do the right thing now,
but a comment noting that would be helpful.

> + return pte_marker_handle_uffd_wp(vmf);
> +
> + /* This is an unknown pte marker */
> + return VM_FAULT_SIGBUS;
> }
>
> /*
> @@ -3968,6 +4008,7 @@ vm_fault_t do_set_pmd(struct vm_fault *vmf, struct page *page)
> void do_set_pte(struct vm_fault *vmf, struct page *page, unsigned long addr)
> {
> struct vm_area_struct *vma = vmf->vma;
> + bool uffd_wp = is_pte_marker_uffd_wp(vmf->orig_pte);
> bool write = vmf->flags & FAULT_FLAG_WRITE;
> bool prefault = vmf->address != addr;
> pte_t entry;
> @@ -3982,6 +4023,8 @@ void do_set_pte(struct vm_fault *vmf, struct page *page, unsigned long addr)
>
> if (write)
> entry = maybe_mkwrite(pte_mkdirty(entry), vma);
> + if (unlikely(uffd_wp))
> + entry = pte_mkuffd_wp(pte_wrprotect(entry));
> /* copy-on-write page */
> if (write && !(vma->vm_flags & VM_SHARED)) {
> inc_mm_counter_fast(vma->vm_mm, MM_ANONPAGES);
> @@ -4155,9 +4198,21 @@ static vm_fault_t do_fault_around(struct vm_fault *vmf)
> return vmf->vma->vm_ops->map_pages(vmf, start_pgoff, end_pgoff);
> }
>
> +/* Return true if we should do read fault-around, false otherwise */
> +static inline bool should_fault_around(struct vm_fault *vmf)
> +{
> + /* No ->map_pages? No way to fault around... */
> + if (!vmf->vma->vm_ops->map_pages)
> + return false;
> +
> + if (uffd_disable_fault_around(vmf->vma))
> + return false;
> +
> + return fault_around_bytes >> PAGE_SHIFT > 1;
> +}
> +
> static vm_fault_t do_read_fault(struct vm_fault *vmf)
> {
> - struct vm_area_struct *vma = vmf->vma;
> vm_fault_t ret = 0;
>
> /*
> @@ -4165,12 +4220,10 @@ static vm_fault_t do_read_fault(struct vm_fault *vmf)
> * if page by the offset is not ready to be mapped (cold cache or
> * something).
> */
> - if (vma->vm_ops->map_pages && fault_around_bytes >> PAGE_SHIFT > 1) {
> - if (likely(!userfaultfd_minor(vmf->vma))) {
> - ret = do_fault_around(vmf);
> - if (ret)
> - return ret;
> - }
> + if (should_fault_around(vmf)) {
> + ret = do_fault_around(vmf);
> + if (ret)
> + return ret;
> }
>
> ret = __do_fault(vmf);
>