Re: [PATCH v3 11/11] mm: rename vm_ops->find_special_page() to vm_ops->find_normal_page()

[Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>]

On Mon, Aug 11, 2025 at 01:26:31PM +0200, David Hildenbrand wrote:
> ... and hide it behind a kconfig option. There is really no need for
> any !xen code to perform this check.
>
> The naming is a bit off: we want to find the "normal" page when a PTE
> was marked "special". So it's really not "finding a special" page.
>
> Improve the documentation, and add a comment in the code where XEN ends
> up performing the pte_mkspecial() through a hypercall. More details can
> be found in commit 923b2919e2c3 ("xen/gntdev: mark userspace PTEs as
> special on x86 PV guests").
>
> Cc: David Vrabel <david.vrabel@xxxxxxxxxx>
> Reviewed-by: Oscar Salvador <osalvador@xxxxxxx>
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
> Reviewed-by: Wei Yang <richard.weiyang@xxxxxxxxx>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>

Oh I already reviewed it. But anyway, may as well say - THANKS fof this
it's great again :)

> ---
>  drivers/xen/Kconfig              |  1 +
>  drivers/xen/gntdev.c             |  5 +++--
>  include/linux/mm.h               | 18 +++++++++++++-----
>  mm/Kconfig                       |  2 ++
>  mm/memory.c                      | 12 ++++++++++--
>  tools/testing/vma/vma_internal.h | 18 +++++++++++++-----
>  6 files changed, 42 insertions(+), 14 deletions(-)
>
> diff --git a/drivers/xen/Kconfig b/drivers/xen/Kconfig
> index 24f485827e039..f9a35ed266ecf 100644
> --- a/drivers/xen/Kconfig
> +++ b/drivers/xen/Kconfig
> @@ -138,6 +138,7 @@ config XEN_GNTDEV
>       depends on XEN
>       default m
>       select MMU_NOTIFIER
> +     select FIND_NORMAL_PAGE
>       help
>         Allows userspace processes to use grants.
>
> diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
> index 1f21607656182..26f13b37c78e6 100644
> --- a/drivers/xen/gntdev.c
> +++ b/drivers/xen/gntdev.c
> @@ -321,6 +321,7 @@ static int find_grant_ptes(pte_t *pte, unsigned long 
> addr, void *data)
>       BUG_ON(pgnr >= map->count);
>       pte_maddr = arbitrary_virt_to_machine(pte).maddr;
>
> +     /* Note: this will perform a pte_mkspecial() through the hypercall. */
>       gnttab_set_map_op(&map->map_ops[pgnr], pte_maddr, flags,
>                         map->grants[pgnr].ref,
>                         map->grants[pgnr].domid);
> @@ -528,7 +529,7 @@ static void gntdev_vma_close(struct vm_area_struct *vma)
>       gntdev_put_map(priv, map);
>  }
>
> -static struct page *gntdev_vma_find_special_page(struct vm_area_struct *vma,
> +static struct page *gntdev_vma_find_normal_page(struct vm_area_struct *vma,
>                                                unsigned long addr)
>  {
>       struct gntdev_grant_map *map = vma->vm_private_data;
> @@ -539,7 +540,7 @@ static struct page *gntdev_vma_find_special_page(struct 
> vm_area_struct *vma,
>  static const struct vm_operations_struct gntdev_vmops = {
>       .open = gntdev_vma_open,
>       .close = gntdev_vma_close,
> -     .find_special_page = gntdev_vma_find_special_page,
> +     .find_normal_page = gntdev_vma_find_normal_page,
>  };
>
>  /* ------------------------------------------------------------------ */
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 8ca7d2fa71343..3868ca1a25f9c 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -657,13 +657,21 @@ struct vm_operations_struct {
>       struct mempolicy *(*get_policy)(struct vm_area_struct *vma,
>                                       unsigned long addr, pgoff_t *ilx);
>  #endif
> +#ifdef CONFIG_FIND_NORMAL_PAGE
>       /*
> -      * Called by vm_normal_page() for special PTEs to find the
> -      * page for @addr.  This is useful if the default behavior
> -      * (using pte_page()) would not find the correct page.
> +      * Called by vm_normal_page() for special PTEs in @vma at @addr. This
> +      * allows for returning a "normal" page from vm_normal_page() even
> +      * though the PTE indicates that the "struct page" either does not exist
> +      * or should not be touched: "special".
> +      *
> +      * Do not add new users: this really only works when a "normal" page
> +      * was mapped, but then the PTE got changed to something weird (+
> +      * marked special) that would not make pte_pfn() identify the originally
> +      * inserted page.
>        */
> -     struct page *(*find_special_page)(struct vm_area_struct *vma,
> -                                       unsigned long addr);
> +     struct page *(*find_normal_page)(struct vm_area_struct *vma,
> +                                      unsigned long addr);
> +#endif /* CONFIG_FIND_NORMAL_PAGE */
>  };
>
>  #ifdef CONFIG_NUMA_BALANCING
> diff --git a/mm/Kconfig b/mm/Kconfig
> index e443fe8cd6cf2..59a04d0b2e272 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -1381,6 +1381,8 @@ config PT_RECLAIM
>
>         Note: now only empty user PTE page table pages will be reclaimed.
>
> +config FIND_NORMAL_PAGE
> +     def_bool n
>
>  source "mm/damon/Kconfig"
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 6f806bf3cc994..002c28795d8b7 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -639,6 +639,12 @@ static void print_bad_page_map(struct vm_area_struct 
> *vma,
>   * trivial. Secondly, an architecture may not have a spare page table
>   * entry bit, which requires a more complicated scheme, described below.
>   *
> + * With CONFIG_FIND_NORMAL_PAGE, we might have the "special" bit set on
> + * page table entries that actually map "normal" pages: however, that page
> + * cannot be looked up through the PFN stored in the page table entry, but
> + * instead will be looked up through vm_ops->find_normal_page(). So far, this
> + * only applies to PTEs.
> + *
>   * A raw VM_PFNMAP mapping (ie. one that is not COWed) is always considered a
>   * special mapping (even if there are underlying and valid "struct pages").
>   * COWed pages of a VM_PFNMAP are always normal.
> @@ -679,8 +685,10 @@ static inline struct page *__vm_normal_page(struct 
> vm_area_struct *vma,
>  {
>       if (IS_ENABLED(CONFIG_ARCH_HAS_PTE_SPECIAL)) {
>               if (unlikely(special)) {
> -                     if (vma->vm_ops && vma->vm_ops->find_special_page)
> -                             return vma->vm_ops->find_special_page(vma, 
> addr);
> +#ifdef CONFIG_FIND_NORMAL_PAGE
> +                     if (vma->vm_ops && vma->vm_ops->find_normal_page)
> +                             return vma->vm_ops->find_normal_page(vma, addr);
> +#endif /* CONFIG_FIND_NORMAL_PAGE */
>                       if (vma->vm_flags & (VM_PFNMAP | VM_MIXEDMAP))
>                               return NULL;
>                       if (is_zero_pfn(pfn) || is_huge_zero_pfn(pfn))
> diff --git a/tools/testing/vma/vma_internal.h 
> b/tools/testing/vma/vma_internal.h
> index 3639aa8dd2b06..cb1c2a8afe265 100644
> --- a/tools/testing/vma/vma_internal.h
> +++ b/tools/testing/vma/vma_internal.h
> @@ -467,13 +467,21 @@ struct vm_operations_struct {
>       struct mempolicy *(*get_policy)(struct vm_area_struct *vma,
>                                       unsigned long addr, pgoff_t *ilx);
>  #endif
> +#ifdef CONFIG_FIND_NORMAL_PAGE
>       /*
> -      * Called by vm_normal_page() for special PTEs to find the
> -      * page for @addr.  This is useful if the default behavior
> -      * (using pte_page()) would not find the correct page.
> +      * Called by vm_normal_page() for special PTEs in @vma at @addr. This
> +      * allows for returning a "normal" page from vm_normal_page() even
> +      * though the PTE indicates that the "struct page" either does not exist
> +      * or should not be touched: "special".
> +      *
> +      * Do not add new users: this really only works when a "normal" page
> +      * was mapped, but then the PTE got changed to something weird (+
> +      * marked special) that would not make pte_pfn() identify the originally
> +      * inserted page.
>        */
> -     struct page *(*find_special_page)(struct vm_area_struct *vma,
> -                                       unsigned long addr);
> +     struct page *(*find_normal_page)(struct vm_area_struct *vma,
> +                                      unsigned long addr);
> +#endif /* CONFIG_FIND_NORMAL_PAGE */
>  };
>
>  struct vm_unmapped_area_info {
> --
> 2.50.1
>