Re: [PATCH V8 33/44] memremap_pages: Introduce pgmap_protection_available()

[Dan Williams]

On Thu, Jan 27, 2022 at 9:55 AM <ira.weiny@xxxxxxxxx> wrote:
>
> From: Ira Weiny <ira.weiny@xxxxxxxxx>
>
> Users will need to specify that they want their dev_pagemap pages
> protected by specifying a flag in (struct dev_pagemap)->flags.  However,
> it is more efficient to know if that protection is available prior to
> requesting it and failing the mapping.
>
> Define pgmap_protection_available() for users to check if protection is
> available to be used.  The name of pgmap_protection_available() was
> specifically chosen to isolate the implementation of the protection from
> higher level users.  However, the current implementation simply calls
> pks_available() to determine if it can support protection.
>
> It was considered to have users specify the flag and check if the
> dev_pagemap object returned was protected or not.  But this was
> considered less efficient than a direct check beforehand.
>
> Signed-off-by: Ira Weiny <ira.weiny@xxxxxxxxx>
>
> ---
> Changes for V8
>         Split this out to it's own patch.
>         s/pgmap_protection_enabled/pgmap_protection_available
> ---
>  include/linux/mm.h | 13 +++++++++++++
>  mm/memremap.c      | 11 +++++++++++
>  2 files changed, 24 insertions(+)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index e1a84b1e6787..2ae99bee6e82 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1143,6 +1143,19 @@ static inline bool is_pci_p2pdma_page(const struct page *page)
>                 page->pgmap->type == MEMORY_DEVICE_PCI_P2PDMA;
>  }
>
> +#ifdef CONFIG_DEVMAP_ACCESS_PROTECTION
> +
> +bool pgmap_protection_available(void);
> +
> +#else
> +
> +static inline bool pgmap_protection_available(void)
> +{
> +       return false;
> +}
> +
> +#endif /* CONFIG_DEVMAP_ACCESS_PROTECTION */
> +
>  /* 127: arbitrary random number, small enough to assemble well */
>  #define folio_ref_zero_or_close_to_overflow(folio) \
>         ((unsigned int) folio_ref_count(folio) + 127u <= 127u)
> diff --git a/mm/memremap.c b/mm/memremap.c
> index 6aa5f0c2d11f..c13b3b8a0048 100644
> --- a/mm/memremap.c
> +++ b/mm/memremap.c
> @@ -6,6 +6,7 @@
>  #include <linux/memory_hotplug.h>
>  #include <linux/mm.h>
>  #include <linux/pfn_t.h>
> +#include <linux/pkeys.h>
>  #include <linux/swap.h>
>  #include <linux/mmzone.h>
>  #include <linux/swapops.h>
> @@ -63,6 +64,16 @@ static void devmap_managed_enable_put(struct dev_pagemap *pgmap)
>  }
>  #endif /* CONFIG_DEV_PAGEMAP_OPS */
>
> +#ifdef CONFIG_DEVMAP_ACCESS_PROTECTION
> +
> +bool pgmap_protection_available(void)
> +{
> +       return pks_available();
> +}
> +EXPORT_SYMBOL_GPL(pgmap_protection_available);

Any reason this was chosen to be an out-of-line function? Doesn't this
defeat the performance advantages of static_cpu_has()?

> +
> +#endif /* CONFIG_DEVMAP_ACCESS_PROTECTION */
> +
>  static void pgmap_array_delete(struct range *range)
>  {
>         xa_store_range(&pgmap_array, PHYS_PFN(range->start), PHYS_PFN(range->end),
> --
> 2.31.1
>