Re: [PATCH RFC v3 22/35] arm64: mte: Enable tag storage if CMA areas have been activated

[Evgenii Stepanov <eugenis@google.com>]

On Thu, Jan 25, 2024 at 8:44 AM Alexandru Elisei
<alexandru.elisei@arm.com> wrote:
>
> Before enabling MTE tag storage management, make sure that the CMA areas
> have been successfully activated. If a CMA area fails activation, the pages
> are kept as reserved. Reserved pages are never used by the page allocator.
>
> If this happens, the kernel would have to manage tag storage only for some
> of the memory, but not for all memory, and that would make the code
> unreasonably complicated.
>
> Choose to disable tag storage management altogether if a CMA area fails to
> be activated.
>
> Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
> ---
>
> Changes since v2:
>
> * New patch.
>
>  arch/arm64/include/asm/mte_tag_storage.h | 12 ++++++
>  arch/arm64/kernel/mte_tag_storage.c      | 50 ++++++++++++++++++++++++
>  2 files changed, 62 insertions(+)
>
> diff --git a/arch/arm64/include/asm/mte_tag_storage.h b/arch/arm64/include/asm/mte_tag_storage.h
> index 3c2cd29e053e..7b3f6bff8e6f 100644
> --- a/arch/arm64/include/asm/mte_tag_storage.h
> +++ b/arch/arm64/include/asm/mte_tag_storage.h
> @@ -6,8 +6,20 @@
>  #define __ASM_MTE_TAG_STORAGE_H
>
>  #ifdef CONFIG_ARM64_MTE_TAG_STORAGE
> +
> +DECLARE_STATIC_KEY_FALSE(tag_storage_enabled_key);
> +
> +static inline bool tag_storage_enabled(void)
> +{
> +       return static_branch_likely(&tag_storage_enabled_key);
> +}
> +
>  void mte_init_tag_storage(void);
>  #else
> +static inline bool tag_storage_enabled(void)
> +{
> +       return false;
> +}
>  static inline void mte_init_tag_storage(void)
>  {
>  }
> diff --git a/arch/arm64/kernel/mte_tag_storage.c b/arch/arm64/kernel/mte_tag_storage.c
> index 9a1a8a45171e..d58c68b4a849 100644
> --- a/arch/arm64/kernel/mte_tag_storage.c
> +++ b/arch/arm64/kernel/mte_tag_storage.c
> @@ -19,6 +19,8 @@
>
>  #include <asm/mte_tag_storage.h>
>
> +__ro_after_init DEFINE_STATIC_KEY_FALSE(tag_storage_enabled_key);
> +
>  struct tag_region {
>         struct range mem_range; /* Memory associated with the tag storage, in PFNs. */
>         struct range tag_range; /* Tag storage memory, in PFNs. */
> @@ -314,3 +316,51 @@ void __init mte_init_tag_storage(void)
>         num_tag_regions = 0;
>         pr_info("MTE tag storage region management disabled");
>  }
> +
> +static int __init mte_enable_tag_storage(void)
> +{
> +       struct range *tag_range;
> +       struct cma *cma;
> +       int i, ret;
> +
> +       if (num_tag_regions == 0)
> +               return 0;
> +
> +       for (i = 0; i < num_tag_regions; i++) {
> +               tag_range = &tag_regions[i].tag_range;
> +               cma = tag_regions[i].cma;
> +               /*
> +                * CMA will keep the pages as reserved when the region fails
> +                * activation.
> +                */
> +               if (PageReserved(pfn_to_page(tag_range->start)))
> +                       goto out_disabled;
> +       }
> +
> +       static_branch_enable(&tag_storage_enabled_key);
> +       pr_info("MTE tag storage region management enabled");
> +
> +       return 0;
> +
> +out_disabled:
> +       for (i = 0; i < num_tag_regions; i++) {
> +               tag_range = &tag_regions[i].tag_range;
> +               cma = tag_regions[i].cma;
> +
> +               if (PageReserved(pfn_to_page(tag_range->start)))
> +                       continue;
> +
> +               /* Try really hard to reserve the tag storage. */
> +               ret = cma_alloc(cma, range_len(tag_range), 8, true);
> +               /*
> +                * Tag storage is still in use for data, memory and/or tag
> +                * corruption will ensue.
> +                */
> +               WARN_ON_ONCE(ret);

cma_alloc returns (page *), so this condition needs to be inverted,
and the type of `ret` changed.
Not sure how it slipped through, this is a compile error with clang.

> +       }
> +       num_tag_regions = 0;
> +       pr_info("MTE tag storage region management disabled");
> +
> +       return -EINVAL;
> +}
> +arch_initcall(mte_enable_tag_storage);
> --
> 2.43.0
>