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Date: Mon, 5 Feb 2024 16:30:17 +0000
From: Alexandru Elisei <alexandru.elisei@arm.com>
To: Evgenii Stepanov <eugenis@google.com>
Cc: catalin.marinas@arm.com, will@kernel.org, oliver.upton@linux.dev,
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Subject: Re: [PATCH RFC v3 22/35] arm64: mte: Enable tag storage if CMA areas
 have been activated
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Hi Evgenii,

On Fri, Feb 02, 2024 at 02:30:00PM -0800, Evgenii Stepanov wrote:
> 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.

Checked just now, it's a warning with gcc, I must have missed it. Will fix.

Thanks,
Alex

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