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Message-ID: <ca069bdf-3d53-41d9-adf1-f7a8b245f57b@redhat.com>
Date:   Wed, 29 Nov 2023 13:48:51 +0100
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Subject: Re: [PATCH RFC v2 19/27] mm: mprotect: Introduce PAGE_FAULT_ON_ACCESS
 for mprotect(PROT_MTE)
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To:     Alexandru Elisei <alexandru.elisei@arm.com>
Cc:     catalin.marinas@arm.com, will@kernel.org, oliver.upton@linux.dev,
        maz@kernel.org, james.morse@arm.com, suzuki.poulose@arm.com,
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References: <20231119165721.9849-1-alexandru.elisei@arm.com>
 <20231119165721.9849-20-alexandru.elisei@arm.com>
 <1c79ad05-cb52-4820-b2aa-bbe07ff82b19@redhat.com> <ZWcmuzUcpVeUnlk2@raptor>
From:   David Hildenbrand <david@redhat.com>
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In-Reply-To: <ZWcmuzUcpVeUnlk2@raptor>
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On 29.11.23 12:55, Alexandru Elisei wrote:
> Hi,
> 
> On Tue, Nov 28, 2023 at 06:55:18PM +0100, David Hildenbrand wrote:
>> On 19.11.23 17:57, Alexandru Elisei wrote:
>>> To enable tagging on a memory range, userspace can use mprotect() with the
>>> PROT_MTE access flag. Pages already mapped in the VMA don't have the
>>> associated tag storage block reserved, so mark the PTEs as
>>> PAGE_FAULT_ON_ACCESS to trigger a fault next time they are accessed, and
>>> reserve the tag storage on the fault path.
>>
>> That sounds alot like fake PROT_NONE. Would there be a way to unify hat
> 
> Yes, arm64 basically defines PAGE_FAULT_ON_ACCESS as PAGE_NONE |
> PTE_TAG_STORAGE_NONE.
> 
>> handling and simply reuse pte_protnone()? For example, could we special case
>> on VMA flags?
>>
>> Like, don't do NUMA hinting in these special VMAs. Then, have something
>> like:
>>
>> if (pte_protnone(vmf->orig_pte))
>> 	return handle_pte_protnone(vmf);
>>
>> In there, special case on the VMA flags.
> 
> Your suggestion from the follow-up reply that an arch should know if it needs to
> do something was spot on, arm64 can use the software bit in the translation
> table entry for that.
> 
> So what you are proposing is this:
> 
> * Rename do_numa_page->handle_pte_protnone
> * At some point in the do_numa_page (now renamed to handle_pte_protnone) flow,
>    decide if pte_protnone() has been set for an arch specific reason or because
>    of automatic NUMA balancing.
> * if pte_protnone() has been set by an architecture, then let the architecture
>    handle the fault.
> 
> If I understood you correctly, that's a good idea, and should be easy to
> implement.

yes! :)

-- 
Cheers,

David / dhildenb