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Date:   Tue, 28 Nov 2023 18:55:18 +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>,
        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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        bristot@redhat.com, vschneid@redhat.com, mhiramat@kernel.org,
        rppt@kernel.org, hughd@google.com
Cc:     pcc@google.com, steven.price@arm.com, anshuman.khandual@arm.com,
        vincenzo.frascino@arm.com, eugenis@google.com, kcc@google.com,
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References: <20231119165721.9849-1-alexandru.elisei@arm.com>
 <20231119165721.9849-20-alexandru.elisei@arm.com>
From:   David Hildenbrand <david@redhat.com>
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In-Reply-To: <20231119165721.9849-20-alexandru.elisei@arm.com>
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Precedence: bulk

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 
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.

I *suspect* that handle_page_missing_tag_storage() stole (sorry :P) some 
code from the prot_none handling path. At least the recovery path and 
writability handling looks like it better be located shared in 
handle_pte_protnone() as well.

That might take some magic out of this patch.

-- 
Cheers,

David / dhildenb