linux-kernel - Re: [PATCH RFC v2 19/27] mm: mprotect: Introduce PAGE_FAULT_ON_ACCESS for mprotect(PROT

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Date:   Tue, 28 Nov 2023 18:55:18 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Alexandru Elisei <alexandru.elisei@....com>,
        catalin.marinas@....com, will@...nel.org, oliver.upton@...ux.dev,
        maz@...nel.org, james.morse@....com, suzuki.poulose@....com,
        yuzenghui@...wei.com, arnd@...db.de, akpm@...ux-foundation.org,
        mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, vschneid@...hat.com, mhiramat@...nel.org,
        rppt@...nel.org, hughd@...gle.com
Cc:     pcc@...gle.com, steven.price@....com, anshuman.khandual@....com,
        vincenzo.frascino@....com, eugenis@...gle.com, kcc@...gle.com,
        hyesoo.yu@...sung.com, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, kvmarm@...ts.linux.dev,
        linux-fsdevel@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-mm@...ck.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH RFC v2 19/27] mm: mprotect: Introduce PAGE_FAULT_ON_ACCESS
 for mprotect(PROT_MTE)

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