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

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Date:   Wed, 29 Nov 2023 13:48:51 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Alexandru Elisei <alexandru.elisei@....com>
Cc:     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, 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 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