linux-kernel - Re: [PATCH v12 3/8] arm64: mte: Sync tags for pages where PTE is untagged

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Message-ID: <20210520130224.GE12251@arm.com>
Date:   Thu, 20 May 2021 14:02:25 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Steven Price <steven.price@....com>
Cc:     Marc Zyngier <maz@...nel.org>, Will Deacon <will@...nel.org>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        kvmarm@...ts.cs.columbia.edu, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, Dave Martin <Dave.Martin@....com>,
        Mark Rutland <mark.rutland@....com>,
        Thomas Gleixner <tglx@...utronix.de>, qemu-devel@...gnu.org,
        Juan Quintela <quintela@...hat.com>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>,
        Richard Henderson <richard.henderson@...aro.org>,
        Peter Maydell <peter.maydell@...aro.org>,
        Haibo Xu <Haibo.Xu@....com>, Andrew Jones <drjones@...hat.com>
Subject: Re: [PATCH v12 3/8] arm64: mte: Sync tags for pages where PTE is
 untagged

On Thu, May 20, 2021 at 01:25:50PM +0100, Catalin Marinas wrote:
> On Thu, May 20, 2021 at 12:55:21PM +0100, Steven Price wrote:
> > The problem I hit is one of include dependencies:
> > 
> > is_swap_pte() is defined (as a static inline) in
> > include/linux/swapops.h. However the definition depends on
> > pte_none()/pte_present() which are defined in pgtable.h - so there's a
> > circular dependency.
> > 
> > Open coding is_swap_pte() in set_pte_at() works, but it's a bit ugly.
> > Any ideas on how to improve on the below?
> > 
> > 	if (system_supports_mte() && pte_present(pte) &&
> > 	    pte_access_permitted(pte, false) && !pte_special(pte)) {
> > 		pte_t old_pte = READ_ONCE(*ptep);
> > 		/*
> > 		 * We only need to synchronise if the new PTE has tags enabled
> > 		 * or if swapping in (in which case another mapping may have
> > 		 * set tags in the past even if this PTE isn't tagged).
> > 		 * (!pte_none() && !pte_present()) is an open coded version of
> > 		 * is_swap_pte()
> > 		 */
> > 		if (pte_tagged(pte) || (!pte_none(pte) && !pte_present(pte)))
> > 			mte_sync_tags(old_pte, pte);
> > 	}
> 
> That's why I avoided testing my suggestion ;). I think we should just
> add !pte_none() in there with a comment that it may be a swap pte and
> use the is_swap_pte() again on the mte_sync_tags() path. We already have
> the pte_present() check.

Correction - pte_present() checks the new pte only, we need another for
the old pte. So it looks like we'll open-code is_swap_pte().

-- 
Catalin