Re: [PATCH V12] mm/debug: Add tests validating architecture page table helpers

From: Catalin Marinas <catalin.marinas@arm.com>
To: Qian Cai <cai@lca.pw>
Cc: Anshuman Khandual <Anshuman.Khandual@arm.com>,
	Mark Rutland <Mark.Rutland@arm.com>,
	linux-ia64@vger.kernel.org, linux-sh@vger.kernel.org,
	Peter Zijlstra <peterz@infradead.org>,
	James Hogan <jhogan@kernel.org>,
	Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
	Heiko Carstens <heiko.carstens@de.ibm.com>,
	Michal Hocko <mhocko@kernel.org>,
	linux-mm@kvack.org, Dave Hansen <dave.hansen@intel.com>,
	Paul Mackerras <paulus@samba.org>,
	sparclinux@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
	linux-s390@vger.kernel.org, Michael Ellerman <mpe@ellerman.id.au>,
	x86@kernel.org, Russell King - ARM Linux <linux@armlinux.org.uk>,
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	Steven Price <Steven.Price@arm.com>,
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	linuxppc-dev@lists.ozlabs.org,
	"David S. Miller" <davem@davemloft.net>
Subject: Re: [PATCH V12] mm/debug: Add tests validating architecture page table helpers
Date: Tue, 28 Jan 2020 17:47:09 +0000	[thread overview]
Message-ID: <20200128174709.GK655507@arrakis.emea.arm.com> (raw)
In-Reply-To: <14882A91-17DE-4ABD-ABF2-08E7CCEDF660@lca.pw>

On Mon, Jan 27, 2020 at 09:11:53PM -0500, Qian Cai wrote:
> On Jan 27, 2020, at 8:28 PM, Anshuman Khandual <Anshuman.Khandual@arm.com> wrote:
> > This adds tests which will validate architecture page table helpers and
> > other accessors in their compliance with expected generic MM semantics.
> > This will help various architectures in validating changes to existing
> > page table helpers or addition of new ones.
[...]
> What’s the value of this block of new code? It only supports x86 and
> arm64 which are supposed to be good now. Did those tests ever find any
> regression or this is almost only useful for new architectures which
> only happened once in a few years?

The primary goal here is not finding regressions but having clearly
defined semantics of the page table accessors across architectures. x86
and arm64 are a good starting point and other architectures will be
enabled as they are aligned to the same semantics.

See for example this past discussion:

https://lore.kernel.org/linux-mm/20190628102003.GA56463@arrakis.emea.arm.com/

These tests should act as the 'contract' between the generic mm code and
the architecture port. Without clear semantics, some bugs may be a lot
subtler than a boot failure.

FTR, I fully support this patch (and I should get around to review it
properly; thanks for the reminder ;)).

-- 
Catalin