Re: [PATCH v2 03/13] mm: Add generic p?d_large() macros

From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Mike Rapoport <rppt@linux.ibm.com>
Cc: "Steven Price" <steven.price@arm.com>,
	"Mark Rutland" <Mark.Rutland@arm.com>,
	"Dave Hansen" <dave.hansen@linux.intel.com>,
	"Arnd Bergmann" <arnd@arndb.de>,
	"Ard Biesheuvel" <ard.biesheuvel@linaro.org>,
	"Peter Zijlstra" <peterz@infradead.org>,
	"Catalin Marinas" <catalin.marinas@arm.com>,
	x86@kernel.org, "Will Deacon" <will.deacon@arm.com>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	"Jérôme Glisse" <jglisse@redhat.com>,
	"Ingo Molnar" <mingo@redhat.com>,
	"Borislav Petkov" <bp@alien8.de>,
	"Andy Lutomirski" <luto@kernel.org>,
	"H. Peter Anvin" <hpa@zytor.com>,
	"James Morse" <james.morse@arm.com>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	linux-arm-kernel@lists.infradead.org, "Liang,
	Kan" <kan.liang@linux.intel.com>
Subject: Re: [PATCH v2 03/13] mm: Add generic p?d_large() macros
Date: Fri, 1 Mar 2019 15:30:32 +0300	[thread overview]
Message-ID: <20190301123031.rw3dswcoaa2x7haq@kshutemo-mobl1> (raw)
In-Reply-To: <20190301115300.GE5156@rapoport-lnx>

On Fri, Mar 01, 2019 at 01:53:01PM +0200, Mike Rapoport wrote:
> Him Kirill,
> 
> On Fri, Feb 22, 2019 at 12:06:18AM +0300, Kirill A. Shutemov wrote:
> > On Thu, Feb 21, 2019 at 05:16:46PM +0000, Steven Price wrote:
> > > >> Note that in terms of the new page walking code, these new defines are
> > > >> only used when walking a page table without a VMA (which isn't currently
> > > >> done), so architectures which don't use p?d_large currently will work
> > > >> fine with the generic versions. They only need to provide meaningful
> > > >> definitions when switching to use the walk-without-a-VMA functionality.
> > > > 
> > > > How other architectures would know that they need to provide the helpers
> > > > to get walk-without-a-VMA functionality? This looks very fragile to me.
> > > 
> > > Yes, you've got a good point there. This would apply to the p?d_large
> > > macros as well - any arch which (inadvertently) uses the generic version
> > > is likely to be fragile/broken.
> > > 
> > > I think probably the best option here is to scrap the generic versions
> > > altogether and simply introduce a ARCH_HAS_PXD_LARGE config option which
> > > would enable the new functionality to those arches that opt-in. Do you
> > > think this would be less fragile?
> > 
> > These helpers are useful beyond pagewalker.
> > 
> > Can we actually do some grinding and make *all* archs to provide correct
> > helpers? Yes, it's tedious, but not that bad.
> 
> Many architectures simply cannot support non-leaf entries at the higher
> levels. I think letting the use a generic helper actually does make sense.

I disagree.

It's makes sense if the level doesn't exists on the arch.

But if the level exists, it will be less frugile to ask the arch to
provide the helper. Even if it is dummy always-false.

-- 
 Kirill A. Shutemov