linux-kernel - Re: [PATCH v2 17/31] arm64: System calls handling

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Message-ID: <20120903114831.GA3313@mbp>
Date:	Mon, 3 Sep 2012 12:48:31 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Will Deacon <Will.Deacon@....com>
Subject: Re: [PATCH v2 17/31] arm64: System calls handling

Hi Arnd,

On Wed, Aug 22, 2012 at 06:13:10PM +0100, Catalin Marinas wrote:
> On Wed, Aug 22, 2012 at 01:27:14PM +0100, Arnd Bergmann wrote:
> > On Wednesday 22 August 2012, Catalin Marinas wrote:
> > > But what's more important - moving this wrapper to glibc causes issues
> > > with the page size. We support both 4KB and 64KB pages on 64-bit systems
> > > (the latter without compat support). The kernel is in a better position
> > > to do the shift by a compile-time constant. Glibc would need to enquire
> > > the actual page size to do the shift before calling sys_mmap_pgoff. If
> > > we assume in glibc that the shift is always 12, we need another wrapper
> > > in the kernel anyway for 64KB page configuration. So passing the offset
> > > in bytes worked best for us.
> > 
> > Right, the kernel interface should really be independent of the page
> > size, as sys_mmap2 normally is, and sys_mmap2 is not provided here.
> 
> sys_mmap2 is indeed independent of the page size on most architectures
> assuming that the last argument represents the offset in units of 4096.
> The cris and ia64 seem to differ (one being 8K, the other variable).
> 
> sys_mmap is also independent of the page size.
> 
> But using sys_mmap2 for a 64-bit architecture, especially when the page
> size is not always 4K, does not bring any advantages. We end up doing a
> shift by 12 in glibc and another shift by (PAGE_SHIFT - 12) in the
> kernel wrapper. Unless I missed your point, I don't see the reason for
> using sys_mmap2 on a 64-bit architecture, apart from it being newer (and
> compat support should not have any relevance, we have different syscall
> tables anyway).

I forgot about this at the KS and we haven't got to a clear conclusion.

Do we (1) stick with the sys_mmap() for 64-bit systems and avoid offset
conversion in both glibc and kernel or (2) use sys_mmap2() with a 12
shift in glibc and (PAGE_SHIFT - 12) in the kernel wrapper?

I personally prefer (1) as it doesn't require a kernel wrapper and we
avoid the double shifting. A reason for (2) would be if we ever need
file offsets greater than 16EB.

-- 
Catalin
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