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

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Message-ID: <20120821220145.GA27934@arm.com>
Date:	Tue, 21 Aug 2012 23:01:45 +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

On Tue, Aug 21, 2012 at 09:14:01PM +0100, Arnd Bergmann wrote:
> On Tuesday 21 August 2012, Catalin Marinas wrote:
> > > > +asmlinkage long sys_mmap(unsigned long addr, unsigned long len,
> > > > +                    unsigned long prot, unsigned long flags,
> > > > +                    unsigned long fd, off_t off)
> > > > +{
> > > > +   if (offset_in_page(off) != 0)
> > > > +           return -EINVAL;
> > > > +
> > > > +   return sys_mmap_pgoff(addr, len, prot, flags, fd, off >> PAGE_SHIFT);
> > > > +}
> > > 
> > > I think
> > > 
> > > #define sys_mmap sys_mmap_pgoff 
> > 
> > There are slightly different semantics with the last argument of
> > sys_mmap() which takes a byte offset. The sys_mmap_pgoff() function
> > takes the offset shifted by PAGE_SHIFT (which is the same as sys_mmap2).
> > 
> > Looking at the other architectures, it makes sense to use a generic
> > sys_mmap() implementation similar to the one above (or the ia-64, seems
> > to be the most complete).
> 
> Why that? The generic sys_mmap_pgoff was specifically added so new architectures
> could just use that instead of having their own wrappers, see f8b72560.

As I understand, sys_mmap_pgoff can be used instead of sys_mmap2 on new
32-bit architectures. But on 64-bit architectures we don't have
sys_mmap2, only sys_mmap with the difference that the last argument is
the offset in bytes (and multiple of PAGE_SIZE) rather than in pages. So
unless we change the meaning of this last argument for sys_mmap, we
cannot just define it to sys_mmap_pgoff.

Since the other 64-bit architectures seem to have a sys_mmap wrapper
that does this:

	sys_mmap_pgoff(..., off >> PAGE_SHIFT);

I think AArch64 should also use the same sys_mmap convention. We can
make this wrapper generic.

-- 
Catalin
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