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Date:      Mon, 22 Apr 2002 16:22:58 -0400 (EDT)
From:      Andrew Gallatin <gallatin@cs.duke.edu>
To:        Kenneth Culver <culverk@alpha.yumyumyum.org>
Cc:        freebsd-hackers@freebsd.org, <freebsd-emulation@freebsd.org>
Subject:   Re: implementing linux mmap2 syscall
Message-ID:  <15556.28962.259876.509643@grasshopper.cs.duke.edu>
In-Reply-To: <20020422155927.E22719-100000@alpha.yumyumyum.org>
References:  <15556.6399.62081.426193@grasshopper.cs.duke.edu> <20020422155927.E22719-100000@alpha.yumyumyum.org>

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Kenneth Culver writes:
 > > To me, it looks like mmap2 takes an offset that's a page index, rather
 > > than a byte position.   Since linux passes the offset with a 32-bit
 > > long, rather than a 64-bit off_t like we do, they need to do this in
 > > order to be able to map offsets larger than 4GB into a file.
 > >
 > > For linux_mmap2, I'd think we want to do roughly the same things as
 > > linux_mmap, but with bsd_args.pos = ctob((off_t)linux_args.pos)
 > >
 > > Drew
 > >
 > >
 > AHH, ok I was wondering where PAGE_SHIFT was for FreeBSD. I guess ctob
 > does what I need it to. I think that's probably why it still wasn't
 > working yet... I think it also has to be page aligned before you pass it
 > in though, I have to look at linux's do_mmap_pgoff() (I think that's the
 > right function name) to see if it's expecting an already page-aligned arg,
 > or if it's aligning it before it uses it.

The name implies that do_mmap_pgoff() takes page-shift'ed args.
An offset specified as a page-shift is page-aligned by definition.
Eg, when you call ctob(pgoff) this turns out to be (pgoff <<
PAGE_SHIFT) bytes. 

Drew



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