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Date: Sat, 10 Aug 2002 11:42:44 -0700 (PDT)
From: Linus Torvalds <torvalds@transmeta.com>
To: Jamie Lokier <lk@tantalophile.demon.co.uk>
cc: Andrew Morton <akpm@zip.com.au>, lkml <linux-kernel@vger.kernel.org>
Subject: Re: [patch 6/12] hold atomic kmaps across generic_file_read
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On Sat, 10 Aug 2002, Jamie Lokier wrote:
> 
> Don't forget to include the need for mmap(... MAP_ANON ...) prior to the
> read.

Ahhah! But I _don't_.

Yes, with read() you have to do a brk() or mmap(MAP_ANON) (and brk() is 
the _much_ faster of the two).

But with mmap() you need to do a fstat() and a munmap() (while with read
you just re-use the area, and we'd do the right thing thanks to the
COW-ness of the pages).

So I don't think the MAP_ANON thing is a loss for the read.

And read() is often the much nicer interface, simply because you don't
need to worry about the size of the file up-front etc.

Also, because of the delayed nature of mmap()/fault, it has some strange
behaviour if somebody is editing your file in the middle of the compile -
with read() you might get strange syntax errors if somebody changes the
file half-way, but with mmap() your preprocessor may get a SIGSEGV in the
middle just because the file was truncated..

In general, I think read() tends to be the right (and simpler) interface
to use if you don't explicitly want to take advantage of the things mmap
offers (on-demand mappings, no-write-back pageouts, VM coherency etc).

		Linus

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