linux-kernel - MAP_PRIVATE that stays private even on external write

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Date:	02 Dec 2008 10:06:45 GMT
From:	"Miquel van Smoorenburg" <miquels@...tron.nl>
To:	linux-kernel@...r.kernel.org
Subject: MAP_PRIVATE that stays private even on external write

I have an application that mmaps blocks from a file and then
sends it out over a TCP socket.

The contents of the file may be overwritten while the blocks
are sent out, but there is a very low chance of that.

To prevent this, I can ofcourse just use read() all blocks into
memory in advance, check consistency, and go ahead .. but since
there may be a lot of network connections active this might cost
a lot of memory.

What I am looking for is a MAP_PRIVATE type flag that, when another
process modifies pages of the file (through mmap() or write())
makes sure that my mapping never sees that.

I found out that mmap(MAP_PRIVATE|MAP_POPULATE) does exactly what
I want, but ofcourse it reads in the entire mapping all at once,
which is equivalent to plain read().

There has been talk of a MAP_SNAPSHOT flag before, e.g.
http://lkml.indiana.edu/hypermail/linux/kernel/0407.1/0416.htm

Has anyone ever looked at implementing something like this ?

Mike.
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