Re: MAP_PRIVATE that stays private even on external write

From: Peter Zijlstra
Date: Tue Dec 02 2008 - 05:28:23 EST

On Tue, 2008-12-02 at 10:06 +0000, Miquel van Smoorenburg wrote:
> 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.
> Has anyone ever looked at implementing something like this ?

I suppose that needs a snapshot filesystem for backing, and we don't
have such a creature (yet). I suppose BTRFS might be able to pull that
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