Re: [discuss] [PATCH] Inconsistent mmap()/mremap() flags

[Hugh Dickins <hugh@veritas.com>]

On Tue, 2 Oct 2007, Andi Kleen wrote:
> 
> > First call mmap with a low hint address, the new size you'll be wanting
> > from the mremap, PROT_NONE, MAP_ANONYMOUS, -1, 0.  Then call mremap with
> > old address, old size, new size, MREMAP_MAYMOVE|MREMAP_FIXED, and new
> > address as returned by the preparatory mmap.
> 
> That's racy unfortunately in a multithreaded process. They would need to loop.

Perhaps.  Though I don't see what your loop would be doing;
and the mapping established by the first thread would only
be vulnerable to another thread if that were really set on
interfering (an un-FIXED mmap by another thread will keep
away from the area assigned to the first).

Certainly a two-stage procedure has to be weaker than one stage,
but it is just how MAP_FIXED is normally used (isn't it?): first
stake out an arena for all that's needed without MAP_FIXED, then
fit into it the actual mappings required using MAP_FIXED.  Blind
use of MAP_FIXED is always in danger of unmapping something vital.

But whether the two-stage procedure is good enough for Thayne's
purpose, he'll have to judge for himself.

Hugh

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