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

Thayne Harbaugh
2007-10-02T15:28:46Z

On Tue, 2007-10-02 at 15:16 +0100, Hugh Dickins wrote:
> 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.

I think my eyes have been opened enough so that I can get things to work
- it's certainly better in many respects than using MAP_32BIT with its
many limitations.

Thank you.