Re: [PATCH] mm: per-thread vma caching

[Linus Torvalds <torvalds@linux-foundation.org>]

On Fri, Feb 21, 2014 at 12:57 PM, Davidlohr Bueso <davidlohr@hp.com> wrote:
>
> Btw, one concern I had is regarding seqnum overflows... if such
> scenarios should happen we'd end up potentially returning bogus vmas and
> getting bus errors and other sorts of issues. So we'd have to flush the
> caches, but, do we care? I guess on 32bit systems it could be a bit more
> possible to trigger given enough forking.

I guess we should do something like

    if (unlikely(!++seqnum))
        flush_vma_cache()

just to not have to worry about it.

And we can either use a "#ifndef CONFIG_64BIT" to disable it for the
64-bit case (because no, we really don't need to worry about overflow
in 64 bits ;), or just decide that a 32-bit sequence number actually
packs better in the structures, and make it be an "u32" even on 64-bit
architectures?

It looks like a 32-bit sequence number might pack nicely next to the

    unsigned brk_randomized:1;

but I didn't actually go and look at the context there to see what
else is there..

                   Linus

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