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From: Davidlohr Bueso <davidlohr@hp.com>
Subject: Re: [PATCH] mm: per-thread vma caching
Date: Fri, 21 Feb 2014 20:55:55 -0800
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To: Linus Torvalds <torvalds@linux-foundation.org>
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On Fri, 2014-02-21 at 13:24 -0800, Linus Torvalds wrote:
> 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;

And probably specially so for structures like task and mm. I hadn't
considered the benefits of packing vs overflowing. So we can afford
flushing all tasks's vmacache every 4 billion forks.