Re: [RFC][PATCH 6/8] mm: handle_speculative_fault()

[Peter Zijlstra]

On Thu, 2010-01-07 at 10:44 -0800, Linus Torvalds wrote:
> 
> On Thu, 7 Jan 2010, Linus Torvalds wrote:
> > 
> > For example: there's no real reason why we take mmap_sem for writing when 
> > extending an existing vma. And while 'brk()' is a very oldfashioned way of 
> > doing memory management, it's still quite common. So rather than looking 
> > at subtle lockless algorithms, why not look at doing the common cases of 
> > an extending brk? Make that one take the mmap_sem for _reading_, and then 
> > do the extending of the brk area with a simple cmpxchg or something?
> 
> I didn't use cmpxchg, because we actually want to update both 
> 'current->brk' _and_ the vma->vm_end atomically, so here's a totally 
> untested patch that uses the page_table_lock spinlock for it instead (it 
> could be a new spinlock, not worth it).
> 
> It's also totally untested and might be horribly broken. But you get the 
> idea.
> 
> We could probably do things like this in regular mmap() too for the 
> "extend a mmap" case. brk() is just especially simple.

I haven't yet looked at the patch, but isn't expand_stack() kinda like
what you want? That serializes using anon_vma_lock().

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