Re: [patch 2/3] MAP_NOZERO - implement sys_brk2()

[Davide Libenzi <davidel@xmailserver.org>]

On Wed, 27 Jun 2007, Hugh Dickins wrote:

> On Tue, 26 Jun 2007, Ulrich Drepper wrote:
> > On 6/26/07, Davide Libenzi <davidel@xmailserver.org> wrote:
> > 
> > > OTOH glibc could implement __morecore using mmap(MAP_NOZERO), and hence
> > > brk2() would not be needed, no?
> > 
> > No.  mmap calls create individual VMAs which gets expensive.  There
> > are also some hardware drivers which get more expensive the more VMAs
> > there are.  I want to go away as much as possible from mmap for
> > malloc.
> 
> Not so: if an mmap can be done by extending either adjacent vma (prot
> and flags and file and offset all match up), that's what's done and no
> separate vma is created.  (And adjacent vmas get merged when mprotect
> removes the difference in protection.)
> 
> I don't think there's any such reason to prefer brk to mmap.  But you
> may have encountered something which we in the kernel are thinking of
> as an insignificant corner case, which is actually breaking things up
> badly in practice (I recall the kernel's internal VM_ACCOUNT flag,
> relating to non-overcommit accounting, which might get turned on at
> any time, sometimes preventing a vma merge you'd otherwise expect).
> Please let me know if you've a test case which shows more vmas than
> expected.

The only way I can see vma fragmentation happen in that case, is if 
userspace uses a mixture of mmaps and mallocs, and flags+prots of the two 
does not match. The glibc allocator seems to support it just fine. There's 
a macro where you specify if the heaps are contiguous or not.


- Davide


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