Re: [PATCH] calloc should not duplicate malloc logic.

[Siddhesh Poyarekar <siddhesh at redhat dot com>]

On Wed, Feb 26, 2014 at 06:24:12PM +0100, OndÅej BÃlka wrote:
> I did not said so, I said there could be minor regression and asked if
> that is problem. Also there is problem of synchronization as independent
> modifications would conflict and I want minimize these issue before they
> happen

Right, and I opined that it is a problem.  There need to be more
opinions on this so that a consensus can be achieved.

> > You haven't understood why this is a bad idea.  The pages will have
> > been zeroed twice with your patch, once by mmap and again with memset.
> > 
> I understood you perfecly and you are wrong here. Current implementation
> also zeroes memory twice as this example demonstrates. Your point is
> invalid here.

Your example does not demonstrate that.  Your example only
demonstrates that the allocated memory is being touched, which is
obvious since the metadata is written to the chunk.  I am surprised
that the kernel decided to page in the entire block for it though.

If a chunk is mmapped, it is only memset if perturb_byte is set:

  /* Two optional cases in which clearing not necessary */
  if (chunk_is_mmapped (p))
    {
      if (__builtin_expect (perturb_byte, 0))
        return memset (mem, 0, sz);

      return mem;
    }

Siddhesh

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