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Re: malloc using mprotect

On Fri, Aug 10, 2001 at 12:38:33PM +0200, Wolfram Gloger wrote:
> top_pad is actually set to 0 by default.  Maybe we should set it to
> 32k, 64k or something like that?


> > mprotect does not do any segment merging in 2.4, so each single page
> > consumes one vma.
> Ahem.. oh my god!  Let me make sure I understand this:
> - mmap(...128k..) still takes one VMA, not 32.
> however
> - calling mmap(..4k..) successively _even at consecutive addresses_
>   now always takes 32 VMAs

No, that will actually take one VMA too, provided it is anonymous private
mapping. mmap does some minimal merging
        /* Can we just expand an old anonymous mapping? */
        if (addr && !file && !(vm_flags & VM_SHARED)) {
                struct vm_area_struct * vma = find_vma(mm, addr-1);
                if (vma && vma->vm_end == addr && !vma->vm_file &&
                    vma->vm_flags == vm_flags) {
                        vma->vm_end = addr + len;
                        goto out;
while mprotect does not do any.

> When exactly was this introduced and why?

AFAIK there were some locking issues and stuff like that why merge_segments
was removed.


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