[RFC] Strange do_munmap in mmap_region

[Wang YanQing <udknight@gmail.com>]

Hi Mel Gorman and all.

I have read do_mmap_pgoff and mmap_region more than one hour,
but still can't catch sense about below code in mmap_region:

"
        /* Clear old maps */
        error = -ENOMEM;
munmap_back:
        if (find_vma_links(mm, addr, addr + len, &prev, &rb_link, &rb_parent)) {
                if (do_munmap(mm, addr, len))
                        return -ENOMEM;
                goto munmap_back;
        }
"

How can we just do_munmap overlapping vma without check its vm_flags
and new vma's vm_flags? I must miss some important things, but I can't
figure out.

You give below comment about the code in "understand the linux memory manager":)

"
If a VMA was found and it is part of the new mmapping, this removes the
 old mmapping because the new one will cover both 
"

But if new mmapping has different vm_flags or others' property, how
can we just say the new one will cover both?

I appreicate any clue and explanation about this headache question.

Thanks.






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