About performance related to do_map

About performance related to do_map

[Hua Ji]

Folks,

When reading source codes, get a question. Thanks in advance.

Function do_map() ./mm/mmap.c,

After/if we get a vma by using **get_unmapped_area**, why we still
double-check
the vma area by using do_munmap()? 

This call to do_munmap()is ONLY NECESSARY when (flags & MAP_FIXED) is
**TRUE**.

The do_munmap will bring some EXTRA cost with the look up the vma linked
list or/and AVL tree.

Also, why not check it first before we create a new vma area? We don't have
to create a vma first and then release it afterwards when lateron we find
out that this vma is overlapped by some other vma area already.

Mike
------------------------------------------------
 /* Obtain the address to map to. we verify (or select) it and ensure
	 * that it represents a valid section of the address space.
	 */
	if (flags & MAP_FIXED) {
		if (addr & ~PAGE_MASK)
			return -EINVAL;
	} else {
		addr = get_unmapped_area(addr, len);
		if (!addr)
			return -ENOMEM;
	}
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