what stops someone from calling kmalloc() with __GFP_HIGHMEM??

["Robert P. J. Day" <rpjday@xxxxxxxxxxxxxx>]

  according to love's book on kernel development, p. 190:

"You cannot specify __GFP_HIGHMEM to either __get_free_pages() or
kmalloc().  Because these both return a logical address, and not a
page structure, it is possible these functions would allocate memory
that is not currently mapped in the kernel's virtual address space
and, thus, does not have a logical address."

  but, AFIACT, there's nothing to stop you from doing such a thing.
if you look in mm/page_alloc.c, you can read:

fastcall unsigned long __get_free_pages(gfp_t gfp_mask, unsigned int order)
{
        struct page * page;
        page = alloc_pages(gfp_mask, order);
        if (!page)
                return 0;
        return (unsigned long) page_address(page);
}

  as you can see, there's no check on whether __GFP_HIGHMEM was
specified -- the routine simply calls alloc_pages(), which *is*
allowed to accept __GFP_HIGHMEM as a zone modifier.

  so what's to stop someone from calling __get_free_pages()
incorrectly with __GFP_HIGHMEM?

rday
-- 
========================================================================
Robert P. J. Day
Linux Consulting, Training and Annoying Kernel Pedantry
Waterloo, Ontario, CANADA

http://fsdev.net/wiki/index.php?title=Main_Page
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