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    SubjectRe: [PATCH] Add alloc_pages_exact() and free_pages_exact()
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    On Tue, 2008-06-24 at 11:40 -0500, Timur Tabi wrote:
    > +void *alloc_pages_exact(size_t size, gfp_t gfp_mask)
    > +{
    > + unsigned int order = get_order(size);
    > + unsigned long addr;
    > +
    > + addr = __get_free_pages(gfp_mask, order);
    > + if (addr) {
    > + unsigned long alloc_end = addr + (PAGE_SIZE << order);
    > + unsigned long used = addr + PAGE_ALIGN(size);
    > +
    > + split_page(virt_to_page(addr), order);
    > + while (used < alloc_end) {
    > + free_page(used);
    > + used += PAGE_SIZE;
    > + }
    > + }
    > +
    > + return (void *)addr;
    > +}

    Hi Timur,

    This looks like a really good idea. It looks pretty good to me, no
    functional problems. My brain had a really hard time parsing that code
    for some reason, though. Could just be a lack of coffee.

    I think the thing that confused me was trying to figure out if
    'alloc_end' was the end of what we *did* allocate from
    __get_free_pages() or if it was the *goal* allocation end.

    'used' also seemed like a slightly strange variable name because it
    points to the memory which is about to be freed and ends up *unused*.

    I'll offer this up just in case you like it better. For me, it is
    easier to parse, and should do the exact same thing. I also think it's
    slightly nicer to do the arithmetic on 'struct page *' rather than
    vaddrs in 'unsigned long'. It is _slightly_ cheaper not having to do a
    virt_to_page() on each free_page() call. The same would go for the free
    side as well.

    All of the 'struct page *' arithmetic is OK since it is all done inside
    one MAX_ORDER area.

    void *alloc_pages_exact(size_t size, gfp_t gfp_mask)
    {
    unsigned int order = get_order(size);
    void *alloc;
    struct page *surplus_start;
    struct page *surplus_end;
    struct page *page;

    size = PAGE_ALIGN(size);

    alloc = (void *)__get_free_pages(gfp_mask, order);
    if (!alloc)
    return NULL;

    /* Turn the big allocation into a bunch of single pages */
    split_page(virt_to_page(alloc), order);

    surplus_start = virt_to_page(alloc + size);
    surplus_end = surplus_start + (1 << order);

    for (page = surplus_start; page < surplus_end; page++)
    __free_page(page);

    return alloc;
    }

    -- Dave

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