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    SubjectRe: [PATCH] MM : alloc_large_system_hash() can free some memory for non power-of-two bucketsize
    On Fri, 18 May 2007 11:54:54 +0200 Eric Dumazet <dada1@cosmosbay.com> wrote:

    > alloc_large_system_hash() is called at boot time to allocate space for several large hash tables.
    >
    > Lately, TCP hash table was changed and its bucketsize is not a power-of-two anymore.
    >
    > On most setups, alloc_large_system_hash() allocates one big page (order > 0) with __get_free_pages(GFP_ATOMIC, order). This single high_order page has a power-of-two size, bigger than the needed size.

    Watch the 200-column text, please.

    > We can free all pages that wont be used by the hash table.
    >
    > On a 1GB i386 machine, this patch saves 128 KB of LOWMEM memory.
    >
    > TCP established hash table entries: 32768 (order: 6, 393216 bytes)
    >
    > Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
    > ---
    > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
    > index ae96dd8..2e0ba08 100644
    > --- a/mm/page_alloc.c
    > +++ b/mm/page_alloc.c
    > @@ -3350,6 +3350,20 @@ void *__init alloc_large_system_hash(const char *tablename,
    > for (order = 0; ((1UL << order) << PAGE_SHIFT) < size; order++)
    > ;
    > table = (void*) __get_free_pages(GFP_ATOMIC, order);
    > + /*
    > + * If bucketsize is not a power-of-two, we may free
    > + * some pages at the end of hash table.
    > + */
    > + if (table) {
    > + unsigned long alloc_end = (unsigned long)table +
    > + (PAGE_SIZE << order);
    > + unsigned long used = (unsigned long)table +
    > + PAGE_ALIGN(size);
    > + while (used < alloc_end) {
    > + free_page(used);
    > + used += PAGE_SIZE;
    > + }
    > + }
    > }
    > } while (!table && size > PAGE_SIZE && --log2qty);
    >

    It went BUG.

    static inline int put_page_testzero(struct page *page)
    {
    VM_BUG_ON(atomic_read(&page->_count) == 0);
    return atomic_dec_and_test(&page->_count);
    }

    http://userweb.kernel.org/~akpm/s5000523.jpg
    http://userweb.kernel.org/~akpm/config-vmm.txt
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