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    SubjectRe: [rfc] increase struct page size?!
    On Fri, 18 May 2007, Nick Piggin wrote:
    >> If we add 8 bytes to struct page on 64-bit machines, it becomes 64 bytes,
    >> which is quite a nice number for cache purposes.
    >> However we don't have to let those 8 bytes go to waste: we can use them
    >> to store the virtual address of the page, which kind of makes sense for
    >> 64-bit, because they can likely to use complicated memory models.

    On Fri, May 18, 2007 at 04:42:10PM +0100, Hugh Dickins wrote:
    > Sooner rather than later, don't we need those 8 bytes to expand from
    > atomic_t to atomic64_t _count and _mapcount? Not that we really need
    > all 64 bits of both, but I don't know how to work atomically with less.
    > (Why do I have this sneaking feeling that you're actually wanting
    > to stick something into the lower bits of page->virtual?)

    I wonder how close we get to overflow on ->_mapcount and ->_count.
    (untested/uncompiled).


    -- wli


    Index: mm-2.6.21/include/linux/mm.h
    ===================================================================
    --- mm-2.6.21.orig/include/linux/mm.h 2007-05-19 10:17:17.682653270 -0700
    +++ mm-2.6.21/include/linux/mm.h 2007-05-19 10:38:52.376433663 -0700
    @@ -248,6 +248,24 @@
    * routine so they can be sure the page doesn't go away from under them.
    */

    +static inline struct page *compound_head(struct page *page)
    +{
    + if (unlikely(PageTail(page)))
    + return page->first_page;
    + return page;
    +}
    +
    +static inline int page_count(struct page *page)
    +{
    + return atomic_read(&compound_head(page)->_count);
    +}
    +
    +#ifdef CONFIG_ATOMIC_HIWATER
    +unsigned long count_hiwater(void);
    +int put_page_testzero(struct page *);
    +int get_page_unless_zero(struct page *);
    +void get_page(struct page *);
    +#else /* !CONFIG_ATOMIC_HIWATER */
    /*
    * Drop a ref, return true if the refcount fell to zero (the page has no users)
    */
    @@ -267,24 +285,13 @@
    return atomic_inc_not_zero(&page->_count);
    }

    -static inline struct page *compound_head(struct page *page)
    -{
    - if (unlikely(PageTail(page)))
    - return page->first_page;
    - return page;
    -}
    -
    -static inline int page_count(struct page *page)
    -{
    - return atomic_read(&compound_head(page)->_count);
    -}
    -
    static inline void get_page(struct page *page)
    {
    page = compound_head(page);
    VM_BUG_ON(atomic_read(&page->_count) == 0);
    atomic_inc(&page->_count);
    }
    +#endif /* !CONFIG_ATOMIC_HIWATER */

    static inline struct page *virt_to_head_page(const void *x)
    {
    Index: mm-2.6.21/mm/Makefile
    ===================================================================
    --- mm-2.6.21.orig/mm/Makefile 2007-05-18 09:58:43.851524250 -0700
    +++ mm-2.6.21/mm/Makefile 2007-05-18 09:58:59.484415118 -0700
    @@ -31,4 +31,4 @@
    obj-$(CONFIG_MIGRATION) += migrate.o
    obj-$(CONFIG_SMP) += allocpercpu.o
    obj-$(CONFIG_QUICKLIST) += quicklist.o
    -
    +obj-$(CONFIG_ATOMIC_HIWATER) += atomic-hiwater.o
    Index: mm-2.6.21/mm/atomic-hiwater.c
    ===================================================================
    --- /dev/null 1970-01-01 00:00:00.000000000 +0000
    +++ mm-2.6.21/mm/atomic-hiwater.c 2007-05-19 10:46:09.713356074 -0700
    @@ -0,0 +1,63 @@
    +#include <linux/mm_types.h>
    +#include <linux/percpu.h>
    +#include <linux/module.h>
    +#include <linux/irqflags.h>
    +#include <linux/page-flags.h>
    +#include <linux/mm.h>
    +
    +static DEFINE_PER_CPU(unsigned long, __count_hiwater);
    +
    +unsigned long count_hiwater(void)
    +{
    + int cpu;
    + unsigned long *hiwater, count = 0;
    +
    + for_each_online_cpu(cpu) {
    + hiwater = &per_cpu(__count_hiwater, cpu);
    + if (*hiwater > count)
    + count = *hiwater;
    + }
    + return count;
    +}
    +EXPORT_SYMBOL_GPL(count_hiwater);
    +
    +static void update_count_hiwater(unsigned long count)
    +{
    + unsigned long flags, *hiwater;
    +
    + local_irq_save(flags);
    + hiwater = &__get_cpu_var(__count_hiwater);
    + if (unlikely(count > *hiwater))
    + *hiwater = count;
    + local_irq_restore(flags);
    +}
    +
    +int get_page_unless_zero(struct page *page)
    +{
    + int ret;
    +
    + VM_BUG_ON(PageCompound(page));
    + ret = atomic_inc_not_zero(&page->_count);
    + update_count_hiwater(atomic_read(&page->_count));
    + return ret;
    +}
    +EXPORT_SYMBOL(get_page_unless_zero);
    +
    +void get_page(struct page *page)
    +{
    + page = compound_head(page);
    + VM_BUG_ON(atomic_read(&page->_count) == 0);
    + update_count_hiwater(atomic_inc_return(&page->_count));
    +}
    +EXPORT_SYMBOL(get_page);
    +
    +int put_page_testzero(struct page *page)
    +{
    + int count;
    +
    + VM_BUG_ON(atomic_read(&page->_count) == 0);
    + count = atomic_dec_return(&page->_count);
    + update_count_hiwater(count);
    + return count;
    +}
    +EXPORT_SYMBOL(put_page_testzero);
    Index: mm-2.6.21/mm/Kconfig
    ===================================================================
    --- mm-2.6.21.orig/mm/Kconfig 2007-05-19 10:20:24.361291479 -0700
    +++ mm-2.6.21/mm/Kconfig 2007-05-19 10:22:17.231723598 -0700
    @@ -168,3 +168,8 @@
    depends on QUICKLIST
    default "1"

    +config ATOMIC_HIWATER
    + bool "Track page reference count high watermarks."
    + default n
    + help
    + This option tracks the largest reference counts seen for a page.
    Index: mm-2.6.21/mm/vmstat.c
    ===================================================================
    --- mm-2.6.21.orig/mm/vmstat.c 2007-05-19 10:34:41.382130313 -0700
    +++ mm-2.6.21/mm/vmstat.c 2007-05-19 10:38:38.519644010 -0700
    @@ -607,6 +607,29 @@
    return v + *pos;
    }

    +#ifdef CONFIG_ATOMIC_HIWATER
    +static void *vmstat_next(struct seq_file *m, void *arg, loff_t *pos)
    +{
    + (*pos)++;
    + if (*pos == ARRAY_SIZE(vmstat_text))
    + return (void *)~0UL;
    + else if (*pos > ARRAY_SIZE(vmstat_text))
    + return NULL;
    + return (unsigned long *)m->private + *pos;
    +}
    +
    +static int vmstat_show(struct seq_file *m, void *arg)
    +{
    + unsigned long *l = arg;
    + unsigned long off = l - (unsigned long *)m->private;
    +
    + if (off < ARRAY_SIZE(vmstat_text))
    + seq_printf(m, "%s %lu\n", vmstat_text[off], *l);
    + else
    + seq_printf(m, "count_hiwater %lu\n", count_hiwater());
    + return 0;
    +}
    +#else /* !CONFIG_ATOMIC_HIWATER */
    static void *vmstat_next(struct seq_file *m, void *arg, loff_t *pos)
    {
    (*pos)++;
    @@ -623,6 +646,7 @@
    seq_printf(m, "%s %lu\n", vmstat_text[off], *l);
    return 0;
    }
    +#endif /* !CONFIG_ATOMIC_HIWATER */

    static void vmstat_stop(struct seq_file *m, void *arg)
    {
    -
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