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    SubjectRe: MM patches against 2.5.31
    Christian Ehrhardt wrote:
    > ...
    > So what we want CPUB do instead is
    > spin_lock(lru_lock);
    > page = list_entry(lru)
    > page_cache_get(page);
    > res = (page_count (page) == 1)
    > if (res) {
    > atomic_dec (&page->count);
    > continue; /* with next page */
    > }
    > ...
    > page_cache_release (page);
    > I.e. we want to detect _atomically_ that we just raised the page count
    > from zero to one. My patch actually has a solution that implements the
    > needed atomic operation above by means of the atomic functions that we
    > currently have on all archs (it's called get_page_testzero and
    > should probably called get_page_testone).
    > The more I think about this the more I think this is the way to go.

    Yes, I think that would provide a minimal fix to the problem.
    (I'd prefer a solution in which presence on the LRU contributes
    to page->count, because that means I can dump a load of expensive
    page_cache_get-inside-lru-lock instances, but whatever)

    You had:

    -#define put_page_testzero(p) atomic_dec_and_test(&(p)->count)
    -#define page_count(p) atomic_read(&(p)->count)
    -#define set_page_count(p,v) atomic_set(&(p)->count, v)
    +#define put_page_testzero(p) atomic_add_negative(-1, &(p)->count)
    +#define page_count(p) (1+atomic_read(&(p)->count))
    +#define set_page_count(p,v) atomic_set(&(p)->count, v-1)
    +#define get_page_testzero(p) atomic_inc_and_test(&(p)->count)

    So the page count is actually offset by -1, and that is hidden by
    the macros. Fair enough.

    atomic_add_negative() is not implemented on quite a number of
    architectures (sparc64, mips, ppc, sh, cris, 68k, alpha..), so
    some legwork is needed there. Looks to be pretty simple though;
    alpha, ppc and others already have atomic_add_return().
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      Last update: 2005-03-22 13:28    [W:0.030 / U:3.012 seconds]
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