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SubjectRe: MM patches against 2.5.31
On Mon, Aug 26, 2002 at 12:24:50PM -0700, Andrew Morton wrote:
> The flaw is in doing the put_page_testzero() outside of any locking
> which would prevent other CPUs from finding and "rescuing" the zero-recount
> page.
>
> CPUA:
> if (put_page_testzero()) {
> /* Here's the window */
> spin_lock(lru_lock);
> list_del(page->lru);
>
> CPUB:
>
> spin_lock(lru_lock);
> page = list_entry(lru);
> page_cache_get(page); /* If this goes from 0->1, we die */
> ...
> page_cache_release(page); /* double free */

So what we want CPUB do instead is

spin_lock(lru_lock);
page = list_entry(lru)

START ATOMIC
page_cache_get(page);
res = (page_count (page) == 1)
END ATOMIC

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.

regards Christian

--
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