On Mon, 21 May 2007 01:08:13 -0700
William Lee Irwin III <[email protected]> wrote:
> Now that I've been informed of the ->_count and ->_mapcount issues,
> I'd say that they're grave and should be corrected even at the cost
> of sizeof(struct page).
As long we handle 4 KB pages, adding 64 bits per page means 0.2 % of overhead. Ouch...
We currently have an overhead of 1.36 % for mem_map
Maybe we can still use 32 bits counters, and make sure non root users cannot
make these counters exceed 2^30. (I believe high order bit has already a meaning,
check page_mapped() definition)
We could use a special atomic_inc_if_not_huge() function, that could revert to
normal atomic_inc() on machines with less than 32 GB (using alternative_() variant)
On small setups (or 32 bits arches), atomic_inc_if_not_huge() would unconditionnally
increment the counter.
#if !defined(BIG_MACHINES)
static int inline atomic_inc_if_not_huge(atomic_t *v)
{
atomic_inc(v);
return 1;
}
#else
extern int atomic_inc_if_not_huge(atomic_t *v);
#endif
/* in a .c file */
/* could be patched at boot time if available memory < 32GB (or other limit) */
#if defined(BIG_MACHINES)
#define MAP_LIMIT_COUNT (2<<30)
int atomic_inc_if_not_huge(atomic_t *v);
{
/* lazy test, we dont care enough to do a real atomic read-modify-write */
if (unlikely(atomic_read(v) >= MAP_LIMIT_COUNT)) {
if (non_root_user())
return 0;
}
atomic_inc(v);
return 1;
}
#endif
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