On Sat 20-06-20 08:59:58, Jaewon Kim wrote:
[...]
> @@ -3502,19 +3525,12 @@ bool __zone_watermark_ok(struct zone *z, unsigned int
> order, unsigned long mark,
> const bool alloc_harder = (alloc_flags & (ALLOC_HARDER|ALLOC_OOM));
>
> /* free_pages may go negative - that's OK */
> - free_pages -= (1 << order) - 1;
> + free_pages -= __zone_watermark_unusable_free(z, order, alloc_flags);
>
> if (alloc_flags & ALLOC_HIGH)
> min -= min / 2;
>
> - /*
> - * If the caller does not have rights to ALLOC_HARDER then subtract
> - * the high-atomic reserves. This will over-estimate the size of the
> - * atomic reserve but it avoids a search.
> - */
> - if (likely(!alloc_harder)) {
> - free_pages -= z->nr_reserved_highatomic;
> - } else {
> + if (unlikely(alloc_harder)) {
> /*
> * OOM victims can try even harder than normal ALLOC_HARDER
> * users on the grounds that it's definitely going to be in
[...]
> @@ -3582,25 +3591,22 @@ static inline bool zone_watermark_fast(struct zone
> *z, unsigned int order,
> unsigned long mark, int highest_zoneidx,
> unsigned int alloc_flags)
> {
> - long free_pages = zone_page_state(z, NR_FREE_PAGES);
> - long cma_pages = 0;
> + long free_pages;
> + long unusable_free;
>
> -#ifdef CONFIG_CMA
> - /* If allocation can't use CMA areas don't use free CMA pages */
> - if (!(alloc_flags & ALLOC_CMA))
> - cma_pages = zone_page_state(z, NR_FREE_CMA_PAGES);
> -#endif
> + free_pages = zone_page_state(z, NR_FREE_PAGES);
> + unusable_free = __zone_watermark_unusable_free(z, order, alloc_flags);
>
> /*
> * Fast check for order-0 only. If this fails then the reserves
> - * need to be calculated. There is a corner case where the check
> - * passes but only the high-order atomic reserve are free. If
> - * the caller is !atomic then it'll uselessly search the free
> - * list. That corner case is then slower but it is harmless.
> + * need to be calculated.
> */
> - if (!order && (free_pages - cma_pages) >
> - mark + z->lowmem_reserve[highest_zoneidx])
> - return true;
> + if (!order) {
> + long fast_free = free_pages - unusable_free;
> +
> + if (fast_free > mark + z->lowmem_reserve[highest_zoneidx])
> + return true;
> + }
There is no user of unusable_free for order > 0. With you current code
__zone_watermark_unusable_free would be called twice for high order
allocations unless compiler tries to be clever..
But more importantly, I have hard time to follow why we need both
zone_watermark_fast and zone_watermark_ok now. They should be
essentially the same for anything but order == 0. For order 0 the
only difference between the two is that zone_watermark_ok checks for
ALLOC_HIGH resp ALLOC_HARDER, ALLOC_OOM. So what is exactly fast about
the former and why do we need it these days?
>
> return __zone_watermark_ok(z, order, mark, highest_zoneidx, alloc_flags,
> free_pages);
> --
> 2.17.1
>
--
Michal Hocko
SUSE Labs
