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SubjectRe: [PATCH 4/6] mm/page_alloc: Remove unnecessary page == NULL check in rmqueue
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On 5/19/22 14:13, Mel Gorman wrote:
> On Thu, May 19, 2022 at 12:57:01PM +0200, Vlastimil Babka wrote:
>> On 5/12/22 10:50, Mel Gorman wrote:
>>> The VM_BUG_ON check for a valid page can be avoided with a simple
>>> change in the flow. The ZONE_BOOSTED_WATERMARK is unlikely in general
>>> and even more unlikely if the page allocation failed so mark the
>>> branch unlikely.
>>
>> Hm, so that makes a DEBUG_VM config avoid the check. On the other hand,
>> it puts it on the path returning from rmqueue_pcplist() for all configs,
>> and that should be the fast path. So unless things further change in the
>> following patches, it doesn't seem that useful?
>>
>
> You're right -- the fast path ends up with both a if
> (page) and if (!page) checks. Andrew, can you drop the patch
> mm-page_alloc-remove-unnecessary-page-==-null-check-in-rmqueue.patch from
> your tree please?
>
> Originally the flow was important when I was writing the patch and later
> became unnecessary. However, it reminded me of another problem I thought
> of when writing this and then forgotten to note it in the changelog. If
> the page allocation fails then ZONE_BOOSTED_WATERMARK should still be
> tested and cleared before waking kswapd. It could happen if an allocation
> attempt tried to fallback to another migratetype and still fail to find
> a suitable page. This is true whether going through the PCP lists or not.
>
> So what do you think of me adding this patch to a follow-up series?

LGTM.

>
> --8<--
> mm/page_alloc: Remove mistaken page == NULL check in rmqueue
>
> If a page allocation fails, the ZONE_BOOSTER_WATERMARK should be tested,
> cleared and kswapd woken whether the allocation attempt was via the PCP
> or directly via the buddy list.
>
> Remove the page == NULL so the ZONE_BOOSTED_WATERMARK bit is checked
> unconditionally. As it is unlikely that ZONE_BOOSTED_WATERMARK is set,
> mark the branch accordingly.
>
> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> ---
> mm/page_alloc.c | 4 +---
> 1 file changed, 1 insertion(+), 3 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 1c4c54503a5d..61d5bc2efffe 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3765,12 +3765,10 @@ struct page *rmqueue(struct zone *preferred_zone,
>
> page = rmqueue_buddy(preferred_zone, zone, order, alloc_flags,
> migratetype);
> - if (unlikely(!page))
> - return NULL;
>
> out:
> /* Separate test+clear to avoid unnecessary atomics */
> - if (test_bit(ZONE_BOOSTED_WATERMARK, &zone->flags)) {
> + if (unlikely(test_bit(ZONE_BOOSTED_WATERMARK, &zone->flags))) {
> clear_bit(ZONE_BOOSTED_WATERMARK, &zone->flags);
> wakeup_kswapd(zone, 0, 0, zone_idx(zone));
> }
>

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