Re: [PATCH] mm: Add nr_free_highatomimic to fix incorrect watermatk routine

[Vlastimil Babka]

On 8/30/19 11:25 AM, Sangwoo wrote:
> The highatomic migrate block can be increased to 1% of Total memory.
> And, this is for only highorder ( > 0 order). So, this block size is
> excepted during check watermark if allocation type isn't alloc_harder.
> 
> It has problem. The usage of highatomic is already calculated at NR_FREE_PAGES.
> So, if we except total block size of highatomic, it's twice minus size of allocated
> highatomic.
> It's cause allocation fail although free pages enough.

This is known, the comment in __zone_watermark_order says "This will
over-estimate the size of the atomic reserve but it avoids a search."
It was discussed during review and wasn't considered a large issue
thanks to unreserving on demand before OOM happens.

> @@ -919,6 +923,9 @@ static inline void __free_one_page(struct page *page,
>  	VM_BUG_ON(migratetype == -1);
>  	if (likely(!is_migrate_isolate(migratetype)))
>  		__mod_zone_freepage_state(zone, 1 << order, migratetype);
> +	if (is_migrate_highatomic(migratetype) ||
> +		is_migrate_highatomic_page(page))
> +		__mod_zone_page_state(zone, NR_FREE_HIGHATOMIC_PAGES, 1 << order);

I suspect the counter will eventually get imbalanced, at the least due
to merging a highatomic pageblock and non-highatomic pageblock. To get
it right, it would have to be complicated in a similar way that we
handle MIGRATE_ISOLATED and MIGRATE_CMA. It wasn't considered serious
enough to warrant these complications.