Re: [PATCH v3] mm/page_reporting: Add page_reporting_delay_ms module parameter

["David Hildenbrand (Arm)" <david@kernel.org>]

>>> With 2s, we wait 2s before we start reporting immediately by kicking the
>>> workqueue immediately.
>>>
>>> So "0" does not mean "report all the time", rather "start reporting immediately
>>> as we are notified about a pageblock to report".
> 
> No, it's whenever pageblock_order is specified, defaulting to a pageblock.
> 
> But I guess in practice fine.
> 
> There's fun like this in virtio_balloon.c btw:
> 
> 		/*
> 		 * The default page reporting order is @pageblock_order, which
> 		 * corresponds to 512MB in size on ARM64 when 64KB base page
> 		 * size is used. The page reporting won't be triggered if the
> 		 * freeing page can't come up with a free area like that huge.
> 		 * So we specify the page reporting order to 5, corresponding
> 		 * to 2MB. It helps to avoid THP splitting if 4KB base page
> 		 * size is used by host.
> 		 *
> 		 * Ideally, the page reporting order is selected based on the
> 		 * host's base page size. However, it needs more work to report
> 		 * that value. The hard-coded order would be fine currently.
> 		 */
> #if defined(CONFIG_ARM64) && defined(CONFIG_ARM64_64K_PAGES)
> 		vb->pr_dev_info.order = 5;
> #endif

Yes, it's really about "a large chunk of memory was freed". On 64k " a large
chunk" is unfortunately order 5 == 2MiB (so the same as on 4k where a pageblock
is 2M).

-- 
Cheers,

David