linux-kernel - Re: [PATCH v1 1/6] mm/page_alloc: tweak comments in has_unmovable

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Date:   Tue, 28 Jul 2020 16:07:49 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Baoquan He <bhe@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        "Michael S . Tsirkin" <mst@...hat.com>
Subject: Re: [PATCH v1 1/6] mm/page_alloc: tweak comments in
 has_unmovable_pages()

On 28.07.20 15:48, Baoquan He wrote:
> On 06/30/20 at 04:26pm, David Hildenbrand wrote:
>> Let's move the split comment regarding bootmem allocations and memory
>> holes, especially in the context of ZONE_MOVABLE, to the PageReserved()
>> check.
>>
>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>> Cc: Michal Hocko <mhocko@...e.com>
>> Cc: Michael S. Tsirkin <mst@...hat.com>
>> Signed-off-by: David Hildenbrand <david@...hat.com>
>> ---
>>  mm/page_alloc.c | 22 ++++++----------------
>>  1 file changed, 6 insertions(+), 16 deletions(-)
>>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 48eb0f1410d47..bd3ebf08f09b9 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -8207,14 +8207,6 @@ struct page *has_unmovable_pages(struct zone *zone, struct page *page,
>>  	unsigned long iter = 0;
>>  	unsigned long pfn = page_to_pfn(page);
>>  
>> -	/*
>> -	 * TODO we could make this much more efficient by not checking every
>> -	 * page in the range if we know all of them are in MOVABLE_ZONE and
>> -	 * that the movable zone guarantees that pages are migratable but
>> -	 * the later is not the case right now unfortunatelly. E.g. movablecore
>> -	 * can still lead to having bootmem allocations in zone_movable.
>> -	 */
>> -
>>  	if (is_migrate_cma_page(page)) {
>>  		/*
>>  		 * CMA allocations (alloc_contig_range) really need to mark
>> @@ -8233,6 +8225,12 @@ struct page *has_unmovable_pages(struct zone *zone, struct page *page,
>>  
>>  		page = pfn_to_page(pfn + iter);
>>  
>> +		/*
>> +		 * Both, bootmem allocations and memory holes are marked
>> +		 * PG_reserved and are unmovable. We can even have unmovable
>> +		 * allocations inside ZONE_MOVABLE, for example when
>> +		 * specifying "movable_core".
>                                ~~~~ should be 'movablecore', we don't
> have kernel parameter 'movable_core'.

Agreed!

> 
> Otherwise, this looks good to me. Esp the code comment at below had been
> added very long time ago and obsolete.
> 
> Reviewed-by: Baoquan He <bhe@...hat.com>
> 
> By the way, David, do you know what is the situation of having unmovable
> allocations inside ZONE_MOVABLE when specifying 'movablecore'? I quickly
> went through find_zone_movable_pfns_for_nodes(), but didn't get why.
> Could you tell a little more detail about it?

As far as I understand, it can happen that we have memblock allocations
during boot that fall into an area the kernel later configures to span
the movable zone (via movable_core).

>
> Thanks
> Baoquan


-- 
Thanks,

David / dhildenb