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

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Message-ID: <8a8c2306-3066-103a-17e1-22fb3b8484ba@redhat.com>
Date:   Wed, 29 Jul 2020 14:29:27 +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 29.07.20 12:47, Baoquan He wrote:
> On 07/28/20 at 04:07pm, David Hildenbrand wrote:
>> 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).
> 
> Seems yes, thanks a lot. Wondering who is still using
> movablecore|kernelcore in what use case.
> 

AFAIK, it's the only (guaranteed) way to get ZONE_MOVABLE without
involving memory hotplug. As I learned, the movable zone is also
interesting beyond memory hotunplug. It limits unmovable fragmentation
and e.g., makes THP/huge pages more likely/easier to allocate.

-- 
Thanks,

David / dhildenb