linux-kernel - Re: [RFC PATCH 0/3] Use pageblock_order for cma and alloc_contig

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Message-ID: <1c67bb96-24db-f5a6-7520-3d97e54e5192@suse.cz>
Date:   Tue, 30 Nov 2021 10:11:36 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Zi Yan <ziy@...dia.com>
Cc:     David Hildenbrand <david@...hat.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Michael Ellerman <mpe@...erman.id.au>,
        Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        linuxppc-dev@...ts.ozlabs.org,
        virtualization@...ts.linux-foundation.org,
        iommu@...ts.linux-foundation.org,
        Mel Gorman <mgorman@...hsingularity.net>
Subject: Re: [RFC PATCH 0/3] Use pageblock_order for cma and
 alloc_contig_range alignment.

On 11/29/21 23:08, Zi Yan wrote:
> On 23 Nov 2021, at 12:32, Vlastimil Babka wrote:
> 
>> On 11/23/21 17:35, Zi Yan wrote:
>>> On 19 Nov 2021, at 10:15, Zi Yan wrote:
>>>>>> From what my understanding, cma required alignment of
>>>>>> max(MAX_ORDER - 1, pageblock_order), because when MIGRATE_CMA was introduced,
>>>>>> __free_one_page() does not prevent merging two different pageblocks, when
>>>>>> MAX_ORDER - 1 > pageblock_order. But current __free_one_page() implementation
>>>>>> does prevent that.
>>>>>
>>>>> But it does prevent that only for isolated pageblock, not CMA, and yout
>>>>> patchset doesn't seem to expand that to CMA? Or am I missing something.
>>>>
>>>> Yeah, you are right. Originally, I thought preventing merging isolated pageblock
>>>> with other types of pageblocks is sufficient, since MIGRATE_CMA is always
>>>> converted from MIGRATE_ISOLATE. But that is not true. I will rework the code.
>>>> Thanks for pointing this out.
>>>>
>>>
>>> I find that two pageblocks with different migratetypes, like MIGRATE_RECLAIMABLE
>>> and MIGRATE_MOVABLE can be merged into a single free page after I checked
>>> __free_one_page() in detail and printed pageblock information during buddy page
>>> merging.
>>
>> Yes, that can happen.
>>
>> I am not sure what consequence it will cause. Do you have any idea?
>>
>> For MIGRATE_RECLAIMABLE or MIGRATE_MOVABLE or even MIGRATE_UNMOVABLE it's
>> absolutely fine. As long as these pageblocks are fully free (and they are if
>> it's a single free page spanning 2 pageblocks), they can be of any of these
>> type, as they can be reused as needed without causing fragmentation.
>>
>> But in case of MIGRATE_CMA and MIGRATE_ISOLATE, uncontrolled merging would
>> break the specifics of those types. That's why the code is careful for
>> MIGRATE_ISOLATE, and MIGRATE_CMA was until now done in MAX_ORDER granularity.
> 
> Thanks for the explanation. Basically migratetypes that can fall back to each
> other can be merged into a single free page, right?

Yes.

> How about MIGRATE_HIGHATOMIC? It should not be merged with other migratetypes
> from my understanding.

Hmm it shouldn't minimally because it has an accounting that would become
broken. So it should prevent merging or make sure the reservations are with
MAX_ORDER granularity, but seems that neither is true? CCing Mel.

> --
> Best Regards,
> Yan, Zi
>