Message-ID: <52dbf824-76be-cc34-3983-d45510b1b618@suse.cz>
Date:   Tue, 23 Nov 2021 18:32:37 +0100
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To:     Zi Yan <ziy@nvidia.com>
Cc:     David Hildenbrand <david@redhat.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        Michael Ellerman <mpe@ellerman.id.au>,
        Christoph Hellwig <hch@lst.de>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Robin Murphy <robin.murphy@arm.com>,
        linuxppc-dev@lists.ozlabs.org,
        virtualization@lists.linux-foundation.org,
        iommu@lists.linux-foundation.org
References: <20211115193725.737539-1-zi.yan@sent.com>
 <3083463d-978b-fbe6-dadf-670d400ed437@suse.cz>
 <AEFF28CF-0ED8-450F-96A4-A6CD59CB1F3D@nvidia.com>
 <BF8FB68A-6E1D-4465-8A2B-884FC034660B@nvidia.com>
From:   Vlastimil Babka <vbabka@suse.cz>
Subject: Re: [RFC PATCH 0/3] Use pageblock_order for cma and
 alloc_contig_range alignment.
In-Reply-To: <BF8FB68A-6E1D-4465-8A2B-884FC034660B@nvidia.com>
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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.