linux-kernel - Re: [RFC PATCH v3 3/8] mm: migrate: allocate the right size of non hugetlb or THP compound pages.

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Date:   Thu, 13 Jan 2022 10:46:36 -0500
From:   Zi Yan <ziy@...dia.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     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, Vlastimil Babka <vbabka@...e.cz>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Eric Ren <renzhengeek@...il.com>
Subject: Re: [RFC PATCH v3 3/8] mm: migrate: allocate the right size of non hugetlb or THP compound pages.

On 12 Jan 2022, at 6:04, David Hildenbrand wrote:

> On 05.01.22 22:47, Zi Yan wrote:
>> From: Zi Yan <ziy@...dia.com>
>>
>> alloc_migration_target() is used by alloc_contig_range() and non-LRU
>> movable compound pages can be migrated. Current code does not allocate the
>> right page size for such pages. Check THP precisely using
>> is_transparent_huge() and add allocation support for non-LRU compound
>> pages.
>
> IIRC, we don't have any non-lru migratable pages that are coumpound
> pages. Read: not used and not supported :)

OK, but nothing prevents one writing a driver that allocates compound
pages and provides address_space->migratepage() and address_space->isolate_page().

Actually, to test this series, I write a kernel module that allocates
an order-10 page, gives it a fake address_space with migratepage() and
isolate_page(), __SetPageMovable() on it, then call alloc_contig_range()
on the page range. Apparently, my kernel module is not supported by
the kernel, thus, I added this patch.

Do you have an alternative test to my kernel module, so that I do not
even need this patch myself?

> Why is this required in the context of this series?

It might not be required. I will drop it.

--
Best Regards,
Yan, Zi

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