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

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Date:   Fri, 19 Nov 2021 13:33:04 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Zi Yan <ziy@...dia.com>, David Hildenbrand <david@...hat.com>,
        linux-mm@...ck.org
Cc:     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
Subject: Re: [RFC PATCH 0/3] Use pageblock_order for cma and
 alloc_contig_range alignment.

On 11/15/21 20:37, Zi Yan wrote:
> From: Zi Yan <ziy@...dia.com>
> 
> Hi David,
> 
> You suggested to make alloc_contig_range() deal with pageblock_order instead of
> MAX_ORDER - 1 and get rid of MAX_ORDER - 1 dependency in virtio_mem[1]. This
> patchset is my attempt to achieve that. Please take a look and let me know if
> I am doing it correctly or not.
> 
> 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.


> It should be OK to just align cma to pageblock_order.
> alloc_contig_range() relies on MIGRATE_CMA to get free pages, so it can use
> pageblock_order as alignment too.
> 
> In terms of virtio_mem, if I understand correctly, it relies on
> alloc_contig_range() to obtain contiguous free pages and offlines them to reduce
> guest memory size. As the result of alloc_contig_range() alignment change,
> virtio_mem should be able to just align PFNs to pageblock_order.
> 
> Thanks.
> 
> 
> [1] https://lore.kernel.org/linux-mm/28b57903-fae6-47ac-7e1b-a1dd41421349@redhat.com/
> 
> Zi Yan (3):
>   mm: cma: alloc_contig_range: use pageblock_order as the single
>     alignment.
>   drivers: virtio_mem: use pageblock size as the minimum virtio_mem
>     size.
>   arch: powerpc: adjust fadump alignment to be pageblock aligned.
> 
>  arch/powerpc/include/asm/fadump-internal.h |  4 +---
>  drivers/virtio/virtio_mem.c                |  6 ++----
>  include/linux/mmzone.h                     |  5 +----
>  kernel/dma/contiguous.c                    |  2 +-
>  mm/cma.c                                   |  6 ++----
>  mm/page_alloc.c                            | 12 +++++-------
>  6 files changed, 12 insertions(+), 23 deletions(-)
>