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Date:   Thu, 14 Sep 2023 11:00:29 +0100
From:   Mel Gorman <mgorman@techsingularity.net>
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Kefeng Wang <wangkefeng.wang@huawei.com>,
        Zi Yan <ziy@nvidia.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/6] mm: page_alloc: fix up block types when merging
 compatible blocks
Message-ID: <20230914100029.eyb45y45jei2md3h@techsingularity.net>
References: <20230911195023.247694-1-hannes@cmpxchg.org>
 <20230911195023.247694-3-hannes@cmpxchg.org>
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On Mon, Sep 11, 2023 at 03:41:43PM -0400, Johannes Weiner wrote:
> The buddy allocator coalesces compatible blocks during freeing, but it
> doesn't update the types of the subblocks to match. When an allocation
> later breaks the chunk down again, its pieces will be put on freelists
> of the wrong type. This encourages incompatible page mixing (ask for
> one type, get another), and thus long-term fragmentation.
> 
> Update the subblocks when merging a larger chunk, such that a later
> expand() will maintain freelist type hygiene.
> 
> v2:
> - remove spurious change_pageblock_range() move (Zi Yan)
> 
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

I'm not 100% convinced on the amount of harm this causes but given that
it's a relatively rare condition, I didn't think about the consequences
too deeply. The patch certainly has merit so;

Acked-by: Mel Gorman <mgorman@techsingularity.net>

-- 
Mel Gorman
SUSE Labs