linux-kernel - Re: [PATCH] mm/page_alloc: change all pageblocks migrate type on coalescing

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Message-ID: <20251214160606.GA905277@cmpxchg.org>
Date: Sun, 14 Dec 2025 11:06:06 -0500
From: Johannes Weiner <hannes@...xchg.org>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Alexander Gordeev <agordeev@...ux.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Marc Hartmayer <mhartmay@...ux.ibm.com>, linux-mm@...ck.org,
	linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/page_alloc: change all pageblocks migrate type on
 coalescing

On Fri, Dec 12, 2025 at 04:46:46PM +0100, Vlastimil Babka wrote:
> On 12/12/25 16:14, Alexander Gordeev wrote:
> > When a page is freed it coalesces with a buddy into a higher
> > order page while possible. When the buddy page migrate type
> > differs, it is expected to be updated to match the one of the
> > page being freed.
> > 
> > However, only the first pageblock of the buddy page is updated,
> > while the rest of the pageblocks are left unchanged.
> > 
> > That causes warnings in later expand() and other code paths
> > (like below), since an inconsistency between migration type
> > of the list containing the page and the page-owned pageblocks
> > migration types is introduced.

Absolutely right, and the fix makes sense to me. Thanks!

> Hm I guess we haven't seen this before because it's common that
> pageblock_order is just one below MAX_ORDER so we're only merging two
> pageblocks. But your arch/config must be different to expose it. In any case
> LGTM, thanks.

+1

> Acked-by: Vlastimil Babka <vbabka@...e.cz>

Acked-by: Johannes Weiner <hannes@...xchg.org>

The warning makes the issue loud, but your patch is arguably fixing an
earlier commit that introduces type updates during merges.  How about:

Fixes: e6cf9e1c4cde ("mm: page_alloc: fix up block types when merging compatible blocks")
Cc: stable@...nel.org