linux-kernel - Re: [PATCH] mm/page_isolation: clarify FIXME around shrink

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Message-ID: <7e43294a-03d5-42e5-bec2-5fbcfab97b69@redhat.com>
Date: Mon, 20 Oct 2025 21:26:42 +0200
From: David Hildenbrand <david@...hat.com>
To: Manish Kumar <manish1588@...il.com>, akpm@...ux-foundation.org
Cc: vbabka@...e.cz, surenb@...gle.com, mhocko@...e.com, jackmanb@...gle.com,
 hannes@...xchg.org, ziy@...dia.com, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/page_isolation: clarify FIXME around shrink_slab() in
 memory hotplug

On 15.10.25 19:50, Manish Kumar wrote:
> The existing FIXME comment notes that memory hotplug doesn't invoke
> shrink_slab() directly. This patch adds context explaining that this is
> an intentional design choice to avoid recursion or deadlocks in the
> memory reclaim path, as slab shrinking is handled by vmscan.
> 
> Signed-off-by: Manish Kumar <manish1588@...il.com>
> ---
>   mm/page_isolation.c | 10 ++++++++--
>   1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
> index b2fc5266e3d2..2ca20c3f0a97 100644
> --- a/mm/page_isolation.c
> +++ b/mm/page_isolation.c
> @@ -176,10 +176,16 @@ static int set_migratetype_isolate(struct page *page, int migratetype, int isol_
>   
>   	/*
>   	 * FIXME: Now, memory hotplug doesn't call shrink_slab() by itself.
> -	 * We just check MOVABLE pages.
> +	 *
> +	 * This is an intentional limitation: invoking shrink_slab() from a
> +	 * hotplug path can cause reclaim recursion or deadlock if the normal
> +	 * memory reclaim (vmscan) path is already active. Slab shrinking is
> +	 * handled by the vmscan reclaim code under normal operation, so hotplug
> +	 * avoids direct calls into shrink_slab() to prevent reentrancy issues.

This is the first time I hear about this reentrance issue.

How did you come up with this explanation?

-- 
Cheers

David / dhildenb