linux-kernel - Re: [PATCH] mm: hugetlb: avoid fallback for specific node allocation of 1G pages

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Message-ID: <Z6sTLjKlX39s0Mo_@localhost.localdomain>
Date: Tue, 11 Feb 2025 10:06:54 +0100
From: Oscar Salvador <osalvador@...e.de>
To: Luiz Capitulino <luizcap@...hat.com>
Cc: linux-kernel@...r.kernel.org, yaozhenguo1@...il.com,
	muchun.song@...ux.dev, linux-mm@...ck.org,
	akpm@...ux-foundation.org, david@...hat.com, rppt@...nel.org,
	fvdl@...gle.com
Subject: Re: [PATCH] mm: hugetlb: avoid fallback for specific node allocation
 of 1G pages

On Mon, Feb 10, 2025 at 10:48:56PM -0500, Luiz Capitulino wrote:
> When using the HugeTLB kernel command-line to allocate 1G pages from
> a specific node, such as:
> 
>    default_hugepagesz=1G hugepages=1:1
> 
> If node 1 happens to not have enough memory for the requested number of
> 1G pages, the allocation falls back to other nodes. A quick way to
> reproduce this is by creating a KVM guest with a memory-less node and
> trying to allocate 1 1G page from it. Instead of failing, the allocation
> will fallback to other nodes.
> 
> This defeats the purpose of node specific allocation. Also, specific
> node allocation for 2M pages don't have this behavior: the allocation
> will just fail for the pages it can't satisfy.
> 
> This issue happens because HugeTLB calls memblock_alloc_try_nid_raw()
> for 1G boot-time allocation as this function falls back to other nodes
> if the allocation can't be satisfied. Use memblock_alloc_exact_nid_raw()
> instead, which ensures that the allocation will only be satisfied from
> the specified node.
> 
> Fixes: b5389086ad7b ("hugetlbfs: extend the definition of hugepages parameter to support node allocation")
> 
> Signed-off-by: Luiz Capitulino <luizcap@...hat.com>

Acked-by: Oscar Salvador <osalvador@...e.de>

This was discussed yesterday in [1], ccing Frank for awareness.

[1] https://patchwork.kernel.org/project/linux-mm/patch/20250206185109.1210657-6-fvdl@google.com/


-- 
Oscar Salvador
SUSE Labs