linux-kernel - Re: [PATCHv4 09/14] mm/hugetlb: Remove fake head pages

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Message-Id: <25C01EB2-FC77-43A5-A737-7BD3D2D98EDE@linux.dev>
Date: Wed, 28 Jan 2026 10:43:13 +0800
From: Muchun Song <muchun.song@...ux.dev>
To: Kiryl Shutsemau <kas@...nel.org>
Cc: Oscar Salvador <osalvador@...e.de>,
 Mike Rapoport <rppt@...nel.org>,
 Vlastimil Babka <vbabka@...e.cz>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Zi Yan <ziy@...dia.com>,
 Baoquan He <bhe@...hat.com>,
 Michal Hocko <mhocko@...e.com>,
 Johannes Weiner <hannes@...xchg.org>,
 Jonathan Corbet <corbet@....net>,
 kernel-team@...a.com,
 linux-mm@...ck.org,
 linux-kernel@...r.kernel.org,
 linux-doc@...r.kernel.org,
 Andrew Morton <akpm@...ux-foundation.org>,
 David Hildenbrand <david@...nel.org>,
 Matthew Wilcox <willy@...radead.org>,
 Usama Arif <usamaarif642@...il.com>,
 Frank van der Linden <fvdl@...gle.com>
Subject: Re: [PATCHv4 09/14] mm/hugetlb: Remove fake head pages



> On Jan 27, 2026, at 22:51, Kiryl Shutsemau <kas@...nel.org> wrote:
> 
> On Thu, Jan 22, 2026 at 03:00:03PM +0800, Muchun Song wrote:
>>> + if (pfn)
>>> + 	return pfn_to_page(pfn);
>>> +
>>> + tail = alloc_pages_node(node, GFP_KERNEL | __GFP_ZERO, 0);
>>> + if (!tail)
>>> + 	return NULL;
>>> +
>>> + p = page_to_virt(tail);
>>> + for (int i = 0; i < PAGE_SIZE / sizeof(struct page); i++)
>>> + 	prep_compound_tail(p + i, NULL, order);
>>> +
>>> + spin_lock(&hugetlb_lock);
>> 
>> hugetlb_lock is considered a contended lock, better not to abuse it.
>> cmpxchg() is enought in this case.
> 
> We hit the lock once per node (excluding races). Its contribution to the
> lock contention is negligible. spin_lock() is easier to follow. I will
> keep it.

I don't think cmpxchg() is hard to follow. It’s precisely because of
your abuse that interrupts still have to be disabled here—hugetlb_lock
must be an irq-off lock. Are you really going to use spin_lock_irq just
because “it feels simpler” to you?

> 
> Ack for the rest of your comments.
> 
> 
> -- 
>  Kiryl Shutsemau / Kirill A. Shutemov