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

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Message-Id: <7BD81F46-74A0-4991-8B32-67E5DBC682B3@linux.dev>
Date: Thu, 29 Jan 2026 11:04:53 +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 28, 2026, at 20:59, Kiryl Shutsemau <kas@...nel.org> wrote:
> 
> On Wed, Jan 28, 2026 at 10:43:13AM +0800, Muchun Song wrote:
>> 
>> 
>>> 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?
> 
> I looked again at it and reconsidered. I will use cmpxchg(), but mostly
> because hugetlb_lock is a bad fit to protect anything in pg_data_t.
> vmemmap_tails can be used by code outside hugetlb.
> 
> Here's the fixup.
> 
> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> index 29e9bbb43178..63e7ca85c8c9 100644
> --- a/mm/hugetlb_vmemmap.c
> +++ b/mm/hugetlb_vmemmap.c
> @@ -512,18 +512,11 @@ static struct page *vmemmap_get_tail(unsigned int order, int node)
> 	for (int i = 0; i < PAGE_SIZE / sizeof(struct page); i++)
> 		prep_compound_tail(p + i, NULL, order);
> 
> - 	spin_lock(&hugetlb_lock);
> - 	if (!NODE_DATA(node)->vmemmap_tails[idx]) {
> - 		pfn = PHYS_PFN(virt_to_phys(p));
> - 		NODE_DATA(node)->vmemmap_tails[idx] = pfn;
> - 		tail = NULL;
> - 	} else {
> - 		pfn = NODE_DATA(node)->vmemmap_tails[idx];
> - 	}
> - 	spin_unlock(&hugetlb_lock);
> -
> - 	if (tail)
> + 	pfn = PHYS_PFN(virt_to_phys(p));
> + 	if (cmpxchg(&NODE_DATA(node)->vmemmap_tails[idx], 0, pfn)) {
> 		__free_page(tail);
> + 		pfn = READ_ONCE(NODE_DATA(node)->vmemmap_tails[idx]);
> + 	}

Simpler than before.

> 
> 	return pfn_to_page(pfn);
> }
> -- 
>  Kiryl Shutsemau / Kirill A. Shutemov