linux-kernel - Re: [PATCHv4 05/14] mm: Rework compound_head() for power-of-2 sizeof(struct page)

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Message-Id: <85016E74-E7D6-4AA1-8754-24B86496CEFE@linux.dev>
Date: Thu, 22 Jan 2026 19:52:52 +0800
From: Muchun Song <muchun.song@...ux.dev>
To: Kiryl Shutsemau <kas@...nel.org>,
 Zi Yan <ziy@...dia.com>
Cc: 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>,
 Oscar Salvador <osalvador@...e.de>,
 Mike Rapoport <rppt@...nel.org>,
 Vlastimil Babka <vbabka@...e.cz>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.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
Subject: Re: [PATCHv4 05/14] mm: Rework compound_head() for power-of-2
 sizeof(struct page)



> On Jan 22, 2026, at 19:29, Kiryl Shutsemau <kas@...nel.org> wrote:
> 
> On Wed, Jan 21, 2026 at 12:12:13PM -0500, Zi Yan wrote:
>>> @@ -1244,8 +1244,8 @@ void snapshot_page(struct page_snapshot *ps, const struct page *page)
>>> again:
>>> memset(&ps->folio_snapshot, 0, sizeof(struct folio));
>>> memcpy(&ps->page_snapshot, page, sizeof(*page));
>>> - head = ps->page_snapshot.compound_info;
>>> - if ((head & 1) == 0) {
>>> + info = ps->page_snapshot.compound_info;
>>> + if ((info & 1) == 0) {
>> 
>> This could be “if (!(info & 1))” like _compound_head(), right?
> 
> Right. But I don't see why it has to change.

Let me briefly explain my programming habits. I guess this might
be Zi Yan's idea.

When I use a variable as a boolean type, my habit is to use the
"!var" operator to check if it is 0 (false). When I use a variable
as a number (with a value range exceeding 0 and 1), and I need to
check if this variable is 0, I usually use "var == 0".

The least significant bit here seems to be used as a boolean type.

> 
> 
> -- 
>  Kiryl Shutsemau / Kirill A. Shutemov