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([2a02:6b6f:e750:1800:450:cba3:aec3:a1fd]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-42f7cbe9065sm15517384f8f.8.2025.12.06.09.36.06 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sat, 06 Dec 2025 09:36:06 -0800 (PST) Message-ID: <996d8153-a69a-4075-aa1a-945ffe3630e9@gmail.com> Date: Sat, 6 Dec 2025 17:36:06 +0000 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 04/11] mm: Rework compound_head() for power-of-2 sizeof(struct page) Content-Language: en-GB To: Kiryl Shutsemau Cc: Andrew Morton , Muchun Song , David Hildenbrand , Oscar Salvador , Mike Rapoport , Vlastimil Babka , Lorenzo Stoakes , Matthew Wilcox , Zi Yan , Baoquan He , Michal Hocko , Johannes Weiner , Jonathan Corbet , kernel-team@meta.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org References: <20251205194351.1646318-1-kas@kernel.org> <20251205194351.1646318-5-kas@kernel.org> <22609798-e84b-46ca-9cb5-649ffba4a2a4@gmail.com> From: Usama Arif In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit >> hmm my understanding reviewing up until this patch of the series is that everything works >> the same as old code when struct page is not a power of 2. Returning page here means you dont >> fix page head when sizeof(struct page) is not a power of 2? > > There's no change for non-power-of-2 sizeof(struct page) as there's no > fake heads because there's no HVO for such cases. Yeah, I forgot that while reviewing :). I see its mentioned in vmemmap_dedup.rst as well. I think might be good to add a reminder in the commit message that HVO doesn't apply to non-power-of-2 sizeof(struct page), but no strong preference :) > > See hugetlb_vmemmap_optimizable_size() as I mentioned in the comment. > >> >>> /* >>> * Only addresses aligned with PAGE_SIZE of struct page may be fake head >>> * struct page. The alignment check aims to avoid access the fields ( >>> @@ -223,10 +230,13 @@ static __always_inline const struct page *page_fixed_fake_head(const struct page >>> * because the @page is a compound page composed with at least >>> * two contiguous pages. >>> */ >>> - unsigned long head = READ_ONCE(page[1].compound_info); >>> + unsigned long info = READ_ONCE(page[1].compound_info); >>> >>> - if (likely(head & 1)) >>> - return (const struct page *)(head - 1); >>> + if (likely(info & 1)) { >>> + unsigned long p = (unsigned long)page; >>> + >>> + return (const struct page *)(p & info); >> >> Would it be worth writing a comment over here similar to what you have in set_compound_head >> to explain why this works? i.e. compound_info contains the mask derived from folio order that >> can be applied to the virtual address to get the head page. > > But this code is about to be deleted. Is it really worth it? Ack, hadn't gotten to that commit.