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Received: from willy by casper.infradead.org with local (Exim 4.97.1 #2 (Red Hat Linux)) id 1sH6Nu-0000000DxX4-0IGK; Tue, 11 Jun 2024 18:38:18 +0000 Date: Tue, 11 Jun 2024 19:38:17 +0100 From: Matthew Wilcox To: Andrew Morton Cc: Andrew Bresticker , David Hildenbrand , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] mm/memory: Don't require head page for do_set_pmd() Message-ID: References: <20240611153216.2794513-1-abrestic@rivosinc.com> <20240611112131.25925b8ee5198668b88de35f@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20240611112131.25925b8ee5198668b88de35f@linux-foundation.org> X-Stat-Signature: 5bmqi6ct3tdk8wnjdqxgif6pzbreyfzt X-Rspamd-Queue-Id: 939B1100008 X-Rspamd-Server: rspam04 X-Rspam-User: X-HE-Tag: 1718131100-987282 X-HE-Meta: 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 zBYy1S4y 00x32M9gjp4I82Qg91KaxNaGQs3anZXQpVk6hZCkIAKCM9RzV8bQ9ovh13NLKpqTB5nO1iBGVbP7H09PA+7QsGWE/R2BcUf4v31dp1gZv17llj7D6XPtPySIdj2cq8yc2uruptJ1OP8G531xRBcFkYwZKUBCiM/6SoKgigQ/u2+dYDEgTE+Y0AqyWUYx0pRcgu/23uY7o43EbwJ9JqLWIGhbUfuIgNdABkKH9RLDIF/xbU4A= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Jun 11, 2024 at 11:21:31AM -0700, Andrew Morton wrote: > On Tue, 11 Jun 2024 19:03:29 +0100 Matthew Wilcox wrote: > > > On Tue, Jun 11, 2024 at 08:32:16AM -0700, Andrew Bresticker wrote: > > > - if (page != &folio->page || folio_order(folio) != HPAGE_PMD_ORDER) > > > + if (folio_order(folio) != HPAGE_PMD_ORDER) > > > return ret; > > > + page = &folio->page; > > > > This works today, but in about six months time it's going to be a pain. > > > > + page = folio_page(folio, 0); > > > > is the one which works today and in the future. > > I was wondering about that. > > hp2:/usr/src/25> fgrep "&folio->page" mm/*.c | wc -l > 84 > hp2:/usr/src/25> fgrep "folio_page(" mm/*.c | wc -l > 35 > > Should these all be converted? What's the general rule here? The rule is ... - If we haven't thought about it, use &folio->page to indicate that somebody needs to think about it. - If the code needs to be modified to split folio and page apart, use &folio->page. - If the code is part of compat code which is going to have to be removed, use &folio->page (eg do_read_cache_page()). To *think* about it, and use folio_page() or folio_file_page(), don't just blindly pass 0 as the second argument. Think about which page within the folio is expected by the function you're working on. Often that is "the first one!" and so folio_page(folio, 0) is the right answer. But that should be justified. It might be the right answer is "Oh, that function should take a folio".