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Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1pGKzx-006AIQ-8y; Fri, 13 Jan 2023 14:25:37 +0000 Date: Fri, 13 Jan 2023 14:25:37 +0000 From: Matthew Wilcox To: Kefeng Wang Cc: akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH -next 1/7] mm: huge_memory: make __do_huge_pmd_anonymous_page() to take a folio Message-ID: References: <20230112083006.163393-1-wangkefeng.wang@huawei.com> <20230112083006.163393-2-wangkefeng.wang@huawei.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230112083006.163393-2-wangkefeng.wang@huawei.com> X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 2242640009 X-Stat-Signature: ckp5yoi31zqrt5u9bs341y73niow1fto X-Rspam-User: X-HE-Tag: 1673619927-931923 X-HE-Meta: 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 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: On Thu, Jan 12, 2023 at 04:30:00PM +0800, Kefeng Wang wrote: > Let's __do_huge_pmd_anonymous_page() take a folio and convert related > functions to use folios. No, this is actively wrong! Andrew, please drop this patch. If we want to support folio sizes larger than PMD size (and I think we do), we need to be able to specify precisely which page in the folio is to be stored at this PTE. The *interface* must remain struct page. We can convert from page to folio within the function, but we *MUST NOT* go the other way. > static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *vmf, > - struct page *page, gfp_t gfp) > + struct folio *folio, gfp_t gfp) > { > struct vm_area_struct *vma = vmf->vma; > + struct page *page = &folio->page; ... ie this is bad and wrong. > @@ -834,7 +835,7 @@ vm_fault_t do_huge_pmd_anonymous_page(struct vm_fault *vmf) > count_vm_event(THP_FAULT_FALLBACK); > return VM_FAULT_FALLBACK; > } > - return __do_huge_pmd_anonymous_page(vmf, &folio->page, gfp); > + return __do_huge_pmd_anonymous_page(vmf, folio, gfp); > } > > static void insert_pfn_pmd(struct vm_area_struct *vma, unsigned long addr, A reasonable person might ask "But Matthew, you allocated a folio here, then you're converting back to a struct page to call __do_huge_pmd_anonymous_page() so isn't this a sensible patch?" And I would say "still no". This is a question of interfaces, and even though __do_huge_pmd_anonymous_page() is static and has precisely one caller today that always allocates a folio of precisely PMD size, I suspect it will need to be more visible in the future and the conversion of the interface from page to folio misleads people.