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[2003:d8:2f3e:5900:27aa:5f4a:b65c:3d3c]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a099ae3ccdsm19537950f8f.38.2025.05.08.00.37.09 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 08 May 2025 00:37:09 -0700 (PDT) Message-ID: <0fd1bf14-a664-49c0-a239-fdc45b8e3a39@redhat.com> Date: Thu, 8 May 2025 09:37:08 +0200 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 2/2] mm: convert do_set_pmd() to take a folio To: Baolin Wang , Matthew Wilcox Cc: akpm@linux-foundation.org, hannes@cmpxchg.org, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, ziy@nvidia.com, vbabka@suse.cz, rppt@kernel.org, surenb@google.com, mhocko@suse.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <8e33c8a65b46170dfd8ba6715d2115856a55b8f6.1746609191.git.baolin.wang@linux.alibaba.com> <88060373-d0b8-404a-b53b-9b5a54f5ba6f@linux.alibaba.com> <569e592c-1c30-4c03-bcee-c1670ac4e159@linux.alibaba.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 08.05.25 04:23, Baolin Wang wrote: > > > On 2025/5/8 00:47, Matthew Wilcox wrote: >> On Wed, May 07, 2025 at 08:36:54PM +0800, Baolin Wang wrote: >>> On 2025/5/7 20:10, Matthew Wilcox wrote: >>>> Because I see nowhere in this patch that you initialise 'page'. >>> >>> Please look at the following code in do_set_pmd(), and the 'page' will be >>> initialized before using. >>> >>> if (thp_disabled_by_hw() || vma_thp_disabled(vma, vma->vm_flags)) >>> return ret; >>> >>> if (!thp_vma_suitable_order(vma, haddr, PMD_ORDER)) >>> return ret; >>> >>> if (folio_order(folio) != HPAGE_PMD_ORDER) >>> return ret; >>> page = &folio->page; >> >> Ah, fair, I missed that. >> >>>> And that's really the important part. You seem to be assuming that a >>>> folio will never be larger than PMD size, and I'm not comfortable with >>> >>> No, I have no this assumption. But do_set_pmd() is used to establish PMD >>> mappings for the PMD-sized folios, and we already have PMD-sized checks to >>> validate the folio size: >>> >>> if (!thp_vma_suitable_order(vma, haddr, PMD_ORDER)) >>> return ret; >>> >>> if (folio_order(folio) != HPAGE_PMD_ORDER) >>> return ret; >>> >>>> that assumption. It's a limitation I put in place a few years ago so we >>>> didn't have to find and fix all those assumptions immediately, but I >>>> imagine that some day we'll want to have larger folios. >>>> >>>> So unless you can derive _which_ page in the folio we want to map from >>> >>> IMO, for PMD mapping of a PMD-sized folio, we do not need to know _which_ >>> page in the folio we want to map, because we'll always map the entire >>> PMD-sized folio. >> >> There's a difference between "Assert that the folio is PMD sized" inside >> the function because we know there are still problems, and "Change the >> interface so we can't specify which page inside the folio is the one >> we're actually interested in". > > Fair enough. So how about adding a new 'folio' parameter to > do_set_pmd(), similar to the set_pte_range() function prototype? > > vm_fault_t do_set_pmd(struct vm_fault *vmf, struct folio *folio, struct > page *page) That's what I used for rmap functions. *Maybe* folio+idx is better: might avoid having to lookup the page in some cases (probably in the future). -- Cheers, David / dhildenb