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[2003:cb:c708:1a00:df86:93fe:6505:d096]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4290566729dsm11335695e9.0.2024.08.07.02.31.56 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 07 Aug 2024 02:31:57 -0700 (PDT) Message-ID: <3230697b-55ea-4776-a5f8-5116366741ad@redhat.com> Date: Wed, 7 Aug 2024 11:31:55 +0200 Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v1 02/11] mm/pagewalk: introduce folio_walk_start() + folio_walk_end() To: Claudio Imbrenda Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-doc@vger.kernel.org, kvm@vger.kernel.org, linux-s390@vger.kernel.org, linux-fsdevel@vger.kernel.org, Andrew Morton , "Matthew Wilcox (Oracle)" , Jonathan Corbet , Christian Borntraeger , Janosch Frank , Heiko Carstens , Vasily Gorbik , Alexander Gordeev , Sven Schnelle , Gerald Schaefer References: <20240802155524.517137-1-david@redhat.com> <20240802155524.517137-3-david@redhat.com> <20240807111754.2148d27e@p-imbrenda.boeblingen.de.ibm.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 07.08.24 11:17, Claudio Imbrenda wrote: > On Fri, 2 Aug 2024 17:55:15 +0200 > David Hildenbrand wrote: > >> We want to get rid of follow_page(), and have a more reasonable way to >> just lookup a folio mapped at a certain address, perform some checks while >> still under PTL, and then only conditionally grab a folio reference if >> really required. >> >> Further, we might want to get rid of some walk_page_range*() users that >> really only want to temporarily lookup a single folio at a single address. >> >> So let's add a new page table walker that does exactly that, similarly >> to GUP also being able to walk hugetlb VMAs. >> >> Add folio_walk_end() as a macro for now: the compiler is not easy to >> please with the pte_unmap()->kunmap_local(). >> >> Note that one difference between follow_page() and get_user_pages(1) is >> that follow_page() will not trigger faults to get something mapped. So >> folio_walk is at least currently not a replacement for get_user_pages(1), >> but could likely be extended/reused to achieve something similar in the >> future. > [...] >> +pmd_table: >> + VM_WARN_ON_ONCE(pud_leaf(*pudp)); > Thanks for the review! > is this warning necessary? can this actually happen? > and if it can happen, wouldn't it be more reasonable to return NULL? The we have to turn this into an unconditional WARN_ON_ONCE() that cannot be compiled out. It's something that should be found early during testing (like I had a bug where I misspelled "CONFIG_PGTABLE_HAS_HUGE_LEAVES" above that took me 2h to debug, so I added it ;) ), and shouldn't need runtime checks. Same for the other one. Thanks! -- Cheers, David / dhildenb