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[2003:cb:c704:1100:65a0:c03a:142a:f914]) by smtp.gmail.com with ESMTPSA id 21-20020a05600c26d500b003d9b87296a9sm10216753wmv.25.2023.01.23.06.20.42 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 23 Jan 2023 06:20:43 -0800 (PST) Message-ID: <77f3fc56-05d8-def0-e518-0906c729e7df@redhat.com> Date: Mon, 23 Jan 2023 15:20:41 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.6.0 Subject: Re: [PATCH v7 2/8] iov_iter: Add a function to extract a page list from an iterator Content-Language: en-US To: David Howells Cc: Al Viro , Christoph Hellwig , Matthew Wilcox , Jens Axboe , Jan Kara , Jeff Layton , Logan Gunthorpe , linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, Christoph Hellwig , John Hubbard , linux-mm@kvack.org References: <7bbcccc9-6ebf-ffab-7425-2a12f217ba15@redhat.com> <246ba813-698b-8696-7f4d-400034a3380b@redhat.com> <20230120175556.3556978-1-dhowells@redhat.com> <20230120175556.3556978-3-dhowells@redhat.com> <3814749.1674474663@warthog.procyon.org.uk> <3903251.1674479992@warthog.procyon.org.uk> <3911637.1674481111@warthog.procyon.org.uk> From: David Hildenbrand Organization: Red Hat In-Reply-To: <3911637.1674481111@warthog.procyon.org.uk> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 23.01.23 14:38, David Howells wrote: > David Hildenbrand wrote: > >> That would be the ideal case: whenever intending to access page content, use >> FOLL_PIN instead of FOLL_GET. >> >> The issue that John was trying to sort out was that there are plenty of >> callsites that do a simple put_page() instead of calling >> unpin_user_page(). IIRC, handling that correctly in existing code -- what was >> pinned must be released via unpin_user_page() -- was the biggest workitem. >> >> Not sure how that relates to your work here (that's why I was asking): if you >> could avoid FOLL_GET, that would be great :) > > Well, it simplifies things a bit. > > I can make the new iov_iter_extract_pages() just do "pin" or "don't pin" and > do no ref-getting at all. Things can be converted over to "unpin the pages or > doing nothing" as they're converted over to using iov_iter_extract_pages() > from iov_iter_get_pages*(). > > The block bio code then only needs a single bit of state: pinned or not > pinned. Unfortunately, I'll have to let BIO experts comment on that :) I only know the MM side of things here. > > For cifs RDMA, do I need to make it pass in FOLL_LONGTERM? And does that need > a special cleanup? Anything that holds pins "possibly forever" should that. vmsplice() is another example that should use it, once properly using FOLL_PIN. [FOLL_GET | FOLL_LONGTERM is not really used/defined with semantics] > > sk_buff fragment handling could still be tricky. I'm thinking that in that > code I'll need to store FOLL_GET/PIN in the bottom two bits of the frag page > pointer. Sometimes it allocates a new page and attaches it (have ref); > sometimes it does zerocopy to/from a page (have pin) and sometimes it may be > pointing to a kernel buffer (don't pin or ref). > > David > -- Thanks, David / dhildenb