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[2620:137:e000::1:20]) by mx.google.com with ESMTP id d25-20020aa7c1d9000000b00459a58216d8si7403483edp.504.2022.11.22.04.57.47; Tue, 22 Nov 2022 04:58:10 -0800 (PST) Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20; Authentication-Results: mx.google.com; spf=pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=fail (p=NONE sp=NONE dis=NONE) header.from=xs4all.nl Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233748AbiKVMZ7 (ORCPT + 91 others); Tue, 22 Nov 2022 07:25:59 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:38244 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232312AbiKVMZ5 (ORCPT ); Tue, 22 Nov 2022 07:25:57 -0500 Received: from ams.source.kernel.org (ams.source.kernel.org [145.40.68.75]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 463B556573; Tue, 22 Nov 2022 04:25:56 -0800 (PST) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id EE7E5B81A4A; Tue, 22 Nov 2022 12:25:54 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id C8585C433C1; Tue, 22 Nov 2022 12:25:47 +0000 (UTC) Message-ID: <6175d780-3307-854c-448a-8e6c7ad0772c@xs4all.nl> Date: Tue, 22 Nov 2022 13:25:45 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.3.0 Subject: Re: [PATCH RFC 16/19] mm/frame-vector: remove FOLL_FORCE usage Content-Language: en-US To: Tomasz Figa , David Hildenbrand , Marek Szyprowski Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, etnaviv@lists.freedesktop.org, dri-devel@lists.freedesktop.org, linux-arm-kernel@lists.infradead.org, linux-samsung-soc@vger.kernel.org, linux-rdma@vger.kernel.org, linux-media@vger.kernel.org, linux-kselftest@vger.kernel.org, Linus Torvalds , Andrew Morton , Jason Gunthorpe , John Hubbard , Peter Xu , Greg Kroah-Hartman , Andrea Arcangeli , Hugh Dickins , Nadav Amit , Vlastimil Babka , Matthew Wilcox , Mike Kravetz , Muchun Song , Lucas Stach , David Airlie , Oded Gabbay , Arnd Bergmann , Mauro Carvalho Chehab References: <20221107161740.144456-1-david@redhat.com> <20221107161740.144456-17-david@redhat.com> From: Hans Verkuil In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Spam-Status: No, score=-6.7 required=5.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,NICE_REPLY_A,RCVD_IN_DNSWL_HI, SPF_HELO_NONE,SPF_PASS autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on lindbergh.monkeyblade.net Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Tomasz, David, On 11/8/22 05:45, Tomasz Figa wrote: > Hi David, > > On Tue, Nov 8, 2022 at 1:19 AM David Hildenbrand wrote: >> >> FOLL_FORCE is really only for debugger access. According to commit >> 707947247e95 ("media: videobuf2-vmalloc: get_userptr: buffers are always >> writable"), the pinned pages are always writable. > > Actually that patch is only a workaround to temporarily disable > support for read-only pages as they seemed to suffer from some > corruption issues in the retrieved user pages. We expect to support > read-only pages as hardware input after. That said, FOLL_FORCE doesn't > sound like the right thing even in that case, but I don't know the > background behind it being added here in the first place. +Hans > Verkuil +Marek Szyprowski do you happen to remember anything about it? I tracked the use of 'force' all the way back to the first git commit (2.6.12-rc1) in the very old video-buf.c. So it is very, very old and the reason is lost in the mists of time. I'm not sure if the 'force' argument of get_user_pages() at that time even meant the same as FOLL_FORCE today. From what I can tell it has just been faithfully used ever since, but I have my doubt that anyone understands the reason behind it since it was never explained. Looking at this old LWN article https://lwn.net/Articles/28548/ suggests that it might be related to calling get_user_pages for write buffers (non-zero write argument) where you also want to be able to read from the buffer. That is certainly something that some drivers need to do post-capture fixups. But 'force' was also always set for read buffers, and I don't know if that was something that was actually needed, or just laziness. I assume that removing FOLL_FORCE from 'FOLL_FORCE|FOLL_WRITE' will still allow drivers to read from the buffer? Regards, Hans > > Best regards, > Tomasz > >> >> FOLL_FORCE in this case seems to be a legacy leftover. Let's just remove >> it. >> >> Cc: Tomasz Figa >> Cc: Marek Szyprowski >> Cc: Mauro Carvalho Chehab >> Signed-off-by: David Hildenbrand >> --- >> drivers/media/common/videobuf2/frame_vector.c | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/drivers/media/common/videobuf2/frame_vector.c b/drivers/media/common/videobuf2/frame_vector.c >> index 542dde9d2609..062e98148c53 100644 >> --- a/drivers/media/common/videobuf2/frame_vector.c >> +++ b/drivers/media/common/videobuf2/frame_vector.c >> @@ -50,7 +50,7 @@ int get_vaddr_frames(unsigned long start, unsigned int nr_frames, >> start = untagged_addr(start); >> >> ret = pin_user_pages_fast(start, nr_frames, >> - FOLL_FORCE | FOLL_WRITE | FOLL_LONGTERM, >> + FOLL_WRITE | FOLL_LONGTERM, >> (struct page **)(vec->ptrs)); >> if (ret > 0) { >> vec->got_ref = true; >> -- >> 2.38.1 >>