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[2003:cb:c705:f100:af38:1d1f:66f4:48c7]) by smtp.gmail.com with ESMTPSA id h17-20020a5d6e11000000b002356c051b9csm12188701wrz.66.2022.11.02.02.14.34 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 02 Nov 2022 02:14:35 -0700 (PDT) Message-ID: <13c0caee-82e9-c295-b1b4-6d2bff132b72@redhat.com> Date: Wed, 2 Nov 2022 10:14:34 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.4.0 Content-Language: en-US To: Jason Gunthorpe Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrew Morton , Mike Kravetz , Peter Xu , John Hubbard , syzbot+f0b97304ef90f0d0b1dc@syzkaller.appspotmail.com References: <20221031152524.173644-1-david@redhat.com> From: David Hildenbrand Organization: Red Hat Subject: Re: [PATCH v1] mm/gup: disallow FOLL_FORCE|FOLL_WRITE on hugetlb mappings In-Reply-To: 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 31.10.22 17:14, Jason Gunthorpe wrote: > On Mon, Oct 31, 2022 at 04:25:24PM +0100, David Hildenbrand wrote: >> Cc: Andrew Morton >> Cc: Mike Kravetz >> Cc: Peter Xu >> Cc: John Hubbard >> Cc: Jason Gunthorpe >> Reported-by: syzbot+f0b97304ef90f0d0b1dc@syzkaller.appspotmail.com >> Signed-off-by: David Hildenbrand >> --- >> >> I assume this has been broken at least since 2014, when mm/gup.c came to >> life. I failed to come up with a suitable Fixes tag quickly. > > I'm worried this would break RDMA over hugetlbfs maps - which is a > real thing people do. > > MikeK do you have test cases? This patch here only silences the warning. The warning+failing is already in 6.0, and so far nobody (besides syzbot) complained. RDMA (due to FOLL_FORCE) would now fail (instead of doing something wrong) on MAP_PRIVATE hugetlb mappings that are R/O. Do we have any actual examples of such RDMA usage? I was able to understand why this case (MAP_PRIVATE, PROT_READ) is important for !hugetlb, but I don't immediately see under which situations this would apply to hugetlb. While we could implement FOLL_FORCE for hugetlb, at least for RDMA we will be moving away from FOLL_FORCE instead --- I'll be posting these patches shortly. So considering upcoming changes, at least RDMA is rather a bad excuse for more widespread FOLL_FORCE support. -- Thanks, David / dhildenb