From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jason Gunthorpe Subject: Re: [PATCH] IB/mthca: Fix how mthca_map_user_db() calls gup Date: Thu, 25 Jan 2018 10:50:48 -0700 Message-ID: <20180125175048.GG10706@ziepe.ca> References: <20180123205459.432-1-dave@stgolabs.net> <1516898063.27592.136.camel@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <1516898063.27592.136.camel@redhat.com> Sender: linux-kernel-owner@vger.kernel.org To: Doug Ledford Cc: Davidlohr Bueso , roland@purestorage.com, linux-rdma@vger.kernel.org, linux-kernel@vger.kernel.org, Davidlohr Bueso List-Id: linux-rdma@vger.kernel.org On Thu, Jan 25, 2018 at 11:34:23AM -0500, Doug Ledford wrote: > > - Should I be wrong about no callers already holding mmap_sem, > > I still think calling gup without the mutex makes sense for > > improved paralellism. Now, if callers can hold the mmap_sem, > > it's wrong to do copy_from_user right before calling mthca_map_user_db. > > So, if I understand you correctly, we (well, you and Al would be more > correct, we haven't looked into the situation yet, so Mellanox people > that worked on this in the day might now, or someone taking the time to > research it could find out) don't have a clear understanding of all the > conditions this function is called under, and so we actually don't know > what the best way forward is to fix it? I looked at it enough to be confident that mthca_map_user_db is never called with mmap_sem held. Also pretty confident that mthca_unmap_user_db is never called with mmap_sem. So how about just grabbing mmap_sem around the call to get_user_pages and no other changes? Jason