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[156.34.48.30]) by smtp.gmail.com with ESMTPSA id n85sm11837193qkn.31.2020.05.25.09.56.37 (version=TLS1_2 cipher=ECDHE-ECDSA-CHACHA20-POLY1305 bits=256/256); Mon, 25 May 2020 09:56:37 -0700 (PDT) Received: from jgg by mlx.ziepe.ca with local (Exim 4.90_1) (envelope-from ) id 1jdGOz-0003i1-9R; Mon, 25 May 2020 13:56:37 -0300 Date: Mon, 25 May 2020 13:56:37 -0300 From: Jason Gunthorpe To: Peter Xu , John Hubbard Cc: Alex Williamson , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, cohuck@redhat.com, cai@lca.pw, Andrea Arcangeli Subject: Re: [PATCH v3 3/3] vfio-pci: Invalidate mmaps and block MMIO access on disabled memory Message-ID: <20200525165637.GG744@ziepe.ca> References: <159017449210.18853.15037950701494323009.stgit@gimli.home> <159017506369.18853.17306023099999811263.stgit@gimli.home> <20200523193417.GI766834@xz-x1> <20200523170602.5eb09a66@x1.home> <20200523235257.GC939059@xz-x1> <20200525122607.GC744@ziepe.ca> <20200525142806.GC1058657@xz-x1> <20200525144651.GE744@ziepe.ca> <20200525151142.GE1058657@xz-x1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200525151142.GE1058657@xz-x1> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, May 25, 2020 at 11:11:42AM -0400, Peter Xu wrote: > On Mon, May 25, 2020 at 11:46:51AM -0300, Jason Gunthorpe wrote: > > On Mon, May 25, 2020 at 10:28:06AM -0400, Peter Xu wrote: > > > On Mon, May 25, 2020 at 09:26:07AM -0300, Jason Gunthorpe wrote: > > > > On Sat, May 23, 2020 at 07:52:57PM -0400, Peter Xu wrote: > > > > > > > > > For what I understand now, IMHO we should still need all those handlings of > > > > > FAULT_FLAG_RETRY_NOWAIT like in the initial version. E.g., IIUC KVM gup will > > > > > try with FOLL_NOWAIT when async is allowed, before the complete slow path. I'm > > > > > not sure what would be the side effect of that if fault() blocked it. E.g., > > > > > the caller could be in an atomic context. > > > > > > > > AFAICT FAULT_FLAG_RETRY_NOWAIT only impacts what happens when > > > > VM_FAULT_RETRY is returned, which this doesn't do? > > > > > > Yes, that's why I think we should still properly return VM_FAULT_RETRY if > > > needed.. because IMHO it is still possible that the caller calls with > > > FAULT_FLAG_RETRY_NOWAIT. > > > > > > My understanding is that FAULT_FLAG_RETRY_NOWAIT majorly means: > > > > > > - We cannot release the mmap_sem, and, > > > - We cannot sleep > > > > Sleeping looks fine, look at any FS implementation of fault, say, > > xfs. The first thing it does is xfs_ilock() which does down_write(). > > Yeah. My wild guess is that maybe fs code will always be without > FAULT_FLAG_RETRY_NOWAIT so it's safe to sleep unconditionally (e.g., I think > the general #PF should be fine to sleep in fault(); gup should be special, but > I didn't observe any gup code called upon file systems)? get_user_pages is called on filesystem backed pages. I have no idea what FAULT_FLAG_RETRY_NOWAIT is supposed to do. Maybe John was able to guess when he reworked that stuff? Jason