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From: Shakeel Butt To: Matthew Wilcox Cc: David Hildenbrand , "lsf-pc@lists.linux-foundation.org" , linux-fsdevel@vger.kernel.org, "linux-mm@kvack.org" , Jeff Layton , Zi Yan , Joanne Koong , Miklos Szeredi Subject: Re: [LSF/MM/BPF TOPIC] Migrating the un-migratable Message-ID: References: <882b566c-34d6-4e68-9447-6c74a0693f18@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Migadu-Flow: FLOW_OUT X-Rspam-User: X-Rspamd-Queue-Id: 9B9794000E X-Rspamd-Server: rspam10 X-Stat-Signature: r4npo9t77x3k6g4qz1tdfupunec8idpa X-HE-Tag: 1738276448-423998 X-HE-Meta: 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 nmG+XWVJ yrlysPtV9UYGK3ww89pE3MI+IsHETTTXPSztWxkSznxHhN6D2sHc7dQ3b8RvgmJinop+rIcwXPjk+0STcVhAy6mHjuh65Vf4jidoDnZxWNFegi2B8S6UZdtONiXtqju1retGxIUXmxlb3uJxwf95JRLRTNg1hywknzaM+aIazEUMDObFHDL6Tj9+XyFPgu6Ydqpx0FHKnFUlR6uTx1YUWFXlECP/pzVOxqwwmk8neVPVU3iTnegrF2Y5WeiSqSM77bZ8hv6/IviJHVSs= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, Jan 30, 2025 at 09:48:42PM +0000, Matthew Wilcox wrote: > On Wed, Jan 29, 2025 at 05:10:03PM +0100, David Hildenbrand wrote: > > As one example, in context of FUSE we recently discovered that folios that > > are under writeback cannot be migrated, and user space in control of when > > writeback will end. Something similar can happen ->readahead() where user > > space is in charge of supplying page content. Networking filesystems in > > general seem to be prone to this as well. > > You're not wrong. The question is whether we're willing to put the > asterisk on "In the presence of a misbehaving server (network or fuse), > our usual guarantees do not apply". I'm not sure it's a soluble > problem, though. Normally writeback (or, as you observed, readahead) > completes just fine and we don't need to use non-movable pages for them. > > But if someone trips over the network cable, anything in flight becomes > unmovable until someone plugs it back in. We've given the DMA address > of the memory to a network adapter, and that's generally a non-revokable > step (maybe the iommu could save us, but at what cost?) > My position is more aligned with Willy's. We definitely should discuss if we can solve the general problem of (im)movability due to writeback or readahead but I think targeting precise problem will be more fruitful. Untrusted fuse server deliberatly causing immovable memory is a real problem as anyone can run fuse server and mount fuse without any permissions. At least to me a misbehaving but trusted server is less of an (practical) issue. (Please correct me if I miss something like this is also a real issue in multi-tenancy world). Joanne has some ideas [1] on fuse specific solution but having a discussion on general solution would be beneficial as well. [1] https://lore.kernel.org/all/CAJnrk1ZCgff6ZWmqKzBXFq5uAEbms46OexA1axWS5v-PCZFqJg@mail.gmail.com/