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Mon, 15 Feb 2021 09:24:15 -0500 (EST) Date: Mon, 15 Feb 2021 09:24:15 -0500 From: Vivek Goyal To: Chirantan Ekbote Subject: Re: [Virtio-fs] [PATCH 07/24] DAX: virtio-fs: Add vhost-user slave commands for mapping Message-ID: <20210215142415.GA3331@redhat.com> References: <20210209190224.62827-1-dgilbert@redhat.com> <20210209190224.62827-8-dgilbert@redhat.com> MIME-Version: 1.0 In-Reply-To: X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=vgoyal@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Received-SPF: pass client-ip=63.128.21.124; envelope-from=vgoyal@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: mst@redhat.com, "Dr. David Alan Gilbert \(git\)" , qemu-devel@nongnu.org, virtio-fs-list , Stefan Hajnoczi , marcandre.lureau@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Mon, Feb 15, 2021 at 07:35:53PM +0900, Chirantan Ekbote wrote: > On Wed, Feb 10, 2021 at 4:04 AM Dr. David Alan Gilbert (git) > wrote: > > > > From: "Dr. David Alan Gilbert" > > + > > +typedef struct { > > + /* Offsets within the file being mapped */ > > + uint64_t fd_offset[VHOST_USER_FS_SLAVE_ENTRIES]; > > + /* Offsets within the cache */ > > + uint64_t c_offset[VHOST_USER_FS_SLAVE_ENTRIES]; > > + /* Lengths of sections */ > > + uint64_t len[VHOST_USER_FS_SLAVE_ENTRIES]; > > + /* Flags, from VHOST_USER_FS_FLAG_* */ > > + uint64_t flags[VHOST_USER_FS_SLAVE_ENTRIES]; > > +} VhostUserFSSlaveMsg; > > + > > Is it too late to change this? This struct allocates space for up to > 8 entries but most of the time the server will only try to set up one > mapping at a time so only 32 out of the 256 bytes in the message are > actually being used. We're just wasting time memcpy'ing bytes that > will never be used. Is there some reason this can't be dynamically > sized? Something like: > > typedef struct { > /* Number of mapping requests */ > uint16_t num_requests; > /* `num_requests` mapping requests */ > MappingRequest requests[]; > } VhostUserFSSlaveMsg; > > typedef struct { > /* Offset within the file being mapped */ > uint64_t fd_offset; > /* Offset within the cache */ > uint64_t c_offset; > /* Length of section */ > uint64_t len; > /* Flags, from VHOST_USER_FS_FLAG_* */ > uint64_t flags; > } MappingRequest; > > The current pre-allocated structure both wastes space when there are > fewer than 8 requests and requires extra messages to be sent when > there are more than 8 requests. I realize that in the grand scheme of > things copying 224 extra bytes is basically not noticeable but it just > irks me that we could fix this really easily before it gets propagated > to too many other places. Sounds like a reasonable idea. We probably will have to dynamically allocate memory for removemapping, hopefully that does not have a performance impact. Vivek