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Tsirkin" , Matthew Wilcox , Benjamin Coddington , linux-kernel@vger.kernel.org, virtualization@lists.linux.dev, houtao1@huawei.com Subject: Re: [PATCH v2 4/6] virtiofs: support bounce buffer backed by scattered pages Message-ID: References: <20240228144126.2864064-1-houtao@huaweicloud.com> <20240228144126.2864064-5-houtao@huaweicloud.com> Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20240228144126.2864064-5-houtao@huaweicloud.com> X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.3 On Wed, Feb 28, 2024 at 10:41:24PM +0800, Hou Tao wrote: > From: Hou Tao > > When reading a file kept in virtiofs from kernel (e.g., insmod a kernel > module), if the cache of virtiofs is disabled, the read buffer will be > passed to virtiofs through out_args[0].value instead of pages. Because > virtiofs can't get the pages for the read buffer, virtio_fs_argbuf_new() > will create a bounce buffer for the read buffer by using kmalloc() and > copy the read buffer into bounce buffer. If the read buffer is large > (e.g., 1MB), the allocation will incur significant stress on the memory > subsystem. > > So instead of allocating bounce buffer by using kmalloc(), allocate a > bounce buffer which is backed by scattered pages. The original idea is > to use vmap(), but the use of GFP_ATOMIC is no possible for vmap(). To > simplify the copy operations in the bounce buffer, use a bio_vec flex > array to represent the argbuf. Also add an is_flat field in struct > virtio_fs_argbuf to distinguish between kmalloc-ed and scattered bounce > buffer. > > Signed-off-by: Hou Tao > --- > fs/fuse/virtio_fs.c | 163 ++++++++++++++++++++++++++++++++++++++++---- > 1 file changed, 149 insertions(+), 14 deletions(-) > > diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c > index f10fff7f23a0f..ffea684bd100d 100644 > --- a/fs/fuse/virtio_fs.c > +++ b/fs/fuse/virtio_fs.c ... > @@ -408,42 +425,143 @@ static void virtio_fs_request_dispatch_work(struct work_struct *work) > } > } > ... > static void virtio_fs_argbuf_copy_from_in_arg(struct virtio_fs_argbuf *argbuf, > unsigned int offset, > const void *src, unsigned int len) > { > - memcpy(argbuf->buf + offset, src, len); > + struct iov_iter iter; > + unsigned int copied; > + > + if (argbuf->is_flat) { > + memcpy(argbuf->f.buf + offset, src, len); > + return; > + } > + > + iov_iter_bvec(&iter, ITER_DEST, argbuf->s.bvec, > + argbuf->s.nr, argbuf->s.size); > + iov_iter_advance(&iter, offset); Hi Hou, Just a random comment, but it seems a little inefficient to reinit and readvance the iter like this on every copy/call. It looks like offset is already incremented in the callers of the argbuf copy helpers. Perhaps iov_iter could be lifted into the callers and passed down, or even just include it in the argbuf structure and init it at alloc time? Brian > + > + copied = _copy_to_iter(src, len, &iter); > + WARN_ON_ONCE(copied != len); > } > > static unsigned int > @@ -451,15 +569,32 @@ virtio_fs_argbuf_out_args_offset(struct virtio_fs_argbuf *argbuf, > const struct fuse_args *args) > { > unsigned int num_in = args->in_numargs - args->in_pages; > + unsigned int offset = fuse_len_args(num_in, > + (struct fuse_arg *)args->in_args); > > - return fuse_len_args(num_in, (struct fuse_arg *)args->in_args); > + if (argbuf->is_flat) > + return offset; > + return round_up(offset, PAGE_SIZE); > } > > static void virtio_fs_argbuf_copy_to_out_arg(struct virtio_fs_argbuf *argbuf, > unsigned int offset, void *dst, > unsigned int len) > { > - memcpy(dst, argbuf->buf + offset, len); > + struct iov_iter iter; > + unsigned int copied; > + > + if (argbuf->is_flat) { > + memcpy(dst, argbuf->f.buf + offset, len); > + return; > + } > + > + iov_iter_bvec(&iter, ITER_SOURCE, argbuf->s.bvec, > + argbuf->s.nr, argbuf->s.size); > + iov_iter_advance(&iter, offset); > + > + copied = _copy_from_iter(dst, len, &iter); > + WARN_ON_ONCE(copied != len); > } > > /* > @@ -1154,7 +1289,7 @@ static unsigned int sg_init_fuse_args(struct scatterlist *sg, > len = fuse_len_args(numargs - argpages, args); > if (len) > total_sgs += virtio_fs_argbuf_setup_sg(req->argbuf, *len_used, > - len, &sg[total_sgs]); > + &len, &sg[total_sgs]); > > if (argpages) > total_sgs += sg_init_fuse_pages(&sg[total_sgs], > @@ -1199,7 +1334,7 @@ static int virtio_fs_enqueue_req(struct virtio_fs_vq *fsvq, > } > > /* Use a bounce buffer since stack args cannot be mapped */ > - req->argbuf = virtio_fs_argbuf_new(args, GFP_ATOMIC); > + req->argbuf = virtio_fs_argbuf_new(args, GFP_ATOMIC, true); > if (!req->argbuf) { > ret = -ENOMEM; > goto out; > -- > 2.29.2 > >