From mboxrd@z Thu Jan 1 00:00:00 1970 References: <5D53D882.5090803@huawei.com> <5D5A4692.9060006@huawei.com> <20190820153926.GC4233@redhat.com> <20190820175610.GD4233@redhat.com> <5D5CA2E2.9000802@huawei.com> <20190821131136.GA2304@redhat.com> From: piaojun Message-ID: <5D5F47A5.3020109@huawei.com> Date: Fri, 23 Aug 2019 09:55:49 +0800 MIME-Version: 1.0 In-Reply-To: <20190821131136.GA2304@redhat.com> Content-Type: text/plain; charset="windows-1252" Content-Transfer-Encoding: 7bit Subject: Re: [Virtio-fs] [PATCH RESEND][RFC] virtiofsd: do not fall back to buffer io when cache=auto/always List-Id: Development discussions about virtio-fs List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Vivek Goyal Cc: virtio-fs@redhat.com On 2019/8/21 21:11, Vivek Goyal wrote: > On Wed, Aug 21, 2019 at 09:48:18AM +0800, piaojun wrote: >> >> >> On 2019/8/21 1:56, Vivek Goyal wrote: >>> On Tue, Aug 20, 2019 at 11:39:26AM -0400, Vivek Goyal wrote: >>>> On Mon, Aug 19, 2019 at 02:49:54PM +0800, piaojun wrote: >>>>> When O_DIRECT flags is set by Guest, virtiofsd will open file with >>>>> O_DIRECT, but unset 'fi->direct_io' which makes Guest go buffer io path. >>>> >>>> Where is virtiofsd unset fi->direct_io flag? All I can see is that it >>>> sets this flag if cache=none? >>>> >>> >>> Ok, I looked at it little bit more closely and I don't think it is a >>> problem. With cache=always/auto, we don't set fi->direct_io. That means >>> server is not enforcing a direct I/O policy on client. >>> >>> Now if client decides to direct I/O by opening file with O_DIRECT, it >>> will still take direct I/O path and call fuse_direct_IO(). Look at >>> fuse_file_aops->fuse_direct_IO(). >> >> The problem is that when Guest open with O_DIRECT, the Host won't set >> open_flags with FOPEN_DIRECT_IO if CACHE_AUTO is set. It makes Guest >> goes into fuse_cache_write_iter() rather than fuse_direct_write_iter(). >> In other words, the Guest's I/O mode will be changed by virtiofsd. > > I think there are two direct I/O paths. One is where server requests > that direct I/O be done on the file and in that case FOPEN_DIRECT_IO > is set. > > Other case is where client initiates direct I/O and in that case > FOPEN_DIRECT_IO is not set and we take regular path which is > capable of doing both direct and buffered I/O. For example, for > reads we do following. > > fuse_file_read_iter() > fuse_cache_read_iter() > generic_file_read_iter() > mapping->a_ops->direct_IO() <--- (If iocb->ki_flags & IOCB_DIRECT) > fuse_direct_IO() > > Similary for write path. > > fuse_file_write_iter() > fuse_cache_write_iter() > generic_file_direct_write() <----- (If iocb->ki_flags & IOCB_DIRECT) > mapping->a_ops->direct_IO() > fuse_direct_IO() > > Can you give it a try. I don't think we have the problem what you > are describing. > I make a try as you said, and it indeed goes direct path in guest even if cache=always. So my doubt has been cleared. Thanks, Jun