On Mon, Sep 27, 2021 at 10:21:48AM +1000, Dave Chinner wrote: > On Thu, Sep 23, 2021 at 09:02:26PM -0400, Vivek Goyal wrote: > > On Fri, Sep 24, 2021 at 08:26:18AM +1000, Dave Chinner wrote: > > > On Thu, Sep 23, 2021 at 03:02:41PM -0400, Vivek Goyal wrote: > > > > On Thu, Sep 23, 2021 at 05:25:23PM +0800, Jeffle Xu wrote: > > > > > We add 'always', 'never', and 'inode' (default). '-o dax' continues to > > > > > operate the same which is equivalent to 'always'. To be consistemt with > > > > > ext4/xfs's tri-state mount option, when neither '-o dax' nor '-o dax=' > > > > > option is specified, the default behaviour is equal to 'inode'. > > > > > > > > So will "-o dax=inode" be used for per file DAX where dax mode comes > > > > from server? > > > > > > > > I think we discussed this. It will be better to leave "-o dax=inode" > > > > alone. It should be used when we are reading dax status from file > > > > attr (like ext4 and xfs). > > > > > > > > And probably create separate option say "-o dax=server" where server > > > > specifies which inode should use dax. > > > > > > That seems like a poor idea to me. > > > > > > The server side already controls what the client side does by > > > controlling the inode attributes that the client side sees. That > > > is, if the server is going to specify whether the client side data > > > access is going to use dax, then the server presents the client with > > > an inode that has the DAX attribute flag set on it. > > > > Hi Dave, > > > > Currently in fuse/virtiofs, DAX is compltely controlled by client. Server > > has no say in it. If client is mounted with "-o dax", dax is enabled on > > all files otherwise dax is disabled on all files. One could think of > > implementing an option on server so that server could deny mmap() > > requests that come from client, but so far nobody asked for such > > an option on server side. > > Can you please refer to the new options "always|never|inode" in > disucssions, because the "-o dax" option is deprecated and we really > need to center discussions around the new options, not the > deprecated one. > > > When you say "inode that has DAX attribute flag set on it", are you > > referring to "S_DAX (in inode->i_flags)" or persistent attr > > "FS_XFLAG_DAX"? > > Neither. The S_DAX flags is the VFS struct inode state that tells > the kernel what to do with that inode. The FS_XFLAG_DAX is the > userspace API status/control flag that represents the current state > of the persistent per-inode DAX control attribute. > > What I'm talking about the persistent attribute that is present > on stable storage. e.g. in the XFS on-disk inode the flag is > XFS_DIFLAG2_DAX which is never seen outside low level XFS code. In > the case of FUSE, this is the persistent server-side DAX attribute > state. The client gets this information from the server via an > inode attribute defined in the FUSE protocol. The client translates > that network protocol attribute to S_DAX status on FS inode > instantiation, and to/from FS_XFLAG_DAX for user control under > dax=inode. > > In the case that the user changes FS_XFLAG_DAX, the FUSE client > needs to communicate that attribute change to the server, where the > server then changes the persistent state of the on-disk inode so > that the next time the client requests that inode, it gets the state > it previously set. Unless, of course, there are server side policy > overrides (never/always). > > IOWs, dax=inode on the client side is essentially "follow server > side policy" because the server maintains the persistent DAX flag > state in the server side inode... > > > As of now S_DAX on client side inode is set by fuse client whenever > > client mounted filesystem with "-o dax". And I think you are assuming > > Of course. Similarly, if the client uses dax=never, the S_DAX is > never set on the VFS inode. But if dax=inode is set, where does the > DAX attribute state that determines S_DAX comes from .....? > > > that DAX attribute of inode is already coming from server. That's not > > the case. In fact that seems to be the proposal. Provide capability > > so that server can specify which inode should be using DAX and which > > inode should not be. > > Yup, that's exactly what I'm talking about - this is "dax=inode" > behaviour, because the client follows what the server tells it > in the inode attributes that arrive over the wire. > > > > In that case, turning off dax on the guest side should be > > > communicated to the fuse server so the server turns off the DAX flag > > > on the server side iff server side policy allows it. > > > > Not sure what do you mean by server turns of DAX flag based on client > > turning off DAX. Server does not have to do anything. If client decides > > to not use DAX (in guest), it will not send FUSE_SETUPMAPPING requests > > to server and that's it. > > Where does the client get it's per-inode DAX policy from if > dax=inode is, like other DAX filesystems, the default behaviour? > > Where is the persistent storage of that per-inode attribute kept? > > > > When the guest > > > then purges it's local inode and reloads it from the server then it > > > will get an inode with the DAX flag set according to server side > > > policy. > > > > So right now we don't have a mechanism for server to specify DAX flag. > > And that's what this patch series is trying to do. > > > > > > > > Hence if the server side is configured with "dax=always" or > > > dax="never" semantics it means the client side inode flag state > > > cannot control DAX mode. That means, regardless of the client side > > > mount options, DAX is operating under always or never policy, > > > > Hmm..., When you say "server side is configured with "dax=always", > > do you mean shared directory on host is mounted with "-o dax=always", > > or you mean some virtiofs server specific option which enables > > dax on all inodes from server side. > > I don't care. That's a server side implementation detail. You can > keep it in a private xattr for all I care. > > > In general, DAX on host and DAX inside guest are completely independent. > > Host filesystem could very well be mounted with dax or without dax and > > that has no affect on guests's capability to be able to enable DAX or > > not. > > If you have both the host and guest accessing the same files with > different modes, then you have a data coherency problem that is > guaranteed to corrupt data. > > > > seems to be exactly what 'dax=inode' behaviour means on the client > > > side - it behaves according to how the server side propagates the > > > DAX attribute to the client for each inode. > > > > Ok. So "-o dax=inode" in fuse will have a different meaning as opposed > > to ext4/xfs. This will mean that server will pass DAX state of inode > > when inode is instantiated and client should honor that. > > No, that's exactly what dax=inode means: DAX behaviour is > per-inode state that users must probe via statx() to determine if > dax is active or not. > > > But what about FS_XFLAG_DAX flag then. Say host file system > > does support this att and fuse/virtiofs allows querying and > > setting this attribute (I don't think it is allowed now). > > FS_XFLAG_DAX is the ioctl-based control API for the client side. > It's not a persistent flag. > > > So will we not create a future conflict where in case of > > fuse/virtiofs "-o dax=inode" means something different and it does > > look at FS_XFLAG_DAX file attr. > > Like many, I suspect you might be mis-understanding the DAX API. > > There are 4 parts to it: > > - persistent filesystem inode state flag > - ioctls to manipulate persistent inode state flag (FS_XFLAG_DAX) > - mount options to override persistent inode state flag > - VFS inode state indicating DAX is active (S_DAX, STATX_ATTR_DAX) > > You seem to be conflating the user API FS_XFLAG_DAX flag with > whatever the filesystem uses to physically storage that state. They > are not the same thing. FS_XFLAG_DAX also has no correlation with > the mount options - we can change the on-disk flag state regardless > of the mount option in use. We can even change the on-disk state > flag on *kernels that don't support DAX*. Changing the on-disk > attribute *does not change S_DAX*. > > That's because the on-disk persistent state is a property of the > filesystem's on-disk format, not a property of the kernel that is > running on that machine. The persistent state flag is managed as a > completely independent filesystem inode attribute, but it only > affects behaviour when the dax=inode mode has been selected. > > Control of the behaviour by this persistent inode flag is what > dax=inode means. It does not define how that attribute flag is > managed, it just defines the policy that an the inode's behaviour > will be determined by it's dax attribute state. So here is my confusion. Say client mounts filesystem with "dax=inode" and file "foo.txt" has FS_XFLAG_DAX set, do I have > > OTOH, S_DAX is used by the kernel to enable DAX access. It is the > _mechanism_, not the policy. We control S_DAX by mount option and/or > the filesystems persistent inode state flag, and behaviour is > determined by these policies at VFS inode instantiation time. Change > the policy, turf the inode out of cache and re-instantiate it, and > S_DAX for that inode is recalculated from the new policy. > > For FUSE, the server provides the persistent storage and nothing > else. The ioctls to manipulate dax state are client side ioctls, as > are the mount options and the S_DAX vfs inode state. Hence for > server side management of the per-inode S_DAX state, the FUSE > protocol needs to be able to pass the per-inode persistent DAX > attribute state to the client and the client needs to honor > that attribute from the server. > > How the FUSE server stores this persistent attribute is up to the > server implementation. If the server wants FUSE access to be > independent of host access, it can't use the persistent inode flags > in the host filesystem - it will need to use it's own xattrs. If the > server wants host access to be coherent with the guest, then it will > need to implement that in a different way. > > But the FUSE client doesn't care about any of this server side > policy mumbo-jumbo - it just needs to do what the server sends > to it in the DAX inode attribute. And that's exactly what dax=inode > means... > > > In summary, there seem to be two use cases. > > > > A. virtiofsd/fuse-server wants do be able to come up with its own > > policy to decide which inodes should use guest. > > > > B. guest client decides which inode use DAX based on FS_XFLAG_DAX > > attr on inode (and server does not play a role). > > > > To be able to different between these two cases, I was suggesting > > using "-o dax=inode" for B and "-o dax=server" for A. > > dax=inode covrees both these cases. All the server side needs to do > is set the inode attribute according to policy, and all the client > needs to do is obey the server side per-inode attribute. IOWs, > client side using "dax=inode" means the server side controls the DAX > behaviour via the FUSE DAX attribute. Hi Dave, Can a filesystem mounted with "-o dax=inode" enable DAX on a file even if FS_XFLAG_DAX attr is not set on the file. I think that's something new which will happen in case of fuse and currently does not happen with ext4/xfs. That's the use case A which wants to enable DAX attribute of a file based on its own policy (irrespective of state of FS_XFLAG_DAX on file). IIUC, you are ok with this. Just want to be sure because this is a subtle change from ext4/xfs behavior which will only enable DAX on a file if FS_XFLAG_DAX is set (with dax=inode). IOW, on ext4/xfs if FS_XFLAG_DAX is not set on file then DAX will not be enabled (dax=inode). I don't want applications to be making assumption that if FS_XFFLAG_DAX is not set on a file, then DAX will not be enabled on that file because that's what exactly fuse/virtiofs can do. > > If the server wants the client to always use DAX, then it always > sets the FUSE inode attribute. If the server says "never use DAX", > then it never sets the FUSE inode attribute. If the server doesn't > want the client to control policy, then it just rejects attempts to > change the per-inode persistent DAX flag via client side > ioctl(FS_XFLAG_DAX) calls. Hence we have use case A completely > covered. Ok, I think the behavior of ioctl(FS_XFLAG_DAX) calls is confusing me most in this context. IIUC, you are suggesting that server will define the policy whether it is supporting ioctl(FS_XFLAG_DAX) or not. This is will part of feature negotiation of fuse protocol. If server decides to support ioctl(FS_XFLAG_DAX), then it should work similar to ext4/xfs and also follow inheritance rules. If server decides to not support (or not enable) ioctl(FS_XFLAG_DAX), then server need to reject any attempt from client to set FS_XFLAG_DAX. And if client queries the current state of FS_XFLAG_DAX, then we need to return it is not set (even if it set on underlying filesystem). That way client will think FS_XFLAG_DAX is not set and will not expect DAX to be necessarily enabled. > > For case B, which is true dax=inode behaviour at the client, then > the server side honours the client side requests to change the > persistent FUSE DAX inode attribute via client side FS_XFLAG_DAX > ioctls. > > See? At the client side, there is absolutely no difference between > server side policy control and true dax=inode support. The client is > completely unaware of server side policies and that's how the client > side should be implemented. The applications have to be prepared for > policy to change dynamically with either dax=server or dax=inode, so > there's no difference to applications running in the guest either. > > Hence I just don't see the justification for a special DAX mode from > an architectural POV. It's no more work to implement per-inode DAX > properly form the start and we don't end up with a special, subtly > different mode. Ok, So I guess initially we could just implement "-o dax=inode" and *not support FS_XFLAG_DAX" API at all. Fuse server will decide which inodes should enable DAX and communicate that to client in a message response as part of protocol. Any attempt by client to set attr FS_XFLAG_DAX should fail (Say -ENOTSUP). And any query of FS_XFLAG_DAX attr should return attr is not set. One could support "FS_XFLAG_DAX" API down the line. And in that case fuser server should allow setting and querying FS_XFLAG_DAX and follow inheritance rules. Also dax flag of inode should be set according to state of FS_XFLAG_DAX attr (and not some internal decision of server). IOW, server will have to follow state of FS_XFLAG_DAX attr in deciding DAX state of inode if it decided to support FS_XFLAG_DAX attr API. Thanks Vivek
