Re: [PATCH v2 0/4] virtiofs,fuse: support per-file DAX

[Vivek Goyal <vgoyal@redhat.com>]

On Wed, Jul 21, 2021 at 08:32:19PM +0800, JeffleXu wrote:
> 
> 
> On 7/21/21 3:18 AM, Vivek Goyal wrote:
> > On Tue, Jul 20, 2021 at 01:25:11PM +0800, JeffleXu wrote:
> >>
> >>
> >> On 7/20/21 5:30 AM, Vivek Goyal wrote:
> >>> On Fri, Jul 16, 2021 at 06:47:49PM +0800, Jeffle Xu wrote:
> >>>> This patchset adds support of per-file DAX for virtiofs, which is
> >>>> inspired by Ira Weiny's work on ext4[1] and xfs[2].
> >>>>
> >>>> There are three related scenarios:
> >>>> 1. Alloc inode: get per-file DAX flag from fuse_attr.flags. (patch 3)
> >>>> 2. Per-file DAX flag changes when the file has been opened. (patch 3)
> >>>> In this case, the dentry and inode are all marked as DONT_CACHE, and
> >>>> the DAX state won't be updated until the file is closed and reopened
> >>>> later.
> >>>> 3. Users can change the per-file DAX flag inside the guest by chattr(1).
> >>>> (patch 4)
> >>>> 4. Create new files under directories with DAX enabled. When creating
> >>>> new files in ext4/xfs on host, the new created files will inherit the
> >>>> per-file DAX flag from the directory, and thus the new created files in
> >>>> virtiofs will also inherit the per-file DAX flag if the fuse server
> >>>> derives fuse_attr.flags from the underlying ext4/xfs inode's per-file
> >>>> DAX flag.
> >>>
> >>> Thinking little bit more about this from requirement perspective. I think
> >>> we are trying to address two use cases here.
> >>>
> >>> A. Client does not know which files DAX should be used on. Only server
> >>>    knows it and server passes this information to client. I suspect
> >>>    that's your primary use case.
> >>
> >> Yes, this is the starting point of this patch set.
> >>
> >>>
> >>> B. Client is driving which files are supposed to be using DAX. This is
> >>>    exactly same as the model ext4/xfs are using by storing a persistent
> >>>    flag on inode. 
> >>>
> >>> Current patches seem to be a hybrid of both approach A and B. 
> >>>
> >>> If we were to implement B, then fuse client probably needs to have the
> >>> capability to query FS_XFLAG_DAX on inode and decide whether to
> >>> enable DAX or not. (Without extra round trip). Or know it indirectly
> >>> by extending GETATTR and requesting this explicitly.
> >>
> >> If guest queries if the file is DAX capable or not by an extra GETATTR,
> >> I'm afraid this will add extra round trip.
> >>
> >> Or if we add the DAX flag (ATTR_DAX) by extending LOOKUP, as done by
> >> this patch set, then the FUSE server needs to set ATTR_DAX according to
> >> the FS_XFLAG_DAX of the backend files, *to make the FS_XFLAG_DAX flag
> >> previously set by FUSE client work*. Then it becomes a *mandatory*
> >> requirement when implementing FUSE server. i.e., it becomes part of the
> >> FUSE protocol rather than implementation specific. FUSE server can still
> >> implement some other algorithm deciding whether to set ATTR_DAX or not,
> >> though it must set ATTR_DAX once the backend file is flagged with
> >> FS_XFLAG_DAX.
> >>
> >> Besides, as you said, FUSE server needs to add one extra
> >> GETFLAGS/FSGETXATTR ioctl per LOOKUP in this case. To eliminate this
> >> cost, we could negotiate the per-file DAX capability during FUSE_INIT.
> >> Only when the per-file DAX capability is negotiated, will the FUSE
> >> server do extra GETFLAGS/FSGETXATTR ioctl and set ATTR_DAX flag when
> >> doing LOOKUP.
> >>
> >>
> >> Personally, I prefer the second way, i.e., by extending LOOKUP (adding
> >> ATTR_DAX), to eliminate the extra round trip.
> > 
> > Negotiating a fuse feature say FUSE_FS_XFLAG_DAX makes sense. If
> > client is mounted with "-o dax=inode", then client will negotitate
> > this feature and if server does not support it, mount can fail.
> > 
> > But this probably will be binding on server that it needs to return
> > the state of FS_XFLAG_DAX attr on inode upon lookup/getattr. I don't
> > this will allow server to implement its own separate policy which
> > does not follow FS_XFLAG_DAX xattr. 
> 
> That means the backend fs must be ext4/xfs supporting per-file DAX feature.

Yes. This probably will be a requirement because we are dependent on
file attr FS_XFLAG_DAX to decide whether to enable DAX or not. And
if underlying filesystem does not support this attr, then it will
not work.

> 
> If given more right to construct its own policy, FUSE server could
> support per-file DAX upon other backend fs, though it will always fail
> when virtiofs wants to set FS_XFLAG_DAX inside guest.
> 
> > 
> > IOW, I don't think server can choose to implement its own policy
> > for enabling dax for "-o dax=inode".
> > 
> > If there is really a need to for something new where server needs
> > to dynamically decide which inodes should use dax (and not use
> > FS_XFLAG_FS), I guess that probably should be a separate mount
> > option say "-o dax=server" and it negotiates a separate feature
> > say FUSE_DAX_SERVER. Once that's negotiated, now both client
> > and server know that DAX will be used on files as determined
> > by server and not necessarily by using file attr FS_XFLAG_DAX.
> > 
> > So is "dax=inode" enough for your needs? What's your requirement,
> > can you give little bit of more details.
> 
> In our use case, the backend fs is something like SquashFS on host. The
> content of the file on host is downloaded *as needed*. When the file is
> not completely ready (completely downloaded), the guest will follow the
> normal IO routine, i.e., by FUSE_READ/FUSE_WRITE request. While the file
> is completely ready, per-file DAX is enabled for this file. IOW the FUSE
> server need to dynamically decide if per-file DAX shall be enabled,
> depending on if the file is completely downloaded.

So you don't want to enable DAX yet because guest might fault on
a section of file which has not been downloaded yet?

I am wondering if somehow user fault handling can help with this.
If we could handle faults for this file in user space, then you
should be able to download that particular page[s] and resolve
the fault?

Thanks
Vivek