[Virtio-fs] Deleting files when using NFS as a shared folder

[Gal Hammer]

On Mon, 2 Aug 2021 at 13:49, Max Reitz <mreitz at redhat.com> wrote:

> On 02.08.21 12:44, Gal Hammer wrote:
> >
> >
> > On Mon, 2 Aug 2021 at 13:36, Dr. David Alan Gilbert
> > <dgilbert at redhat.com <mailto:dgilbert at redhat.com>> wrote:
> >
> >     * Gal Hammer (ghammer at redhat.com <mailto:ghammer at redhat.com>) wrote:
> >     > Hello,
> >     >
> >     > When using NFS as a shared folder (mount type nfs4) with a Linux
> >     guest I
> >     > have the following issue:
> >     >
> >     > Guest:
> >     > $ ls -la /mnt/shared
> >     > total 8
> >     > drwxr-xrwx.  2  135  135 4096 Aug  2 13:08 .
> >     > dr-xr-xr-x. 17 root   root    224 May 23 10:58 ..
> >     > -rw-r--rw-.  1  135  135   27 Aug  2 13:07 readme.txt
> >     >
> >     > Host:
> >     > $ rm readme.txt
> >     >
> >     > Guest:
> >     > $ ls -la /mnt/shared
> >     > total 8
> >     > drwxr-xrwx.  2  135  135 4096 Aug  2 13:10 .
> >     > dr-xr-xr-x. 17 root   root    224 May 23 10:58 ..
> >     > -rw-r--rw-.  1  135  135   27 Aug  2 13:07
> >     .nfs0000000001b600d000000005
> >     >
> >     > Guest:
> >     > $ cat /mnt/shared/readme.txt
> >     > This is a readme.txt file.
> >     >
> >     > So it seems that the virtiofsd has a reference to the file which
> >     the guest
> >     > is not aware of and is unable to send a FUSE_FORGET message.
> >     This results
> >     > in a file not actually deleted (renamed to .nfsXXX) and is still
> >     accessible
> >     > by the guest.
> >     >
> >     > I have a similar problem when deleting a file from a Windows
> >     guest side.
> >     > The FUSE_READDIR(PLUS) commands add a reference count to files
> >     which the OS
> >     > doesn't have a file context for. However I was able to solve it
> >     (for now?)
> >     > by keeping track of returned files' inodes.
> >     >
> >     > Is this behaviour current and by design?
> >
> >     Current problem, not really by design; the problem is the O_PATH
> files
> >     that we have open for the inodes.  I thought if the guest sent the
> >     forget for the file then it got closed.
> >
> >
> > So if I understand then sending forget message for each inode returned
> > by readdir won't solve the problem because you need the open files for
> > inodes?
>
> virtiofsd internally keeps an lo_inode object for every inode that has
> been looked up at some point, and every such lo_inode contains an O_PATH
> fd referencing that inode.  I don’t know by heart what the conditions
> for dropping those lo_inode objects are.
>

I think it depends on the guest's forget message.


> However, once it’s possible to use file handles to reference inodes
> instead of O_PATH fds (already in virtiofsd-rs, for virtiofsd there’s
> this series:
> https://listman.redhat.com/archives/virtio-fs/2021-July/msg00050.html),
> then giving the appropriate options (-o inode_file_handles -o
> modcaps=+dac_read_search) should result in no O_PATH fds being kept
> around anymore, so that deleting an inode on the host will result in the
> inode being truly deleted (unless the guest still has it open).
>

Will the guest will still need to send forget messages with this new
feature?


>
> But with O_PATH fds, it’s kind of by design, I would say.
>

Thanks for the clarification.

    Gal.


>
> Max
>
>
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