Re: [RFC PATCH v3 6/8] fuse: implementation of lookup_handle+statx compound operation

[Joanne Koong]

On Tue, Apr 7, 2026 at 4:06 PM Joanne Koong <joannelkoong@xxxxxxxxx> wrote:
>
> On Tue, Apr 7, 2026 at 2:20 PM Luis Henriques <luis@xxxxxxxxxx> wrote:
> >
> > Hi Joanne,
> >
> > On Tue, Apr 07 2026, Joanne Koong wrote:
> >
> > > On Wed, Feb 25, 2026 at 3:25 AM Luis Henriques <luis@xxxxxxxxxx> wrote:
> > >>
> > >> The implementation of lookup_handle+statx compound operation extends the
> > >> lookup operation so that a file handle is be passed into the kernel.  It
> > >> also needs to include an extra inarg, so that the parent directory file
> > >> handle can be sent to user-space.  This extra inarg is added as an extension
> > >> header to the request.
> > >>
> > >> By having a separate statx including in a compound operation allows the
> > >> attr to be dropped from the lookup_handle request, simplifying the
> > >> traditional FUSE lookup operation.
> > >>
> > >> Signed-off-by: Luis Henriques <luis@xxxxxxxxxx>
> > >> ---
> > >>  fs/fuse/dir.c             | 294 +++++++++++++++++++++++++++++++++++---
> > >>  fs/fuse/fuse_i.h          |  23 ++-
> > >>  fs/fuse/inode.c           |  48 +++++--
> > >>  fs/fuse/readdir.c         |   2 +-
> > >>  include/uapi/linux/fuse.h |  23 ++-
> > >>  5 files changed, 355 insertions(+), 35 deletions(-)
> > >>
> > >> diff --git a/include/uapi/linux/fuse.h b/include/uapi/linux/fuse.h
> > >> index 113583c4efb6..89e6176abe25 100644
> > >> --- a/include/uapi/linux/fuse.h
> > >> +++ b/include/uapi/linux/fuse.h
> > >>
> > >>  enum fuse_opcode {
> > >> @@ -671,6 +676,8 @@ enum fuse_opcode {
> > >>          */
> > >>         FUSE_COMPOUND           = 54,
> > >>
> > >> +       FUSE_LOOKUP_HANDLE      = 55,
> > >> +
> > >>         /* CUSE specific operations */
> > >>         CUSE_INIT               = 4096,
> > >>
> > >> @@ -707,6 +714,20 @@ struct fuse_entry_out {
> > >>         struct fuse_attr attr;
> > >>  };
> > >>
> > >> +struct fuse_entry2_out {
> > >> +       uint64_t        nodeid;
> > >> +       uint64_t        generation;
> > >> +       uint64_t        entry_valid;
> > >> +       uint32_t        entry_valid_nsec;
> > >> +       uint32_t        flags;
> > >> +       uint64_t        spare;
> > >> +};
> > >
> > > Hi Luis,
> > >
> > > Could you explain why we need a new struct fuse_entry2_out instead of
> > > reusing struct fuse_entry_out? From what i see, the only differences
> > > between them are that fuse_entry2_out drops attr_valid,
> > > attr_valid_nsec, and struct fuse_attr. Is this done so that it saves
> > > the ~100 bytes per lookup? Would it be cleaner from an abi perspective
> > > to just reuse fuse_entry_out and ignore the attr fields if they're not
> > > necessary? The reason I'm asking is because I'm looking at how you're
> > > doing the lookup request reply to see if the fuse passthrough stuff
> > > for metadata/directory operations can be combined with it. But I'm not
> > > fully understanding why fuse_entry2_out is needed here.
> > >
> > > I'm also a bit confused by why the compound with statx is needed here,
> > > could you explain this part? I see the call to fuse_statx_to_attr()
> > > after do_lookup_handle_statx(),  but fuse_statx_to_attr() converts the
> > > statx reply right back to a struct fuse_attr for inode setup, so if
> > > FUSE_LOOKUP_HANDLE returned struct fuse_entry_out instead of struct
> > > fuse_entry2_out, doesn't this solve the problem of needing to compound
> > > it with a statx or getattr request? I also noticed that the statx part
> > > uses FUSE_ROOT_ID as a workaround for the node id because the actual
> > > nodeid isn't known yet, this seems like another sign that the
> > > attributes stuff should just be part of the lookup response itself
> > > rather than a separate operation?
> >
> > First of all, thanks a lot for looking into this patchset.  Much
> > appreciated!
> >
> > The main reason for swapping the usage of attr by statx is that statx
> > includes some attributes that attr does not (e.g. btime).  And since I was
> > adding a new FUSE operation, it would be a good time for using statx
> > instead.  (Moreover, as new attributes may be added to statx in the
> > future, the benefits of using statx could eventually be even greater.)
> >
> > This was suggested by Miklos here[0], before converting the whole thing to
> > use compound commands.  So, I was going to use fuse_statx in the _out args
> > for lookup_handle.  However, because the interface was getting a bit
> > complex with extra args (and ext headers!), Miklos ended up suggesting[1]
> > to remove attr completely from the lookup_handle operation, and use
> > compounds instead to have the full functionality.
>
> Thank you for the context and links, Luis!
>
> Using fuse_statx over fuse_getattr makes sense to me for the new
> FUSE_LOOKUP_HANDLE op but the part I am confused about is why it needs
> to be compounded. If we are adding a new struct (struct
> fuse_entry2_out) to the abi, why is it not simpler to just make the
> new struct something like:
>
>   struct fuse_entry_handle_out {

Thinking more about how passthrough will use this... I'm thinking
something like this would be ideal (though please correct me if I'm
completely off track here, Amir):

  struct fuse_entry_handle_out {
      uint64_t    nodeid;
      uint64_t    generation;
      uint64_t    entry_valid;
      uint64_t    attr_valid;
      uint32_t    entry_valid_nsec;
      uint32_t    attr_valid_nsec;
      int32_t     backing_id;
      uint32_t    flags;
      struct fuse_statx statx;
  };

where if the inode is not passed through, the kernel uses the stax
field for the attributes but if the inode is passed through and the
server sets a bitmask specifying that attributes should be derived
from the backing file, then the statx field will be ignored by the
server and kernel).

Thanks,
Joanne

>       uint64_t    nodeid;
>       uint64_t    generation;
>       uint64_t    entry_valid;
>       uint64_t    attr_valid;
>       uint32_t    entry_valid_nsec;
>       uint32_t    attr_valid_nsec;
>       struct fuse_statx stat;
>   };
>
> and avoid the compound stuff altogether? Is it because that would make
> the struct fuse_entry_handle_out too big to be stack-allocated and
> would thus have to be heap allocated? But if I'm recalling correctly,
> the compound requests path requires heap allocations as well. Although
> at that point if we're gonna have to do the heap allocation, then we
> might as well just also embed the struct fuse_file_handle inside
> struct fuse_entry_handle_out?
>
> Thanks,
> Joanne
>
> >
> > Obviously, I may have misunderstood (or mis-implemented) the suggestions
> > that were done.  And hopefully the provided links to the discussion that
> > originated this approach will help.
> >
> > Regarding the usage of FUSE_ROOT_ID as a workaround for the node id, I
> > believe this is a more generic problem which will occur in other compound
> > commands as well.  If we want to create a new file system object and
> > perform some operation with it within the same compound, a similar
> > workaround will be required (or some sort of flag in the compound command
> > to signal this dependency).
> >
> > I hope this helped to clarify a bit your questions.
> >
> > [0] https://lore.kernel.org/all/CAJfpegsoeUH42ZSg_MSEYukbgXOM_83YT8z_sksMj84xPPCMGQ@xxxxxxxxxxxxxx
> > [1] https://lore.kernel.org/all/CAJfpegst6oha7-M+8v9cYpk7MR-9k_PZofJ3uzG39DnVoVXMkA@xxxxxxxxxxxxxx
> >
> > Cheers,
> > --
> > Luís