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

[Luis Henriques]

On Tue, Apr 07 2026, Joanne Koong wrote:

> 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).

If we end up deciding to revert back to not using compound commands I'll
have to take a closer look into this, as it's not clear what the above
means.  For example, when you say:

  "... if the inode is not passed through"

does this "inode" refer to the backing_id?  I guess this is some index to
backing ids being used by FUSE, not exactly an inode, but I'm not very
familiar with the code.

Cheers,
-- 
Luís