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 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 {
>       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?

I'm open to drop the usage of compounds for this, of course.  In fact,
during the v2 discussions I suggested here[0] the usage of a similar
struct fuse_entry_handle_out.  Using compound commands was a way to try to
simplify the interface.  But maybe at that point I was too quick at
jumping into the compound commands suggestion.  It may make sense to
reevaluate this decision if you think it simplifies things, specially for
passthrough.

[0] https://lore.kernel.org/all/87zf6nov6c.fsf@wotan.olymp

And I agree that the stack allocation is unlikely to be a good argument as
I can see that using compound commands can make these stack allocations
even worse, as we need to allocate more structs for more operations.

Miklos, do you have a opinion on this?

Cheers,
-- 
Luís

> 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