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

[Luis Henriques]

On Thu, Apr 09 2026, Jingbo Xu wrote:

> Hi Luis,
>
> On 4/8/26 6:16 PM, Luis Henriques wrote:
>> 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
>> 
>
> Many thanks for your work on this.
>
> Just FYI We are also interested in the kernel-maintained filehandle,
> which can dramatically help reduce the memory footprint on the FUSE
> server side.

Awesome!

> It just seems that the overall design is still not clear right now.  I'd
> like to and would be happy if I could help any.

Thanks you, Jingbo.  I guess that for now we need to revalidate the usage
of compounds or going back to my previous (v2) approach.  If we end up
deciding to drop compound commands, I can try to get a new version of the
patchset before LSFMM so that we can eventually discuss more details
there.

Cheers,
-- 
Luís