linux-kernel - Re: [RFC PATCH v2 3/6] fuse: initial infrastructure for FUSE_LOOKUP

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Message-ID: <CAOQ4uxj_-_zbuCLdWuHQj4fx2sBOn04+-6F2WiC9SRdmcacsDA@mail.gmail.com>
Date: Mon, 15 Dec 2025 18:06:24 +0100
From: Amir Goldstein <amir73il@...il.com>
To: Bernd Schubert <bschubert@....com>
Cc: Luis Henriques <luis@...lia.com>, Miklos Szeredi <miklos@...redi.hu>, 
	"Darrick J. Wong" <djwong@...nel.org>, Kevin Chen <kchen@....com>, 
	Horst Birthelmer <hbirthelmer@....com>, 
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Matt Harvey <mharvey@...ptrading.com>, 
	"kernel-dev@...lia.com" <kernel-dev@...lia.com>
Subject: Re: [RFC PATCH v2 3/6] fuse: initial infrastructure for
 FUSE_LOOKUP_HANDLE support

On Mon, Dec 15, 2025 at 2:36 PM Bernd Schubert <bschubert@....com> wrote:
>
> Hi Luis,
>
> I'm really sorry for late review.
>
> On 12/12/25 19:12, Luis Henriques wrote:
> > This patch adds the initial infrastructure to implement the LOOKUP_HANDLE
> > operation.  It simply defines the new operation and the extra fuse_init_out
> > field to set the maximum handle size.
> >
> > Signed-off-by: Luis Henriques <luis@...lia.com>
> > ---
> >   fs/fuse/fuse_i.h          | 4 ++++
> >   fs/fuse/inode.c           | 9 ++++++++-
> >   include/uapi/linux/fuse.h | 8 +++++++-
> >   3 files changed, 19 insertions(+), 2 deletions(-)
> >
> > diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
> > index 1792ee6f5da6..fad05fae7e54 100644
> > --- a/fs/fuse/fuse_i.h
> > +++ b/fs/fuse/fuse_i.h
> > @@ -909,6 +909,10 @@ struct fuse_conn {
> >       /* Is synchronous FUSE_INIT allowed? */
> >       unsigned int sync_init:1;
> >
> > +     /** Is LOOKUP_HANDLE implemented by fs? */
> > +     unsigned int lookup_handle:1;
> > +     unsigned int max_handle_sz;
> > +
> >       /* Use io_uring for communication */
> >       unsigned int io_uring;
> >
> > diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
> > index ef63300c634f..bc84e7ed1e3d 100644
> > --- a/fs/fuse/inode.c
> > +++ b/fs/fuse/inode.c
> > @@ -1465,6 +1465,13 @@ static void process_init_reply(struct fuse_mount *fm, struct fuse_args *args,
> >
> >                       if (flags & FUSE_REQUEST_TIMEOUT)
> >                               timeout = arg->request_timeout;
> > +
> > +                     if ((flags & FUSE_HAS_LOOKUP_HANDLE) &&
> > +                         (arg->max_handle_sz > 0) &&
> > +                         (arg->max_handle_sz <= FUSE_MAX_HANDLE_SZ)) {
> > +                             fc->lookup_handle = 1;
> > +                             fc->max_handle_sz = arg->max_handle_sz;
>
> I don't have a strong opinion on it, maybe
>
> if (flags & FUSE_HAS_LOOKUP_HANDLE) {
>         if (!arg->max_handle_sz || arg->max_handle_sz > FUSE_MAX_HANDLE_SZ) {
>                 pr_info_ratelimited("Invalid fuse handle size %d\n, arg->max_handle_sz)
>         } else {
>                 fc->lookup_handle = 1;
>                 fc->max_handle_sz = arg->max_handle_sz;

Why do we need both?
This seems redundant.
fc->max_handle_sz != 0 is equivalent to fc->lookup_handle
isnt it?

Thanks,
Amir.

>         }
> }
>
>
> I.e. give developers a warning what is wrong?
>
>
> > +                     }
> >               } else {
> >                       ra_pages = fc->max_read / PAGE_SIZE;
> >                       fc->no_lock = 1;
> > @@ -1515,7 +1522,7 @@ static struct fuse_init_args *fuse_new_init(struct fuse_mount *fm)
> >               FUSE_SECURITY_CTX | FUSE_CREATE_SUPP_GROUP |
> >               FUSE_HAS_EXPIRE_ONLY | FUSE_DIRECT_IO_ALLOW_MMAP |
> >               FUSE_NO_EXPORT_SUPPORT | FUSE_HAS_RESEND | FUSE_ALLOW_IDMAP |
> > -             FUSE_REQUEST_TIMEOUT;
> > +             FUSE_REQUEST_TIMEOUT | FUSE_LOOKUP_HANDLE;
> >   #ifdef CONFIG_FUSE_DAX
> >       if (fm->fc->dax)
> >               flags |= FUSE_MAP_ALIGNMENT;
> > diff --git a/include/uapi/linux/fuse.h b/include/uapi/linux/fuse.h
> > index c13e1f9a2f12..4acf71b407c9 100644
> > --- a/include/uapi/linux/fuse.h
> > +++ b/include/uapi/linux/fuse.h
>
> I forget to do that all the time myself, I think it should also increase the
> minor version here and add add a comment for it.
>
> > @@ -495,6 +495,7 @@ struct fuse_file_lock {
> >   #define FUSE_ALLOW_IDMAP    (1ULL << 40)
> >   #define FUSE_OVER_IO_URING  (1ULL << 41)
> >   #define FUSE_REQUEST_TIMEOUT        (1ULL << 42)
> > +#define FUSE_HAS_LOOKUP_HANDLE       (1ULL << 43)
> >
> >   /**
> >    * CUSE INIT request/reply flags
> > @@ -663,6 +664,7 @@ enum fuse_opcode {
> >       FUSE_TMPFILE            = 51,
> >       FUSE_STATX              = 52,
> >       FUSE_COPY_FILE_RANGE_64 = 53,
> > +     FUSE_LOOKUP_HANDLE      = 54,
> >
> >       /* CUSE specific operations */
> >       CUSE_INIT               = 4096,
> > @@ -908,6 +910,9 @@ struct fuse_init_in {
> >       uint32_t        unused[11];
> >   };
> >
> > +/* Same value as MAX_HANDLE_SZ */
> > +#define FUSE_MAX_HANDLE_SZ 128
> > +
> >   #define FUSE_COMPAT_INIT_OUT_SIZE 8
> >   #define FUSE_COMPAT_22_INIT_OUT_SIZE 24
> >
> > @@ -925,7 +930,8 @@ struct fuse_init_out {
> >       uint32_t        flags2;
> >       uint32_t        max_stack_depth;
> >       uint16_t        request_timeout;
> > -     uint16_t        unused[11];
> > +     uint16_t        max_handle_sz;
> > +     uint16_t        unused[10];
> >   };
>
> No strong opinion either and just given we are slowly running out of
> available space. If we never expect to need more than 256 bytes,
> maybe uint8_t?
>
>
>
> Thanks,
> Bernd
>