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

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Message-ID: <87f48f32-ddc4-4c57-98c1-75bc5e684390@ddn.com>
Date: Mon, 15 Dec 2025 13:36:06 +0000
From: Bernd Schubert <bschubert@....com>
To: Luis Henriques <luis@...lia.com>, Miklos Szeredi <miklos@...redi.hu>
CC: Amir Goldstein <amir73il@...il.com>, "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

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;
	}
}


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