Re: [PATCH v4 18/25] fuse: support buffer copying for kernel addresses

[Bernd Schubert <bschubert@ddn.com>]

On 1/17/26 00:30, Joanne Koong wrote:
> This is a preparatory patch needed to support kernel-managed ring
> buffers in fuse-over-io-uring. For kernel-managed ring buffers, we get
> the vmapped address of the buffer which we can directly use.
> 
> Currently, buffer copying in fuse only supports extracting underlying
> pages from an iov iter and kmapping them. This commit allows buffer
> copying to work directly on a kaddr.
> 
> Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
> ---
>  fs/fuse/dev.c        | 23 +++++++++++++++++------
>  fs/fuse/fuse_dev_i.h |  7 ++++++-
>  2 files changed, 23 insertions(+), 7 deletions(-)
> 
> diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
> index 6d59cbc877c6..ceb5d6a553c0 100644
> --- a/fs/fuse/dev.c
> +++ b/fs/fuse/dev.c
> @@ -848,6 +848,9 @@ void fuse_copy_init(struct fuse_copy_state *cs, bool write,
>  /* Unmap and put previous page of userspace buffer */
>  void fuse_copy_finish(struct fuse_copy_state *cs)
>  {
> +	if (cs->is_kaddr)
> +		return;
> +
>  	if (cs->currbuf) {
>  		struct pipe_buffer *buf = cs->currbuf;
>  
> @@ -873,6 +876,9 @@ static int fuse_copy_fill(struct fuse_copy_state *cs)
>  	struct page *page;
>  	int err;
>  
> +	if (cs->is_kaddr)
> +		return 0;
> +
>  	err = unlock_request(cs->req);
>  	if (err)
>  		return err;
> @@ -931,15 +937,20 @@ static int fuse_copy_do(struct fuse_copy_state *cs, void **val, unsigned *size)
>  {
>  	unsigned ncpy = min(*size, cs->len);
>  	if (val) {
> -		void *pgaddr = kmap_local_page(cs->pg);
> -		void *buf = pgaddr + cs->offset;
> +		void *pgaddr, *buf;
> +		if (!cs->is_kaddr) {
> +			pgaddr = kmap_local_page(cs->pg);
> +			buf = pgaddr + cs->offset;
> +		} else {
> +			buf = cs->kaddr + cs->offset;
> +		}
>  
>  		if (cs->write)
>  			memcpy(buf, *val, ncpy);
>  		else
>  			memcpy(*val, buf, ncpy);
> -
> -		kunmap_local(pgaddr);
> +		if (!cs->is_kaddr)
> +			kunmap_local(pgaddr);
>  		*val += ncpy;
>  	}
>  	*size -= ncpy;
> @@ -1127,7 +1138,7 @@ static int fuse_copy_folio(struct fuse_copy_state *cs, struct folio **foliop,
>  	}
>  
>  	while (count) {
> -		if (cs->write && cs->pipebufs && folio) {
> +		if (cs->write && cs->pipebufs && folio && !cs->is_kaddr) {
>  			/*
>  			 * Can't control lifetime of pipe buffers, so always
>  			 * copy user pages.
> @@ -1139,7 +1150,7 @@ static int fuse_copy_folio(struct fuse_copy_state *cs, struct folio **foliop,
>  			} else {
>  				return fuse_ref_folio(cs, folio, offset, count);
>  			}
> -		} else if (!cs->len) {
> +		} else if (!cs->len && !cs->is_kaddr) {
>  			if (cs->move_folios && folio &&
>  			    offset == 0 && count == size) {
>  				err = fuse_try_move_folio(cs, foliop);
> diff --git a/fs/fuse/fuse_dev_i.h b/fs/fuse/fuse_dev_i.h
> index 134bf44aff0d..aa1d25421054 100644
> --- a/fs/fuse/fuse_dev_i.h
> +++ b/fs/fuse/fuse_dev_i.h
> @@ -28,12 +28,17 @@ struct fuse_copy_state {
>  	struct pipe_buffer *currbuf;
>  	struct pipe_inode_info *pipe;
>  	unsigned long nr_segs;
> -	struct page *pg;
> +	union {
> +		struct page *pg;
> +		void *kaddr;
> +	};
>  	unsigned int len;
>  	unsigned int offset;
>  	bool write:1;
>  	bool move_folios:1;
>  	bool is_uring:1;
> +	/* if set, use kaddr; otherwise use pg */
> +	bool is_kaddr:1;
>  	struct {
>  		unsigned int copied_sz; /* copied size into the user buffer */
>  	} ring;


I'm confused here, how cs->len will get initialized. So far that was
done from fuse_copy_fill?


Thanks,
Bernd