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

[Joanne Koong <joannelkoong@gmail.com>]

On Tue, Jan 27, 2026 at 3:40 PM Bernd Schubert <bschubert@ddn.com> wrote:
>
> 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?

With kaddrs, cs->len is initialized when the copy state is set up (in
setup_fuse_copy_state()) before we do any copying to/from the ring.
The changes for that are in the later patch that adds the ringbuffer
logic ("fuse: add io-uring kernel-managed buffer ring"). The kaddr and
len correspond to the address and length of the buffer that was
selected from the ring buffer (in fuse_uring_select_buffer()).

Thanks,
Joanne
>
>
> Thanks,
> Bernd
>
>