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;
-- 
2.47.3