[RFC PATCH 6/7] fuse: convert direct IO paths to use FOLL_PIN

[John Hubbard <jhubbard@nvidia.com>]

Convert the fuse filesystem to support the new iov_iter_get_pages()
behavior. That routine now invokes pin_user_pages_fast(), which means that
such pages must be released via unpin_user_page(), rather than via
put_page().

This commit also removes any possibility of kernel pages being handled, in
the fuse_get_user_pages() call. Although this may seem like a steep price
to pay, Christoph Hellwig actually recommended it a few years ago for
nearly the same situation [1].

[1] https://lore.kernel.org/kvm/20190724061750.GA19397@infradead.org/

Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 fs/fuse/dev.c  |  5 ++++-
 fs/fuse/file.c | 23 ++++++++---------------
 2 files changed, 12 insertions(+), 16 deletions(-)

diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
index e1b4a846c90d..a93037c96b89 100644
--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -675,7 +675,10 @@ static void fuse_copy_finish(struct fuse_copy_state *cs)
 			flush_dcache_page(cs->pg);
 			set_page_dirty_lock(cs->pg);
 		}
-		put_page(cs->pg);
+		if (cs->pipebufs)
+			put_page(cs->pg);
+		else
+			unpin_user_page(cs->pg);
 	}
 	cs->pg = NULL;
 }
diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index 94747bac3489..395c2fb613fb 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -619,7 +619,7 @@ static void fuse_release_user_pages(struct fuse_args_pages *ap,
 	for (i = 0; i < ap->num_pages; i++) {
 		if (should_dirty)
 			set_page_dirty_lock(ap->pages[i]);
-		put_page(ap->pages[i]);
+		unpin_user_page(ap->pages[i]);
 	}
 }
 
@@ -1382,20 +1382,13 @@ static int fuse_get_user_pages(struct fuse_args_pages *ap, struct iov_iter *ii,
 	size_t nbytes = 0;  /* # bytes already packed in req */
 	ssize_t ret = 0;
 
-	/* Special case for kernel I/O: can copy directly into the buffer */
-	if (iov_iter_is_kvec(ii)) {
-		unsigned long user_addr = fuse_get_user_addr(ii);
-		size_t frag_size = fuse_get_frag_size(ii, *nbytesp);
-
-		if (write)
-			ap->args.in_args[1].value = (void *) user_addr;
-		else
-			ap->args.out_args[0].value = (void *) user_addr;
-
-		iov_iter_advance(ii, frag_size);
-		*nbytesp = frag_size;
-		return 0;
-	}
+	/*
+	 * TODO: this warning can eventually be removed. It is just to help
+	 * developers, during a transitional period, while direct IO code is
+	 * moving over to FOLL_PIN pages.
+	 */
+	if (WARN_ON_ONCE(!iter_is_iovec(ii)))
+		return -EOPNOTSUPP;
 
 	while (nbytes < *nbytesp && ap->num_pages < max_pages) {
 		unsigned npages;
-- 
2.35.1