fuse: allow larger read requests by setting bdi->io_pages

A FUSE server that advertises a large max_pages and max_write (e.g.
max_pages=256, max_write=1MB) cannot currently obtain matching
FUSE_READ request sizes from the kernel.  Buffered sequential writes
arrive at the server at the negotiated max_write size, but a large
buffered read() is split into several smaller FUSE_READ requests.

For a buffered read, filemap_get_pages() -> page_cache_sync_ra() sizes
the read against ractl_max_pages():

	max_pages = ractl->ra->ra_pages;
	if (req_size > max_pages && bdi->io_pages > max_pages)
		max_pages = min(req_size, bdi->io_pages);

fuse leaves bdi->io_pages at the default VM_READAHEAD_PAGES (128KB), so
a 1MB read() (req_size = 256 pages) is clamped to the readahead window
(128KB, or 256KB for POSIX_FADV_SEQUENTIAL), producing four 256KB
FUSE_READ round-trips instead of one.

Set bdi->io_pages to fc->max_pages after feature negotiation.  As the
code above shows, io_pages only raises the limit when the request size
already exceeds the readahead window, so it enlarges explicitly
requested reads without enlarging the speculative readahead window.
This avoids increasing speculative page-cache readahead on behalf of
an unprivileged server.  NFS does the same, setting io_pages from
rpages while leaving ra_pages at the default.

fc->max_pages is already bounded by fc->max_pages_limit (and, for
virtio-fs, by the virtqueue descriptor count), so io_pages inherits
the same bound.

Suggested-by: Joanne Koong <joannelkoong@gmail.com>
Signed-off-by: Jim Harris <jim.harris@nvidia.com>
Assisted-by: Cursor:claude-opus-4.8
Reviewed-by: Joanne Koong <joannelkoong@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
index d975073c6029..f7a0a0860a04 100644
--- a/fs/fuse/inode.c
+++ b/fs/fuse/inode.c
@@ -1416,6 +1416,7 @@ static void process_init_reply(struct fuse_args *args, int error)
 
 		fm->sb->s_bdi->ra_pages =
 				min(fm->sb->s_bdi->ra_pages, ra_pages);
+		fm->sb->s_bdi->io_pages = fc->max_pages;
 		fc->minor = arg->minor;
 		fc->max_write = arg->minor < 5 ? 4096 : arg->max_write;
 		fc->max_write = max_t(unsigned, 4096, fc->max_write);