[PATCH] netfs: Only call folio_start_fscache() one time for each folio

[Dave Wysochanski <dwysocha@xxxxxxxxxx>]

If a network filesystem using netfs implements a clamp_length()
function, it can set subrequest lengths smaller than a page size.
When we loop through the folios in netfs_rreq_unlock_folios() to
set any folios to be written back, we need to make sure we only
call folio_start_fscache() once for each folio.  Otherwise,
this simple testcase:
  mount -o fsc,rsize=1024,wsize=1024 127.0.0.1:/export /mnt/nfs
  dd if=/dev/zero of=/mnt/nfs/file.bin bs=4096 count=1
  1+0 records in
  1+0 records out
  4096 bytes (4.1 kB, 4.0 KiB) copied, 0.0126359 s, 324 kB/s
  cat /mnt/nfs/file.bin > /dev/null

will trigger an oops similar to the following:
...
 page dumped because: VM_BUG_ON_FOLIO(folio_test_private_2(folio))
 ------------[ cut here ]------------
 kernel BUG at include/linux/netfs.h:44!
...
 CPU: 5 PID: 134 Comm: kworker/u16:5 Kdump: loaded Not tainted 6.4.0-rc5
...
 RIP: 0010:netfs_rreq_unlock_folios+0x68e/0x730 [netfs]
...
 Call Trace:
  <TASK>
  netfs_rreq_assess+0x497/0x660 [netfs]
  netfs_subreq_terminated+0x32b/0x610 [netfs]
  nfs_netfs_read_completion+0x14e/0x1a0 [nfs]
  nfs_read_completion+0x2f9/0x330 [nfs]
  rpc_free_task+0x72/0xa0 [sunrpc]
  rpc_async_release+0x46/0x70 [sunrpc]
  process_one_work+0x3bd/0x710
  worker_thread+0x89/0x610
  kthread+0x181/0x1c0
  ret_from_fork+0x29/0x50

Signed-off-by: Dave Wysochanski <dwysocha@xxxxxxxxxx>
---
 fs/netfs/buffered_read.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/netfs/buffered_read.c b/fs/netfs/buffered_read.c
index 3404707ddbe7..0dafd970c1b6 100644
--- a/fs/netfs/buffered_read.c
+++ b/fs/netfs/buffered_read.c
@@ -21,6 +21,7 @@ void netfs_rreq_unlock_folios(struct netfs_io_request *rreq)
 	pgoff_t last_page = ((rreq->start + rreq->len) / PAGE_SIZE) - 1;
 	size_t account = 0;
 	bool subreq_failed = false;
+	bool folio_started;
 
 	XA_STATE(xas, &rreq->mapping->i_pages, start_page);
 
@@ -53,6 +54,7 @@ void netfs_rreq_unlock_folios(struct netfs_io_request *rreq)
 
 		pg_end = folio_pos(folio) + folio_size(folio) - 1;
 
+		folio_started = false;
 		for (;;) {
 			loff_t sreq_end;
 
@@ -60,8 +62,10 @@ void netfs_rreq_unlock_folios(struct netfs_io_request *rreq)
 				pg_failed = true;
 				break;
 			}
-			if (test_bit(NETFS_SREQ_COPY_TO_CACHE, &subreq->flags))
+			if (!folio_started && test_bit(NETFS_SREQ_COPY_TO_CACHE, &subreq->flags)) {
 				folio_start_fscache(folio);
+				folio_started = true;
+			}
 			pg_failed |= subreq_failed;
 			sreq_end = subreq->start + subreq->len - 1;
 			if (pg_end < sreq_end)
-- 
2.31.1