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

In the Linux kernel, the following vulnerability has been resolved:

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

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

echo 3 > /proc/sys/vm/drop_caches

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:

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