[PATCH] generic: add a test to ensure that page is properly filled before write

[Jeff Layton <jlayton@kernel.org>]

We had a broken optimization in cephfs and netfs lib that could cause
part of a page to be improperly zeroed-out when writing to an offset
that was beyond the EOF but in an existing page.

Add a simple test that would have caught this.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 tests/generic/XXX     | 53 +++++++++++++++++++++++++++++++++++++++++++
 tests/generic/XXX.out |  5 ++++
 tests/generic/group   |  1 +
 3 files changed, 59 insertions(+)
 create mode 100755 tests/generic/XXX
 create mode 100644 tests/generic/XXX.out

diff --git a/tests/generic/XXX b/tests/generic/XXX
new file mode 100755
index 000000000000..0ddaaa544609
--- /dev/null
+++ b/tests/generic/XXX
@@ -0,0 +1,53 @@
+#! /bin/bash
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (c) 2021, Jeff Layton <jlayton@redhat.com>
+#
+# FS QA Test No. XXX
+#
+# Open a file and write a little data to it. Unmount (to clean out the cache)
+# and then mount again. Then write some data to it beyond the EOF and ensure
+# the result is correct. Prompted by a bug in ceph_write_begin. See:
+#
+# https://lore.kernel.org/ceph-devel/97002.1623448034@warthog.procyon.org.uk/T/#m9f34b328623b493c529505e12f64f4dfdbddfb27
+#
+seq=`basename $0`
+seqres=$RESULT_DIR/$seq
+echo "QA output created by $seq"
+
+here=`pwd`
+status=1    # failure is the default!
+trap "_cleanup; exit \$status" 0 1 2 3 15
+
+
+_cleanup()
+{
+	cd /
+	rm -f $testfile
+}
+
+# get standard environment, filters and checks
+. ./common/rc
+. ./common/filter
+
+_supported_fs generic
+_require_test
+
+rm -f $seqres.full
+
+testfile="$TEST_DIR/test_write_begin.$$"
+
+# write some data to file and fsync it out
+$XFS_IO_PROG -f -c "pwrite -q 0 32" $testfile
+
+# cycle the mount to clean out the pagecache
+_test_cycle_mount
+
+# now, write to the file (near the end)
+$XFS_IO_PROG -c "pwrite -q 32 32" $testfile
+
+# dump what we think is in there
+echo "The result should be 64 bytes filled with 0xcd:"
+hexdump -C $testfile
+
+status=0
+exit
diff --git a/tests/generic/XXX.out b/tests/generic/XXX.out
new file mode 100644
index 000000000000..4e7653858e51
--- /dev/null
+++ b/tests/generic/XXX.out
@@ -0,0 +1,5 @@
+QA output created by XXX
+The result should be 64 bytes filled with 0xcd:
+00000000  cd cd cd cd cd cd cd cd  cd cd cd cd cd cd cd cd  |................|
+*
+00000040
diff --git a/tests/generic/group b/tests/generic/group
index 9a636b23f243..204a9b548f78 100644
--- a/tests/generic/group
+++ b/tests/generic/group
@@ -641,3 +641,4 @@
 636 auto quick swap
 637 auto quick dir
 638 auto quick rw
+XXX auto quick rw
-- 
2.31.1