[PATCH] generic: add swapfile maxpages regression test

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+#! /bin/bash
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (c) 2021 Oracle.  All Rights Reserved.
+#
+# FS QA Test 727
+#
+# Regression test for "mm/swap: consider max pages in iomap_swapfile_add_extent"
+
+# Xu Yu found that the iomap swapfile activation code failed to constrain
+# itself to activating however many swap pages that the mm asked us for.  This
+# is an deviation in behavior from the classic swapfile code.  It also leads to
+# kernel memory corruption if the swapfile is cleverly constructed.
+#
+. ./common/preamble
+_begin_fstest auto swap
+
+# Override the default cleanup function.
+_cleanup()
+{
+	cd /
+	rm -f $tmp.*
+	test -n "$swapfile" && swapoff $swapfile &> /dev/null
+}
+
+# real QA test starts here
+_supported_fs generic
+_require_scratch_swapfile
+
+_scratch_mkfs >> $seqres.full
+_scratch_mount >> $seqres.full
+
+# Assuming we're not borrowing a FAT16 partition from Windows 3.1, we need an
+# unlikely enough name that we can grep /proc/swaps for this.
+swapfile=$SCRATCH_MNT/386spart.par
+_format_swapfile $swapfile 1m >> $seqres.full
+
+swapfile_pages() {
+	local swapfile="$1"
+
+	grep "$swapfile" /proc/swaps | awk '{print $3}'
+}
+
+_swapon_file $swapfile
+before_pages=$(swapfile_pages "$swapfile")
+swapoff $swapfile
+
+# Extend the length of the swapfile but do not rewrite the header.
+# The subsequent swapon should set up 1MB worth of pages, not 2MB.
+$XFS_IO_PROG -f -c 'pwrite 1m 1m' $swapfile >> $seqres.full
+
+_swapon_file $swapfile
+after_pages=$(swapfile_pages "$swapfile")
+swapoff $swapfile
+
+# Both swapon attempts should have found the same number of pages.
+test "$before_pages" -eq "$after_pages" || \
+	echo "swapon added $after_pages pages, expected $before_pages"
+
+# success, all done
+echo Silence is golden
+status=0
+exit

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+QA output created by 727
+Silence is golden