fusefs: fix a recurse-on-non-recursive lockmgr panic

fuse_vnop_bmap needs to know the file's size in order to calculate the
optimum amount of readahead.  If the file's size is unknown, it must ask
the FUSE server.  But if the file's data was previously cached and the
server reports that its size has shrunk, fusefs must invalidate the
cached data.  That's not possible during VOP_BMAP because the buffer
object is already locked.

Fix the panic by not querying the FUSE server for the file's size during
VOP_BMAP if we don't need it.  That's also a a slight performance
optimization.

PR:		256937
Reported by:	Agata <chogata@moosefs.pro>
Tested by:	Agata <chogata@moosefs.pro>
MFC after:	2 weeks

1 files changed, 86 insertions, 0 deletions

diff --git a/tests/sys/fs/fusefs/bmap.cc b/tests/sys/fs/fusefs/bmap.cc
index 3f392a4447f2..c635f4d7e46f 100644
--- a/tests/sys/fs/fusefs/bmap.cc
+++ b/tests/sys/fs/fusefs/bmap.cc
@@ -75,6 +75,8 @@ void expect_lookup(const char *relpath, uint64_t ino, off_t size)
 }
 };
 
+class BmapEof: public Bmap, public WithParamInterface<int> {};
+
 /*
  * Test FUSE_BMAP
  * XXX The FUSE protocol does not include the runp and runb variables, so those
@@ -165,3 +167,87 @@ TEST_F(Bmap, default_)
 
 	leak(fd);
 }
+
+/*
+ * VOP_BMAP should not query the server for the file's size, even if its cached
+ * attributes have expired.
+ * Regression test for https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=256937
+ */
+TEST_P(BmapEof, eof)
+{
+	/*
+	 * Outline:
+	 * 1) lookup the file, setting attr_valid=0
+	 * 2) Read more than one block, causing the kernel to issue VOP_BMAP to
+	 *    plan readahead.
+	 * 3) Nothing should panic
+	 * 4) Repeat the tests, truncating the file after different numbers of
+	 *    GETATTR operations.
+	 */
+	Sequence seq;
+	const off_t filesize = 2 * m_maxbcachebuf;
+	const ino_t ino = 42;
+	mode_t mode = S_IFREG | 0644;
+	void *contents, *buf;
+	int fd;
+	int ngetattrs;
+
+	ngetattrs = GetParam();
+	contents = calloc(1, filesize);
+	FuseTest::expect_lookup(RELPATH, ino, mode, filesize, 1, 0);
+	expect_open(ino, 0, 1);
+	// Depending on ngetattrs, FUSE_READ could be called with either
+	// filesize or filesize / 2 .
+	EXPECT_CALL(*m_mock, process(
+	ResultOf([=](auto in) {
+		return (in.header.opcode == FUSE_READ &&
+			in.header.nodeid == ino &&
+			in.body.read.offset == 0 &&
+			( in.body.read.size == filesize ||
+			  in.body.read.size == filesize / 2));
+		}, Eq(true)),
+		_)
+	).WillOnce(Invoke(ReturnImmediate([=](auto in, auto& out) {
+		size_t osize = in.body.read.size;
+		out.header.len = sizeof(struct fuse_out_header) + osize;
+		bzero(out.body.bytes, osize);
+	})));
+	EXPECT_CALL(*m_mock, process(
+		ResultOf([](auto in) {
+			return (in.header.opcode == FUSE_GETATTR &&
+				in.header.nodeid == ino);
+		}, Eq(true)),
+		_)
+	).Times(Between(ngetattrs - 1, ngetattrs))
+	.InSequence(seq)
+	.WillRepeatedly(Invoke(ReturnImmediate([=](auto i __unused, auto& out) {
+		SET_OUT_HEADER_LEN(out, attr);
+		out.body.attr.attr_valid = 0;
+		out.body.attr.attr.ino = ino;
+		out.body.attr.attr.mode = S_IFREG | 0644;
+		out.body.attr.attr.size = filesize;
+	})));
+	EXPECT_CALL(*m_mock, process(
+		ResultOf([](auto in) {
+			return (in.header.opcode == FUSE_GETATTR &&
+				in.header.nodeid == ino);
+		}, Eq(true)),
+		_)
+	).InSequence(seq)
+	.WillRepeatedly(Invoke(ReturnImmediate([=](auto i __unused, auto& out) {
+		SET_OUT_HEADER_LEN(out, attr);
+		out.body.attr.attr_valid = 0;
+		out.body.attr.attr.ino = ino;
+		out.body.attr.attr.mode = S_IFREG | 0644;
+		out.body.attr.attr.size = filesize / 2;
+	})));
+
+	buf = calloc(1, filesize);
+	fd = open(FULLPATH, O_RDWR);
+	ASSERT_LE(0, fd) << strerror(errno);
+	read(fd, buf, filesize);
+}
+
+INSTANTIATE_TEST_CASE_P(BE, BmapEof,
+	Values(1, 2, 3)
+);