netfs: zero the tail of a short DIO/unbuffered readgrafted

The buffered read collector zero-fills the tail of a short read that
stops below the inode's i_size (netfs_clear_unread()), so a read that
races an extending write still returns the expected number of bytes.
The non-buffered collector path does no such thing: it just records
how much was transferred.

Add the same zero-fill for the non-buffered case, gated on
NETFS_SREQ_CLEAR_TAIL: a subreq's source sets that flag when a short
result from it is known to be safe to treat as a hole, as opposed to
NETFS_SREQ_HIT_EOF, which means the read genuinely ran off the end of
the file and should be reported short as-is. Only CLEAR_TAIL should
zero-fill here; a real EOF must stay a real short read.

No source currently sets CLEAR_TAIL on an unbuffered/DIO subrequest,
so this is inert on its own -- a following change teaches cifs to set
it in the one case that needs it.

Fixes: e2d46f2ec332 ("netfs: Change the read result collector to only use one work item")
Reviewed-by: David Howells <dhowells@redhat.com>
Reviewed-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Paulo Alcantara <pc@manguebit.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Ronnie Sahlberg <ronniesahlberg@gmail.com>
Cc: Shyam Prasad N <sprasad@microsoft.com>
Cc: Tom Talpey <tom@talpey.com>
Cc: Bharath SM <bharathsm@microsoft.com>
Cc: stable@vger.kernel.org

1 files changed, 285 insertions, 0 deletions

diff --git a/Documentation/dev-tools/gcov.rst b/Documentation/dev-tools/gcov.rst
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index 000000000..075df6a45
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+Using gcov with the Linux kernel
+================================
+
+gcov profiling kernel support enables the use of GCC's coverage testing
+tool gcov_ with the Linux kernel. Coverage data of a running kernel
+is exported in gcov-compatible format via the "gcov" debugfs directory.
+To get coverage data for a specific file, change to the kernel build
+directory and use gcov with the ``-o`` option as follows (requires root)::
+
+    # cd /tmp/linux-out
+    # gcov -o /sys/kernel/debug/gcov/tmp/linux-out/kernel spinlock.c
+
+This will create source code files annotated with execution counts
+in the current directory. In addition, graphical gcov front-ends such
+as lcov_ can be used to automate the process of collecting data
+for the entire kernel and provide coverage overviews in HTML format.
+
+Possible uses:
+
+* debugging (has this line been reached at all?)
+* test improvement (how do I change my test to cover these lines?)
+* minimizing kernel configurations (do I need this option if the
+  associated code is never run?)
+
+.. _gcov: https://gcc.gnu.org/onlinedocs/gcc/Gcov.html
+.. _lcov: https://github.com/linux-test-project/lcov
+
+
+Preparation
+-----------
+
+Configure the kernel with::
+
+        CONFIG_DEBUG_FS=y
+        CONFIG_GCOV_KERNEL=y
+
+and to get coverage data for the entire kernel::
+
+        CONFIG_GCOV_PROFILE_ALL=y
+
+Note that kernels compiled with profiling flags will be significantly
+larger and run slower. Also CONFIG_GCOV_PROFILE_ALL may not be supported
+on all architectures.
+
+Profiling data will only become accessible once debugfs has been
+mounted::
+
+        mount -t debugfs none /sys/kernel/debug
+
+
+Customization
+-------------
+
+To enable profiling for specific files or directories, add a line
+similar to the following to the respective kernel Makefile:
+
+- For a single file (e.g. main.o)::
+
+	GCOV_PROFILE_main.o := y
+
+- For all files in one directory::
+
+	GCOV_PROFILE := y
+
+To exclude files from being profiled even when CONFIG_GCOV_PROFILE_ALL
+is specified, use::
+
+	GCOV_PROFILE_main.o := n
+
+and::
+
+	GCOV_PROFILE := n
+
+Only files which are linked to the main kernel image or are compiled as
+kernel modules are supported by this mechanism.
+
+
+Module specific configs
+-----------------------
+
+Gcov kernel configs for specific modules are described below:
+
+CONFIG_GCOV_PROFILE_RDS:
+        Enables GCOV profiling on RDS for checking which functions or
+        lines are executed. This config is used by the rds selftest to
+        generate coverage reports. If left unset the report is omitted.
+
+
+Files
+-----
+
+The gcov kernel support creates the following files in debugfs:
+
+``/sys/kernel/debug/gcov``
+	Parent directory for all gcov-related files.
+
+``/sys/kernel/debug/gcov/reset``
+	Global reset file: resets all coverage data to zero when
+        written to.
+
+``/sys/kernel/debug/gcov/path/to/compile/dir/file.gcda``
+	The actual gcov data file as understood by the gcov
+        tool. Resets file coverage data to zero when written to.
+
+``/sys/kernel/debug/gcov/path/to/compile/dir/file.gcno``
+	Symbolic link to a static data file required by the gcov
+        tool. This file is generated by gcc when compiling with
+        option ``-ftest-coverage``.
+
+
+Modules
+-------
+
+Kernel modules may contain cleanup code which is only run during
+module unload time. The gcov mechanism provides a means to collect
+coverage data for such code by keeping a copy of the data associated
+with the unloaded module. This data remains available through debugfs.
+Once the module is loaded again, the associated coverage counters are
+initialized with the data from its previous instantiation.
+
+This behavior can be deactivated by specifying the gcov_persist kernel
+parameter::
+
+        gcov_persist=0
+
+At run-time, a user can also choose to discard data for an unloaded
+module by writing to its data file or the global reset file.
+
+
+Separated build and test machines
+---------------------------------
+
+The gcov kernel profiling infrastructure is designed to work out-of-the
+box for setups where kernels are built and run on the same machine. In
+cases where the kernel runs on a separate machine, special preparations
+must be made, depending on where the gcov tool is used:
+
+.. _gcov-test:
+
+a) gcov is run on the TEST machine
+
+    The gcov tool version on the test machine must be compatible with the
+    gcc version used for kernel build. Also the following files need to be
+    copied from build to test machine:
+
+    from the source tree:
+      - all C source files + headers
+
+    from the build tree:
+      - all C source files + headers
+      - all .gcda and .gcno files
+      - all links to directories
+
+    It is important to note that these files need to be placed into the
+    exact same file system location on the test machine as on the build
+    machine. If any of the path components is symbolic link, the actual
+    directory needs to be used instead (due to make's CURDIR handling).
+
+.. _gcov-build:
+
+b) gcov is run on the BUILD machine
+
+    The following files need to be copied after each test case from test
+    to build machine:
+
+    from the gcov directory in sysfs:
+      - all .gcda files
+      - all links to .gcno files
+
+    These files can be copied to any location on the build machine. gcov
+    must then be called with the -o option pointing to that directory.
+
+    Example directory setup on the build machine::
+
+      /tmp/linux:    kernel source tree
+      /tmp/out:      kernel build directory as specified by make O=
+      /tmp/coverage: location of the files copied from the test machine
+
+      [user@build] cd /tmp/out
+      [user@build] gcov -o /tmp/coverage/tmp/out/init main.c
+
+
+Note on compilers
+-----------------
+
+GCC and LLVM gcov tools are not necessarily compatible. Use gcov_ to work with
+GCC-generated .gcno and .gcda files, and use llvm-cov_ for Clang.
+
+.. _gcov: https://gcc.gnu.org/onlinedocs/gcc/Gcov.html
+.. _llvm-cov: https://llvm.org/docs/CommandGuide/llvm-cov.html
+
+Build differences between GCC and Clang gcov are handled by Kconfig. It
+automatically selects the appropriate gcov format depending on the detected
+toolchain.
+
+
+Troubleshooting
+---------------
+
+Problem
+    Compilation aborts during linker step.
+
+Cause
+    Profiling flags are specified for source files which are not
+    linked to the main kernel or which are linked by a custom
+    linker procedure.
+
+Solution
+    Exclude affected source files from profiling by specifying
+    ``GCOV_PROFILE := n`` or ``GCOV_PROFILE_basename.o := n`` in the
+    corresponding Makefile.
+
+Problem
+    Files copied from sysfs appear empty or incomplete.
+
+Cause
+    Due to the way seq_file works, some tools such as cp or tar
+    may not correctly copy files from sysfs.
+
+Solution
+    Use ``cat`` to read ``.gcda`` files and ``cp -d`` to copy links.
+    Alternatively use the mechanism shown in Appendix B.
+
+
+Appendix A: gather_on_build.sh
+------------------------------
+
+Sample script to gather coverage meta files on the build machine
+(see :ref:`Separated build and test machines a. <gcov-test>`):
+
+.. code-block:: sh
+
+    #!/bin/bash
+
+    KSRC=$1
+    KOBJ=$2
+    DEST=$3
+
+    if [ -z "$KSRC" ] || [ -z "$KOBJ" ] || [ -z "$DEST" ]; then
+      echo "Usage: $0 <ksrc directory> <kobj directory> <output.tar.gz>" >&2
+      exit 1
+    fi
+
+    KSRC=$(cd $KSRC; printf "all:\n\t@echo \${CURDIR}\n" | make -f -)
+    KOBJ=$(cd $KOBJ; printf "all:\n\t@echo \${CURDIR}\n" | make -f -)
+
+    find $KSRC $KOBJ \( -name '*.gcno' -o -name '*.[ch]' -o -type l \) -a \
+                     -perm /u+r,g+r | tar cfz $DEST -P -T -
+
+    if [ $? -eq 0 ] ; then
+      echo "$DEST successfully created, copy to test system and unpack with:"
+      echo "  tar xfz $DEST -P"
+    else
+      echo "Could not create file $DEST"
+    fi
+
+
+Appendix B: gather_on_test.sh
+-----------------------------
+
+Sample script to gather coverage data files on the test machine
+(see :ref:`Separated build and test machines b. <gcov-build>`):
+
+.. code-block:: sh
+
+    #!/bin/bash -e
+
+    DEST=$1
+    GCDA=/sys/kernel/debug/gcov
+
+    if [ -z "$DEST" ] ; then
+      echo "Usage: $0 <output.tar.gz>" >&2
+      exit 1
+    fi
+
+    TEMPDIR=$(mktemp -d)
+    echo Collecting data..
+    find $GCDA -type d -exec mkdir -p $TEMPDIR/\{\} \;
+    find $GCDA -name '*.gcda' -exec sh -c 'cat < $0 > '$TEMPDIR'/$0' {} \;
+    find $GCDA -name '*.gcno' -exec sh -c 'cp -d $0 '$TEMPDIR'/$0' {} \;
+    tar czf $DEST -C $TEMPDIR sys
+    rm -rf $TEMPDIR
+
+    echo "$DEST successfully created, copy to build system and unpack with:"
+    echo "  tar xfz $DEST"