[PATCH v2 0/4] make '%pD' print the full path of file

Background
==========
Linus suggested printing the full path of file instead of printing
the components as '%pd'.

Typically, there is no need for printk specifiers to take any real locks
(ie mount_lock or rename_lock). So I introduce a new helper
d_path_unsafe() which is similar to d_path() except it doesn't take any
seqlock/spinlock.

This series is based on Al Viro's d_path() cleanup patches [1] which
lifted the inner lockless loop into a new helper. 

Test
====
The cases I tested:
1. print '%pD' with full path of ext4 file
2. mount a ext4 filesystem upon a ext4 filesystem, and print the file
   with '%pD'
3. all test_print selftests, including the new '%14pD' '%-14pD'
4. kasprintf

TODO
====
I plan to do the followup work after '%pD' behavior is changed.
- s390/hmcdrv: remove the redundant directory path in printing string.
- fs/iomap: simplify the iomap_swapfile_fail() with '%pD'.
- simplify the string printing when file_path() is invoked(in some
  cases, not all).
- change the previous '%pD[2,3,4]' to '%pD'
   
Changelog
=========
v2:
- refine the commit msg/comments (Andy)
- pass the validator check by "make C=1 W=1"
- add the R-b for patch 4/4 from Andy

v1: https://lkml.org/lkml/2021/6/22/680
- remove the RFC tag
- refine the commit msg/comments(by Petr, Andy)
- make using_scratch_space a new parameter of the test case 

RFCv4:
- don't support spec.precision anymore for '%pD'
- add Rasmus's patch into this series
 
RFCv3:
- implement new d_path_unsafe to use [buf, end] instead of stack space for
  filling bytes (by Matthew)
- add new test cases for '%pD'
- drop patch "hmcdrv: remove the redundant directory path" before removing rfc.

RFCv2: 
- implement new d_path_fast based on Al Viro's patches
- add check_pointer check (by Petr)
- change the max full path size to 256 in stack space

RFCv1: https://lkml.org/lkml/2021/5/8/122

Link: https://lkml.org/lkml/2021/5/18/1260 [1]

Jia He (3):
  fs: introduce helper d_path_unsafe()
  lib/vsprintf.c: make '%pD' print the full path of file
  lib/test_printf.c: add test cases for '%pD'

Rasmus Villemoes (1):
  lib/test_printf.c: split write-beyond-buffer check in two

 Documentation/core-api/printk-formats.rst |   5 +-
 fs/d_path.c                               | 122 ++++++++++++++++++++--
 include/linux/dcache.h                    |   1 +
 lib/test_printf.c                         |  54 ++++++++--
 lib/vsprintf.c                            |  40 ++++++-
 5 files changed, 199 insertions(+), 23 deletions(-)

-- 
2.17.1