mm, printk: introduce new format string for flags

In mm we use several kinds of flags bitfields that are sometimes printed
for debugging purposes, or exported to userspace via sysfs.  To make
them easier to interpret independently on kernel version and config, we
want to dump also the symbolic flag names.  So far this has been done
with repeated calls to pr_cont(), which is unreliable on SMP, and not
usable for e.g.  sysfs export.

To get a more reliable and universal solution, this patch extends
printk() format string for pointers to handle the page flags (%pGp),
gfp_flags (%pGg) and vma flags (%pGv).  Existing users of
dump_flag_names() are converted and simplified.

It would be possible to pass flags by value instead of pointer, but the
%p format string for pointers already has extensions for various kernel
structures, so it's a good fit, and the extra indirection in a
non-critical path is negligible.

[linux@rasmusvillemoes.dk: lots of good implementation suggestions]
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Sasha Levin <sasha.levin@oracle.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Mel Gorman <mgorman@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 18 insertions, 0 deletions

diff --git a/Documentation/printk-formats.txt b/Documentation/printk-formats.txt
index 5d1128bf0282..5962949944fd 100644
--- a/Documentation/printk-formats.txt
+++ b/Documentation/printk-formats.txt
@@ -298,6 +298,24 @@ bitmap and its derivatives such as cpumask and nodemask:
 
 	Passed by reference.
 
+Flags bitfields such as page flags, gfp_flags:
+
+	%pGp	referenced|uptodate|lru|active|private
+	%pGg	GFP_USER|GFP_DMA32|GFP_NOWARN
+	%pGv	read|exec|mayread|maywrite|mayexec|denywrite
+
+	For printing flags bitfields as a collection of symbolic constants that
+	would construct the value. The type of flags is given by the third
+	character. Currently supported are [p]age flags, [v]ma_flags (both
+	expect unsigned long *) and [g]fp_flags (expects gfp_t *). The flag
+	names and print order depends on the particular	type.
+
+	Note that this format should not be used directly in TP_printk() part
+	of a tracepoint. Instead, use the show_*_flags() functions from
+	<trace/events/mmflags.h>.
+
+	Passed by reference.
+
 Network device features:
 
 	%pNF	0x000000000000c000