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Date:   Thu, 9 Nov 2017 11:05:31 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Joe Perches <joe@...ches.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm/page_alloc: Avoid KERN_CONT uses in warn_alloc

On Tue 07-11-17 08:03:27, Joe Perches wrote:
> On Tue, 2017-11-07 at 16:43 +0100, Michal Hocko wrote:
> > On Tue 07-11-17 07:34:25, Joe Perches wrote:
> []
> > > I believe, but have not tested, that using a specific width
> > > as an argument to %*pb[l] will constrain the number of
> > > spaces before the '(null)' output in any NULL pointer use.
> > > 
> > > So how about a #define like
> > > 
> > > /*
> > >  * nodemask_pr_args is only used with a "%*pb[l]" format for a nodemask.
> > >  * A NULL nodemask uses 6 to emit "(null)" without leading spaces.
> > >  */
> > > #define nodemask_pr_args(maskp)			\
> > > 	(maskp) ? MAX_NUMNODES : 6,		\
> > > 	(maskp) ? (maskp)->bits : NULL
> > 
> > Why not -1 then?
> 
> I believe it's the field width and not the precision that
> needs to be set.

But the first of the two arguments is the field with specifier, not the
precision. /me confused...

Anyway, the following works as expected when printing the OOM report:
[   47.005321] mem_eater invoked oom-killer: gfp_mask=0x14280ca(GFP_HIGHUSER_MOVABLE|__GFP_ZERO), nodemask=(null), order=0, oom_score_adj=0
[   47.007183] mem_eater cpuset=/ mems_allowed=0-1
[   47.007829] CPU: 3 PID: 3223 Comm: mem_eater Tainted: G        W       4.13.0-pr1-dirty #11

I hope I haven't overlooked anything
---
>From 35aa7742d35d29af88c66dd5e6f8f5d62215f5fd Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko@...e.com>
Date: Thu, 9 Nov 2017 10:58:41 +0100
Subject: [PATCH] mm: simplify nodemask printing

alloc_warn and dump_header have to explicitly handle NULL nodemask which
forces both paths to use pr_cont. We can do better. printk already
handles NULL pointers properly so all what we need is to teach
nodemask_pr_args to handle NULL nodemask carefully. This allows
simplification of both alloc_warn and dump_header and get rid of pr_cont
altogether.

This patch has been motivated by patch from Joe Perches
http://lkml.kernel.org/r/b31236dfe3fc924054fd7842bde678e71d193638.1509991345.git.joe@perches.com

Signed-off-by: Michal Hocko <mhocko@...e.com>
---
 include/linux/nodemask.h |  2 +-
 mm/oom_kill.c            | 12 ++++--------
 mm/page_alloc.c          | 12 +++---------
 3 files changed, 8 insertions(+), 18 deletions(-)

diff --git a/include/linux/nodemask.h b/include/linux/nodemask.h
index cf0b91c3ec12..5d3cc67207ed 100644
--- a/include/linux/nodemask.h
+++ b/include/linux/nodemask.h
@@ -103,7 +103,7 @@ extern nodemask_t _unused_nodemask_arg_;
  *
  * Can be used to provide arguments for '%*pb[l]' when printing a nodemask.
  */
-#define nodemask_pr_args(maskp)		MAX_NUMNODES, (maskp)->bits
+#define nodemask_pr_args(maskp)	(maskp) ? MAX_NUMNODES : 0, (maskp) ? (maskp)->bits : NULL
 
 /*
  * The inline keyword gives the compiler room to decide to inline, or
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 8dd0e088189b..606213a81ceb 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -426,14 +426,10 @@ static void dump_tasks(struct mem_cgroup *memcg, const nodemask_t *nodemask)
 
 static void dump_header(struct oom_control *oc, struct task_struct *p)
 {
-	pr_warn("%s invoked oom-killer: gfp_mask=%#x(%pGg), nodemask=",
-		current->comm, oc->gfp_mask, &oc->gfp_mask);
-	if (oc->nodemask)
-		pr_cont("%*pbl", nodemask_pr_args(oc->nodemask));
-	else
-		pr_cont("(null)");
-	pr_cont(",  order=%d, oom_score_adj=%hd\n",
-		oc->order, current->signal->oom_score_adj);
+	pr_warn("%s invoked oom-killer: gfp_mask=%#x(%pGg), nodemask=%*pbl, order=%d, oom_score_adj=%hd\n",
+		current->comm, oc->gfp_mask, &oc->gfp_mask,
+		nodemask_pr_args(oc->nodemask), oc->order,
+			current->signal->oom_score_adj);
 	if (!IS_ENABLED(CONFIG_COMPACTION) && oc->order)
 		pr_warn("COMPACTION is disabled!!!\n");
 
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index d755434aee94..457e43ed4c10 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3281,20 +3281,14 @@ void warn_alloc(gfp_t gfp_mask, nodemask_t *nodemask, const char *fmt, ...)
 	if ((gfp_mask & __GFP_NOWARN) || !__ratelimit(&nopage_rs))
 		return;
 
-	pr_warn("%s: ", current->comm);
-
 	va_start(args, fmt);
 	vaf.fmt = fmt;
 	vaf.va = &args;
-	pr_cont("%pV", &vaf);
+	pr_warn("%s: %pV, mode:%#x(%pGg), nodemask=%*pbl\n",
+			current->comm, &vaf, gfp_mask, &gfp_mask,
+			nodemask_pr_args(nodemask));
 	va_end(args);
 
-	pr_cont(", mode:%#x(%pGg), nodemask=", gfp_mask, &gfp_mask);
-	if (nodemask)
-		pr_cont("%*pbl\n", nodemask_pr_args(nodemask));
-	else
-		pr_cont("(null)\n");
-
 	cpuset_print_current_mems_allowed();
 
 	dump_stack();
-- 
2.14.2

-- 
Michal Hocko
SUSE Labs

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