linux-kernel - Re: [LKP] [ocfs2] e2ac55b6a8e: ocfs2_init:1612 ERROR: Unable to create ocfs2 debugfs root

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Date:	Mon, 20 Apr 2015 14:10:24 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Chengyu Song <csong84@...ech.edu>,
	Huang Ying <ying.huang@...el.com>,
	LKML <linux-kernel@...r.kernel.org>, LKP ML <lkp@...org>,
	Mark Fasheh <mfasheh@...e.com>,
	Joel Becker <jlbec@...lplan.org>
Subject: Re: [LKP] [ocfs2] e2ac55b6a8e: ocfs2_init:1612 ERROR: Unable to
 create ocfs2 debugfs root

On Mon, 20 Apr 2015 13:50:38 -0700 Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Sun, Apr 19, 2015 at 8:45 PM, Chengyu Song <csong84@...ech.edu> wrote:
> >
> > As suggested in the patch,  -19 (-ENODEV) happens when debugfs is not configured (see include/linux/debugfs.h). So if debugfs is necessary for the functionality, in Kconfig, we should either declare it as a dependency, or auto select it.
> 
> That makes no sense.
> 
> If it used to work before that patch, then this is a regression and
> the patch needs to be reverted.
> 
> Yes, the old code apparently used to set "o2hb_debug_dir" to an error
> pointer when debugfs was compiled out, but since debugfs was compiled
> out, that error pointer was probably never actually *used*. So things
> presumably worked.
> 
> Now, it hangs, according to Huang Ying. If so, that's clearly a
> regression. That means that commit e2ac55b6a8e3 ("ocfs2: incorrect
> check for debugfs returns") needs to be reverted or fixed.
> 
> Andrew?
> 

Yes, that one snuck through.  I think a revert would be best at this
stage, please.



The debugfs interfaces are exceptional, and not very nice.  My
understanding of the general idea is:

- debugfs is just for debug and subsystems shouldn't care whether
  debugfs is present or not.

- if a debugfs call fails, the subsystem shouldn't care - don't log
  it, just ignore it.


The return semantics from things like debugfs_create_dir() are:

NULL: debugfs is available, but something went wrong.  We don't
      tell you what it was.

-ENODEV: debugfs isn't available

-Exxx:  I don't think other errnos are supposed to happen.


So the ofs2 code shouldn't log unless the debugfs calls return NULL. 
And really, they shouldn't log at all, due to the general debugfs
philosophy of "errors should be silently ignored".



A problem with functions like o2hb_debug_init() is that when
CONFIG_DEBUG_FS=n they will still generate significant amounts of code.
 That's fixable with something like

--- a/fs/ocfs2/cluster/heartbeat.c~a
+++ a/fs/ocfs2/cluster/heartbeat.c
@@ -1312,12 +1312,8 @@ static int o2hb_debug_init(void)
 	int ret = -ENOMEM;
 
 	o2hb_debug_dir = debugfs_create_dir(O2HB_DEBUG_DIR, NULL);
-	if (IS_ERR_OR_NULL(o2hb_debug_dir)) {
-		ret = o2hb_debug_dir ?
-			PTR_ERR(o2hb_debug_dir) : -ENOMEM;
-		mlog_errno(ret);
-		goto bail;
-	}
+	if (IS_ERR(o2hb_debug_dir))
+		return 0;
 
 	o2hb_debug_livenodes = o2hb_debug_create(O2HB_DEBUG_LIVENODES,
 						 o2hb_debug_dir,

Here, the compiler shold see that the `return 0' is always taken (due
to the -ENODEV) and the rest of the function will be eliminated.

Or we wrap large pieces of code inside `#ifdef CONFIG_DEBUG_FS'.

Either way, a cleanup here needs some thought and study.  And better
testing, plesae.

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