Re: [patch 07/54] mm/slub: use stackdepot to save stack trace in objects

From: Linus Torvalds <torvalds@linux-foundation.org>
To: Christoph Hellwig <hch@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Christoph Lameter <cl@linux.com>,
	glittao@gmail.com, Joonsoo Kim <iamjoonsoo.kim@lge.com>,
	Linux-MM <linux-mm@kvack.org>,
	mm-commits@vger.kernel.org, Pekka Enberg <penberg@kernel.org>,
	Randy Dunlap <rdunlap@infradead.org>,
	David Rientjes <rientjes@google.com>,
	Vlastimil Babka <vbabka@suse.cz>,
	linux-xfs <linux-xfs@vger.kernel.org>
Subject: Re: [patch 07/54] mm/slub: use stackdepot to save stack trace in objects
Date: Fri, 16 Jul 2021 13:12:02 -0700	[thread overview]
Message-ID: <CAHk-=whnjz19Ln3=s4rDZn4+XER2pmA+pEVrjpwMYGba2rHAQA@mail.gmail.com> (raw)
In-Reply-To: <YPE3l82acwgI2OiV@infradead.org>

On Fri, Jul 16, 2021 at 12:39 AM Christoph Hellwig <hch@infradead.org> wrote:
>
> This somewhat unexpectedly causes a crash when running the xfs/433 test
> in xfstests for me.  Reverting the commit fixes the problem:

I don't see why that would be the case, but I'm inclined to revert
that commit for another reason: the code doesn't seem to match the
description of the commit.

It used to be that CONFIG_SLUB_DEBUG was a config option that was
harmless and that defaulted to 'y' because there was little downside.
In fact, it's not just "default y", it doesn't even *ask* the user
unless CONFIG_EXPERT is on. Because it was fairly harmless. And then
SLOB_DEBUG_ON was that "do you actually want this code _enabled_".

But now it basically force-enables that STACKDEPOT support too, and
then instead of having an _optional_ CONFIG_STACKTRACE, you basically
have that as being forced on you whether you want active debugging or
not.

Maybe that

        select STACKDEPOT if STACKTRACE_SUPPORT

should have been

        select STACKDEPOT if STACKTRACE

because i\t used to be that CONFIG_STACKTRACE was somewhat unusual,
and only enabled for special debug cases (admittedly "CONFIG_TRACING"
likely meant that it was fairly widely enabled).

In contrast, STACKTRACE_SUPPORT is basically "this architecture supports it".

So now it seems STACKDEPOT is enabled basically unconditionally.

So I really don't see why it would cause that xfs issue, but I think
there are multiple reasons to just go "Hmm" on that commit.

Comments?

                Linus