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Date:   Wed, 2 Mar 2022 17:22:48 +0000
From:   Hyeonggon Yoo <42.hyeyoo@gmail.com>
To:     Vlastimil Babka <vbabka@suse.cz>
Cc:     David Rientjes <rientjes@google.com>,
        Christoph Lameter <cl@linux.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Pekka Enberg <penberg@kernel.org>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        patches@lists.linux.dev, linux-kernel@vger.kernel.org,
        Oliver Glitta <glittao@gmail.com>,
        Faiyaz Mohammed <faiyazm@codeaurora.org>
Subject: Re: [PATCH 2/5] mm/slub: use stackdepot to save stack trace in
 objects
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On Wed, Mar 02, 2022 at 05:51:32PM +0100, Vlastimil Babka wrote:
> On 2/27/22 10:44, Hyeonggon Yoo wrote:
> > On Fri, Feb 25, 2022 at 07:03:15PM +0100, Vlastimil Babka wrote:
> >> From: Oliver Glitta <glittao@gmail.com>
> >> 
> >> Many stack traces are similar so there are many similar arrays.
> >> Stackdepot saves each unique stack only once.
> >>
> >> Replace field addrs in struct track with depot_stack_handle_t handle.  Use
> >> stackdepot to save stack trace.
> >>
> > 
> > I think it's not a replacement?
> 
> It is, for the array 'addrs':
> 
> -#ifdef CONFIG_STACKTRACE
> -	unsigned long addrs[TRACK_ADDRS_COUNT];	/* Called from address */
> +#ifdef CONFIG_STACKDEPOT
> +	depot_stack_handle_t handle;
> 
> Not confuse with 'addr' which is the immediate caller and indeed stays
> for redundancy/kernels without stack trace enabled.
>

Oh, my fault. Right. I was confused.
I should read it again.

> >> The benefits are smaller memory overhead and possibility to aggregate
> >> per-cache statistics in the following patch using the stackdepot handle
> >> instead of matching stacks manually.
> >> 
> >> [ vbabka@suse.cz: rebase to 5.17-rc1 and adjust accordingly ]
> >> 
> >> This was initially merged as commit 788691464c29 and reverted by commit
> >> ae14c63a9f20 due to several issues, that should now be fixed.
> >> The problem of unconditional memory overhead by stackdepot has been
> >> addressed by commit 2dba5eb1c73b ("lib/stackdepot: allow optional init
> >> and stack_table allocation by kvmalloc()"), so the dependency on
> >> stackdepot will result in extra memory usage only when a slab cache
> >> tracking is actually enabled, and not for all CONFIG_SLUB_DEBUG builds.
> >> The build failures on some architectures were also addressed, and the
> >> reported issue with xfs/433 test did not reproduce on 5.17-rc1 with this
> >> patch.
> > 
> > This is just an idea and beyond this patch.
> > 
> > After this patch, now we have external storage that records stack traces.
> 
> Well, we had it before this patch too.
>
> > It's possible that some rare stack traces are in stack depot, but
> > not reachable because track is overwritten.
> 
> Yes.
> 
> > I think it's worth implementing a way to iterate through stacks in stack depot?
> 
> The question is for what use case? We might even not know who stored
> them - could have been page_owner, or other stack depot users.

> But the point is usually not to learn about all existing traces, but to
> determine which ones cause an object lifetime bug, or memory leak.

Yeah, this is exactly what I misunderstood.
I thought purpose of free_traces is to show all existing traces.
But I realized today that free trace without alloc trace is not useful.

I'll review v2 with these in mind.
Thank you.

> >> 
> >> Signed-off-by: Oliver Glitta <glittao@gmail.com>
> >> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> >> Cc: David Rientjes <rientjes@google.com>
> >> Cc: Christoph Lameter <cl@linux.com>
> >> Cc: Pekka Enberg <penberg@kernel.org>
> >> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> > 
> 

-- 
Thank you, You are awesome!
Hyeonggon :-)