Re: [PATCH 2/5] mm/slub: use stackdepot to save stack trace in objects

[Vlastimil Babka]

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@xxxxxxxxx>
>> 
>> 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.

>> 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@xxxxxxx: 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.

>> 
>> Signed-off-by: Oliver Glitta <glittao@xxxxxxxxx>
>> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
>> Cc: David Rientjes <rientjes@xxxxxxxxxx>
>> Cc: Christoph Lameter <cl@xxxxxxxxx>
>> Cc: Pekka Enberg <penberg@xxxxxxxxxx>
>> Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
>