Re: [PATCH 2/2] mm/kmemleak: No need to check kmemleak_initialized in set_track_prepare()

[Vlastimil Babka]

On 8/10/23 12:03, Vlastimil Babka wrote:
> On 8/10/23 09:47, Xiaolei Wang wrote:
>> The kmemleak_late_init() is defined as a late_initcall. The current
>> implementation of set_track_prepare() depends on the kmemleak init.
>> That also means there is no call trace for the memory leak which object
>> is created before the kmemleak_late_init().
> 
> So if I understand correctly, we have the following sequence of events durin
> boot
> 
> ...
> A: stack_depot is initialized
> ...
> B: kmemleak is initialized
> ...
> 
> before this patchset, we can miss allocations before B, aftewards only
> before A (which can't be helped), so we now have between A and B.
> 
> That's nice, but it's weird that can record kmemleak when
> !kmemleak_initialized. Why can't it be initialized sooner in that case?

Looking closer, I think what you want could be achieved by kmemleak_init()
setting a variable that is checked in kmemleak_initialized() instead of the
kmemleak_initialized that's set too late.

I think this should work because:
- I assume kmemleak can't record anything before kmemleak_init()
- stack depot early init is requested one way or the other
- mm_core_init() calls stack_depot_early_init() before kmemleak_init()

But I also wonder how kmemleak can even reach set_track_prepare() before
kmemleak_init(), maybe that's the issue?

>> In a previous patch, we have fixed a bug in stack_depot_save() so that
>> it can be invoked even before stack depot is initialized. So there is
>> no reason to check the kmemleak_initialized in set_track_prepare().
>> So delete the kmemleak_initialized judgment in set_track_prepare()
>> 
>> unreferenced object 0xc674ca80 (size 64):
>>   comm "swapper/0", pid 1, jiffies 4294938337 (age 204.880s)
>>   hex dump (first 32 bytes):
>>     80 55 75 c6 80 54 75 c6 00 55 75 c6 80 52 75 c6 .Uu..Tu..Uu..Ru.
>>     00 53 75 c6 00 00 00 00 00 00 00 00 00 00 00 00 .Su..........
>> 
>> Fixes: 56a61617dd22 ("mm: use stack_depot for recording kmemleak's backtrace")
>> Signed-off-by: Xiaolei Wang <xiaolei.wang@xxxxxxxxxxxxx>
>> ---
>>  mm/kmemleak.c | 2 --
>>  1 file changed, 2 deletions(-)
>> 
>> diff --git a/mm/kmemleak.c b/mm/kmemleak.c
>> index a2d34226e3c8..c9f2f816db19 100644
>> --- a/mm/kmemleak.c
>> +++ b/mm/kmemleak.c
>> @@ -610,8 +610,6 @@ static noinline depot_stack_handle_t set_track_prepare(void)
>>  	unsigned long entries[MAX_TRACE];
>>  	unsigned int nr_entries;
>>  
>> -	if (!kmemleak_initialized)
>> -		return 0;
>>  	nr_entries = stack_trace_save(entries, ARRAY_SIZE(entries), 3);
>>  	trace_handle = stack_depot_save(entries, nr_entries, GFP_NOWAIT);
>>  
>