linux-kernel - Re: [PATCH 2/2] mm/kmemleak: No need to check kmemleak_initialized in set_track

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Date:   Thu, 10 Aug 2023 12:03:58 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Xiaolei Wang <xiaolei.wang@...driver.com>, catalin.marinas@....com,
        akpm@...ux-foundation.org, glider@...gle.com, andreyknvl@...il.com,
        zhaoyang.huang@...soc.com
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] mm/kmemleak: No need to check kmemleak_initialized in
 set_track_prepare()

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?

> 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@...driver.com>
> ---
>  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);
>