linux-kernel - Re: [PATCH] debugobject: don't wake up kswapd from fill

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Date:   Thu, 11 May 2023 20:44:58 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:     syzbot <syzbot+fe0c72f0ccbb93786380@...kaller.appspotmail.com>,
        syzkaller-bugs@...glegroups.com, Ingo Molnar <mingo@...e.hu>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH] debugobject: don't wake up kswapd from fill_pool()

On Thu, 11 May 2023 22:47:32 +0900 Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp> wrote:

> syzbot is reporting lockdep warning in fill_pool(), for GFP_ATOMIC is
> (__GFP_HIGH | __GFP_KSWAPD_RECLAIM) which wakes up kswapd.
> Since fill_pool() might be called with arbitrary locks held,
> fill_pool() should not assume that holding pgdat->kswapd_wait is safe.

hm.  But many GFP_ATOMIC allocation attempts are made with locks held. 
Why aren't all such callers buggy, by trying to wake kswapd with locks
held?  What's special about this one?

> Also, __GFP_NORETRY is pointless for !__GFP_DIRECT_RECLAIM allocation
> 
> Reported-by: syzbot <syzbot+fe0c72f0ccbb93786380@...kaller.appspotmail.com>
> Closes: https://syzkaller.appspot.com/bug?extid=fe0c72f0ccbb93786380
> Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
> Fixes: 3ac7fe5a4aab ("infrastructure to debug (dynamic) objects")
> ---
>  lib/debugobjects.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/lib/debugobjects.c b/lib/debugobjects.c
> index 003edc5ebd67..986adca357b4 100644
> --- a/lib/debugobjects.c
> +++ b/lib/debugobjects.c
> @@ -126,7 +126,7 @@ static const char *obj_states[ODEBUG_STATE_MAX] = {
>  
>  static void fill_pool(void)
>  {
> -	gfp_t gfp = GFP_ATOMIC | __GFP_NORETRY | __GFP_NOWARN;
> +	gfp_t gfp = __GFP_HIGH | __GFP_NOWARN;

Does this weaken fill_pool()'s allocation attempt more than necessary? 
We can still pass __GFP_HIGH?