linux-ext4 - Re: [RFC PATCH] jbd2: avoid __GFP_ZERO with SLAB_TYPESAFE_BY

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Date:   Wed, 9 Feb 2022 19:10:10 +0100
From:   Jan Kara <jack@...e.cz>
To:     Qian Cai <quic_qiancai@...cinc.com>
Cc:     Theodore Ts'o <tytso@....edu>, Jan Kara <jack@...e.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Boqun Feng <boqun.feng@...il.com>, linux-ext4@...r.kernel.org,
        rcu@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] jbd2: avoid __GFP_ZERO with SLAB_TYPESAFE_BY_RCU

On Wed 09-02-22 11:57:42, Qian Cai wrote:
> Since the linux-next commit 120aa5e57479 (mm: Check for
> SLAB_TYPESAFE_BY_RCU and __GFP_ZERO slab allocation), we will get a
> boot warning. Avoid it by calling synchronize_rcu() before the zeroing.
> 
> Signed-off-by: Qian Cai <quic_qiancai@...cinc.com>

No, the performance impact of this would be just horrible. Can you
ellaborate a bit why SLAB_TYPESAFE_BY_RCU + __GFP_ZERO is a problem and why
synchronize_rcu() would be needed here before the memset() please? I mean
how is zeroing here any different from the memory just being used?

								Honza

> ---
>  fs/jbd2/journal.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
> index c2cf74b01ddb..323112de5921 100644
> --- a/fs/jbd2/journal.c
> +++ b/fs/jbd2/journal.c
> @@ -2861,15 +2861,18 @@ static struct journal_head *journal_alloc_journal_head(void)
>  #ifdef CONFIG_JBD2_DEBUG
>  	atomic_inc(&nr_journal_heads);
>  #endif
> -	ret = kmem_cache_zalloc(jbd2_journal_head_cache, GFP_NOFS);
> +	ret = kmem_cache_alloc(jbd2_journal_head_cache, GFP_NOFS);
>  	if (!ret) {
>  		jbd_debug(1, "out of memory for journal_head\n");
>  		pr_notice_ratelimited("ENOMEM in %s, retrying.\n", __func__);
> -		ret = kmem_cache_zalloc(jbd2_journal_head_cache,
> +		ret = kmem_cache_alloc(jbd2_journal_head_cache,
>  				GFP_NOFS | __GFP_NOFAIL);
>  	}
> -	if (ret)
> +	if (ret) {
> +		synchronize_rcu();
> +		memset(ret, 0, sizeof(*ret));
>  		spin_lock_init(&ret->b_state_lock);
> +	}
>  	return ret;
>  }
>  
> -- 
> 2.30.2
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR