Date:   Fri, 17 Mar 2023 17:31:04 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     "Uladzislau Rezki (Sony)" <urezki@gmail.com>
Cc:     Theodore Ts'o <tytso@mit.edu>,
        Joel Fernandes <joel@joelfernandes.org>,
        RCU <rcu@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>,
        Oleksiy Avramchenko <oleksiy.avramchenko@sony.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Frederic Weisbecker <frederic@kernel.org>,
        linux-ext4@vger.kernel.org, Lukas Czerner <lczerner@redhat.com>,
        Andreas Dilger <adilger.kernel@dilger.ca>
Subject: Re: [PATCH 1/1] ext4: Replace ext4_kvfree_array_rcu() by
 kvfree_rcu_mightsleep()
Message-ID: <28e1585c-7062-49c2-b08e-a5d33ca06577@paulmck-laptop>
Reply-To: paulmck@kernel.org
References: <20230317071558.335645-1-urezki@gmail.com>
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On Fri, Mar 17, 2023 at 08:15:58AM +0100, Uladzislau Rezki (Sony) wrote:
> The ext4_kvfree_array_rcu() function was introduced in order to
> release some memory after a grace period during resizing of a
> partition. An object that is freed does not contain any rcu_head
> filed.
> 
> To do so, it requires to allocate some extra memory for a special
> structure that has an rcu_head filed and pointer one where a freed
> memory is attached. Finally call_rcu() API is invoked.
> 
> Since we have a single argument of kvfree_rcu() API, we can easily
> replace all that tricky code by one single call that does the same
> but in more efficient way.
> 
> Cc: linux-ext4@vger.kernel.org
> Cc: Lukas Czerner <lczerner@redhat.com>
> Cc: Andreas Dilger <adilger.kernel@dilger.ca>
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>

From an RCU perspective:

Reviewed-by: Paul E. McKenney <paulmck@kernel.org>

> ---
>  fs/ext4/ext4.h    |  1 -
>  fs/ext4/mballoc.c |  2 +-
>  fs/ext4/resize.c  | 31 ++-----------------------------
>  fs/ext4/super.c   |  2 +-
>  4 files changed, 4 insertions(+), 32 deletions(-)
> 
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index 140e1eb300d1..e43d1a2d175e 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -3057,7 +3057,6 @@ extern int ext4_generic_delete_entry(struct inode *dir,
>  extern bool ext4_empty_dir(struct inode *inode);
>  
>  /* resize.c */
> -extern void ext4_kvfree_array_rcu(void *to_free);
>  extern int ext4_group_add(struct super_block *sb,
>  				struct ext4_new_group_data *input);
>  extern int ext4_group_extend(struct super_block *sb,
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index 5b2ae37a8b80..96b4e3317640 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -3055,7 +3055,7 @@ int ext4_mb_alloc_groupinfo(struct super_block *sb, ext4_group_t ngroups)
>  	rcu_assign_pointer(sbi->s_group_info, new_groupinfo);
>  	sbi->s_group_info_size = size / sizeof(*sbi->s_group_info);
>  	if (old_groupinfo)
> -		ext4_kvfree_array_rcu(old_groupinfo);
> +		kvfree_rcu_mightsleep(old_groupinfo);
>  	ext4_debug("allocated s_groupinfo array for %d meta_bg's\n",
>  		   sbi->s_group_info_size);
>  	return 0;
> diff --git a/fs/ext4/resize.c b/fs/ext4/resize.c
> index 6b91443d6bf3..330b81f36e10 100644
> --- a/fs/ext4/resize.c
> +++ b/fs/ext4/resize.c
> @@ -18,33 +18,6 @@
>  
>  #include "ext4_jbd2.h"
>  
> -struct ext4_rcu_ptr {
> -	struct rcu_head rcu;
> -	void *ptr;
> -};
> -
> -static void ext4_rcu_ptr_callback(struct rcu_head *head)
> -{
> -	struct ext4_rcu_ptr *ptr;
> -
> -	ptr = container_of(head, struct ext4_rcu_ptr, rcu);
> -	kvfree(ptr->ptr);
> -	kfree(ptr);
> -}
> -
> -void ext4_kvfree_array_rcu(void *to_free)
> -{
> -	struct ext4_rcu_ptr *ptr = kzalloc(sizeof(*ptr), GFP_KERNEL);
> -
> -	if (ptr) {
> -		ptr->ptr = to_free;
> -		call_rcu(&ptr->rcu, ext4_rcu_ptr_callback);
> -		return;
> -	}
> -	synchronize_rcu();
> -	kvfree(to_free);
> -}
> -
>  int ext4_resize_begin(struct super_block *sb)
>  {
>  	struct ext4_sb_info *sbi = EXT4_SB(sb);
> @@ -931,7 +904,7 @@ static int add_new_gdb(handle_t *handle, struct inode *inode,
>  	n_group_desc[gdb_num] = gdb_bh;
>  	rcu_assign_pointer(EXT4_SB(sb)->s_group_desc, n_group_desc);
>  	EXT4_SB(sb)->s_gdb_count++;
> -	ext4_kvfree_array_rcu(o_group_desc);
> +	kvfree_rcu_mightsleep(o_group_desc);
>  
>  	lock_buffer(EXT4_SB(sb)->s_sbh);
>  	le16_add_cpu(&es->s_reserved_gdt_blocks, -1);
> @@ -994,7 +967,7 @@ static int add_new_gdb_meta_bg(struct super_block *sb,
>  
>  	rcu_assign_pointer(EXT4_SB(sb)->s_group_desc, n_group_desc);
>  	EXT4_SB(sb)->s_gdb_count++;
> -	ext4_kvfree_array_rcu(o_group_desc);
> +	kvfree_rcu_mightsleep(o_group_desc);
>  	return err;
>  }
>  
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index 87aa23d047c9..34d802848d6d 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -3108,7 +3108,7 @@ int ext4_alloc_flex_bg_array(struct super_block *sb, ext4_group_t ngroup)
>  	rcu_assign_pointer(sbi->s_flex_groups, new_groups);
>  	sbi->s_flex_groups_allocated = size;
>  	if (old_groups)
> -		ext4_kvfree_array_rcu(old_groups);
> +		kvfree_rcu_mightsleep(old_groups);
>  	return 0;
>  }
>  
> -- 
> 2.30.2
>