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Date:   Thu, 19 Oct 2023 13:44:39 +0200
From:   Jan Kara <jack@...e.cz>
To:     Baokun Li <libaokun1@...wei.com>
Cc:     linux-ext4@...r.kernel.org, tytso@....edu,
        adilger.kernel@...ger.ca, jack@...e.cz, ritesh.list@...il.com,
        linux-kernel@...r.kernel.org, yi.zhang@...wei.com,
        yangerkun@...wei.com, yukuai3@...wei.com
Subject: Re: [PATCH 3/4] ext4: avoid online resizing failures due to
 oversized flex bg

On Wed 18-10-23 19:42:20, Baokun Li wrote:
> When we online resize an ext4 filesystem with a oversized flexbg_size,
> 
>      mkfs.ext4 -F -G 67108864 $dev -b 4096 100M
>      mount $dev $dir
>      resize2fs $dev 16G
> 
> the following WARN_ON is triggered:
> ==================================================================
> WARNING: CPU: 0 PID: 427 at mm/page_alloc.c:4402 __alloc_pages+0x411/0x550
> Modules linked in: sg(E)
> CPU: 0 PID: 427 Comm: resize2fs Tainted: G  E  6.6.0-rc5+ #314
> RIP: 0010:__alloc_pages+0x411/0x550
> Call Trace:
>  <TASK>
>  __kmalloc_large_node+0xa2/0x200
>  __kmalloc+0x16e/0x290
>  ext4_resize_fs+0x481/0xd80
>  __ext4_ioctl+0x1616/0x1d90
>  ext4_ioctl+0x12/0x20
>  __x64_sys_ioctl+0xf0/0x150
>  do_syscall_64+0x3b/0x90
> ==================================================================
> 
> This is because flexbg_size is too large and the size of the new_group_data
> array to be allocated exceeds MAX_ORDER. Currently, the minimum value of
> MAX_ORDER is 8, the minimum value of PAGE_SIZE is 4096, the corresponding
> maximum number of groups that can be allocated is:
> 
>  (PAGE_SIZE << MAX_ORDER) / sizeof(struct ext4_new_group_data) ≈ 21845
> 
> And the value that is down-aligned to the power of 2 is 16384. Therefore,
> this value is defined as MAX_RESIZE_BG, and the number of groups added
> each time does not exceed this value during resizing, and is added multiple
> times to complete the online resizing. The difference is that the metadata
> in a flex_bg may be more dispersed.
> 
> Signed-off-by: Baokun Li <libaokun1@...wei.com>

Looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

								Honza

> ---
>  fs/ext4/resize.c | 25 +++++++++++++++++--------
>  1 file changed, 17 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/ext4/resize.c b/fs/ext4/resize.c
> index 0a57b199883c..e168a9f59600 100644
> --- a/fs/ext4/resize.c
> +++ b/fs/ext4/resize.c
> @@ -218,10 +218,17 @@ struct ext4_new_flex_group_data {
>  						   in the flex group */
>  	__u16 *bg_flags;			/* block group flags of groups
>  						   in @groups */
> +	ext4_group_t resize_bg;			/* number of allocated
> +						   new_group_data */
>  	ext4_group_t count;			/* number of groups in @groups
>  						 */
>  };
>  
> +/*
> + * Avoiding memory allocation failures due to too many groups added each time.
> + */
> +#define MAX_RESIZE_BG				16384
> +
>  /*
>   * alloc_flex_gd() allocates a ext4_new_flex_group_data with size of
>   * @flexbg_size.
> @@ -236,14 +243,18 @@ static struct ext4_new_flex_group_data *alloc_flex_gd(unsigned int flexbg_size)
>  	if (flex_gd == NULL)
>  		goto out3;
>  
> -	flex_gd->count = flexbg_size;
> -	flex_gd->groups = kmalloc_array(flexbg_size,
> +	if (unlikely(flexbg_size > MAX_RESIZE_BG))
> +		flex_gd->resize_bg = MAX_RESIZE_BG;
> +	else
> +		flex_gd->resize_bg = flexbg_size;
> +
> +	flex_gd->groups = kmalloc_array(flex_gd->resize_bg,
>  					sizeof(struct ext4_new_group_data),
>  					GFP_NOFS);
>  	if (flex_gd->groups == NULL)
>  		goto out2;
>  
> -	flex_gd->bg_flags = kmalloc_array(flexbg_size, sizeof(__u16),
> +	flex_gd->bg_flags = kmalloc_array(flex_gd->resize_bg, sizeof(__u16),
>  					  GFP_NOFS);
>  	if (flex_gd->bg_flags == NULL)
>  		goto out1;
> @@ -1602,8 +1613,7 @@ static int ext4_flex_group_add(struct super_block *sb,
>  
>  static int ext4_setup_next_flex_gd(struct super_block *sb,
>  				    struct ext4_new_flex_group_data *flex_gd,
> -				    ext4_fsblk_t n_blocks_count,
> -				    unsigned int flexbg_size)
> +				    ext4_fsblk_t n_blocks_count)
>  {
>  	struct ext4_sb_info *sbi = EXT4_SB(sb);
>  	struct ext4_super_block *es = sbi->s_es;
> @@ -1627,7 +1637,7 @@ static int ext4_setup_next_flex_gd(struct super_block *sb,
>  	BUG_ON(last);
>  	ext4_get_group_no_and_offset(sb, n_blocks_count - 1, &n_group, &last);
>  
> -	last_group = group | (flexbg_size - 1);
> +	last_group = group | (flex_gd->resize_bg - 1);
>  	if (last_group > n_group)
>  		last_group = n_group;
>  
> @@ -2130,8 +2140,7 @@ int ext4_resize_fs(struct super_block *sb, ext4_fsblk_t n_blocks_count)
>  	/* Add flex groups. Note that a regular group is a
>  	 * flex group with 1 group.
>  	 */
> -	while (ext4_setup_next_flex_gd(sb, flex_gd, n_blocks_count,
> -					      flexbg_size)) {
> +	while (ext4_setup_next_flex_gd(sb, flex_gd, n_blocks_count)) {
>  		if (time_is_before_jiffies(last_update_time + HZ * 10)) {
>  			if (last_update_time)
>  				ext4_msg(sb, KERN_INFO,
> -- 
> 2.31.1
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR

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