linux-kernel - Re: [PATCH] ext4: add ext4_sb_bread_nofail() helper function for ext4_free

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Message-ID: <clyk3sndidaz3plcvojzci7rs7sqmsegga5izrp46l23zaxtoc@sovizlltl6jd>
Date: Mon, 25 Aug 2025 11:52:50 +0200
From: Jan Kara <jack@...e.cz>
To: libaokun@...weicloud.com
Cc: linux-ext4@...r.kernel.org, tytso@....edu, adilger.kernel@...ger.ca, 
	jack@...e.cz, linux-kernel@...r.kernel.org, yi.zhang@...wei.com, 
	yangerkun@...wei.com, libaokun1@...wei.com
Subject: Re: [PATCH] ext4: add ext4_sb_bread_nofail() helper function for
 ext4_free_branches()

On Thu 21-08-25 21:38:57, libaokun@...weicloud.com wrote:
> From: Baokun Li <libaokun1@...wei.com>
> 
> The implicit __GFP_NOFAIL flag in ext4_sb_bread() was removed in commit
> 8a83ac54940d ("ext4: call bdev_getblk() from sb_getblk_gfp()"), meaning
> the function can now fail under memory pressure.
> 
> Most callers of ext4_sb_bread() propagate the error to userspace and do not
> remount the filesystem read-only. However, ext4_free_branches() handles
> ext4_sb_bread() failure by remounting the filesystem read-only.
> 
> This implies that an ext3 filesystem (mounted via the ext4 driver) could be
> forcibly remounted read-only due to a transient page allocation failure,
> which is unacceptable.
> 
> To mitigate this, introduce a new helper function, ext4_sb_bread_nofail(),
> which explicitly uses __GFP_NOFAIL, and use it in ext4_free_branches().
> 
> Fixes: 8a83ac54940d ("ext4: call bdev_getblk() from sb_getblk_gfp()")
> Signed-off-by: Baokun Li <libaokun1@...wei.com>

Looks sane. Feel free to add:

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

								Honza

> ---
>  fs/ext4/ext4.h     | 2 ++
>  fs/ext4/indirect.c | 2 +-
>  fs/ext4/super.c    | 9 +++++++++
>  3 files changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index 106484739932..32315e51639e 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -3134,6 +3134,8 @@ extern struct buffer_head *ext4_sb_bread(struct super_block *sb,
>  					 sector_t block, blk_opf_t op_flags);
>  extern struct buffer_head *ext4_sb_bread_unmovable(struct super_block *sb,
>  						   sector_t block);
> +extern struct buffer_head *ext4_sb_bread_nofail(struct super_block *sb,
> +						sector_t block);
>  extern void ext4_read_bh_nowait(struct buffer_head *bh, blk_opf_t op_flags,
>  				bh_end_io_t *end_io, bool simu_fail);
>  extern int ext4_read_bh(struct buffer_head *bh, blk_opf_t op_flags,
> diff --git a/fs/ext4/indirect.c b/fs/ext4/indirect.c
> index 7de327fa7b1c..39350a18e919 100644
> --- a/fs/ext4/indirect.c
> +++ b/fs/ext4/indirect.c
> @@ -1025,7 +1025,7 @@ static void ext4_free_branches(handle_t *handle, struct inode *inode,
>  			}
>  
>  			/* Go read the buffer for the next level down */
> -			bh = ext4_sb_bread(inode->i_sb, nr, 0);
> +			bh = ext4_sb_bread_nofail(inode->i_sb, nr);
>  
>  			/*
>  			 * A read failure? Report error and clear slot
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index c7d39da7e733..a0e2b396aa76 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -265,6 +265,15 @@ struct buffer_head *ext4_sb_bread_unmovable(struct super_block *sb,
>  	return __ext4_sb_bread_gfp(sb, block, 0, gfp);
>  }
>  
> +struct buffer_head *ext4_sb_bread_nofail(struct super_block *sb,
> +					 sector_t block)
> +{
> +	gfp_t gfp = mapping_gfp_constraint(sb->s_bdev->bd_mapping,
> +			~__GFP_FS) | __GFP_MOVABLE | __GFP_NOFAIL;
> +
> +	return __ext4_sb_bread_gfp(sb, block, 0, gfp);
> +}
> +
>  void ext4_sb_breadahead_unmovable(struct super_block *sb, sector_t block)
>  {
>  	struct buffer_head *bh = bdev_getblk(sb->s_bdev, block,
> -- 
> 2.39.2
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR