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Date: Thu, 4 Jan 2024 13:22:14 +0100
From: Jan Kara <jack@suse.cz>
To: Yu Kuai <yukuai1@huaweicloud.com>
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Subject: Re: [PATCH RFC v3 for-6.8/block 14/17] buffer: add a new helper to
 read sb block
Message-ID: <20240104122214.jndsqygnmljxmj5d@quack3>
References: <20231221085712.1766333-1-yukuai1@huaweicloud.com>
 <20231221085853.1770062-1-yukuai1@huaweicloud.com>
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In-Reply-To: <20231221085853.1770062-1-yukuai1@huaweicloud.com>

On Thu 21-12-23 16:58:53, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@huawei.com>
> 
> Unlike __bread_gfp(), ext4 has special handing while reading sb block:
> 
> 1) __GFP_NOFAIL is not set, and memory allocation can fail;
> 2) If buffer write failed before, set buffer uptodate and don't read
>    block from disk;
> 3) REQ_META is set for all IO, and REQ_PRIO is set for reading xattr;
> 4) If failed, return error ptr instead of NULL;
> 
> This patch add a new helper __bread_gfp2() that will match above 2 and 3(
> 1 will be used, and 4 will still be encapsulated by ext4), and prepare to
> prevent calling mapping_gfp_constraint() directly on bd_inode->i_mapping
> in ext4.
> 
> Signed-off-by: Yu Kuai <yukuai3@huawei.com>

I'm not enthusiastic about this but I guess it is as good as it gets
without larger cleanups in this area. So feel free to add:

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

								Honza

> ---
>  fs/buffer.c                 | 68 ++++++++++++++++++++++++++-----------
>  include/linux/buffer_head.h | 18 +++++++++-
>  2 files changed, 65 insertions(+), 21 deletions(-)
> 
> diff --git a/fs/buffer.c b/fs/buffer.c
> index 967f34b70aa8..188bd36c9fea 100644
> --- a/fs/buffer.c
> +++ b/fs/buffer.c
> @@ -1255,16 +1255,19 @@ void __bforget(struct buffer_head *bh)
>  }
>  EXPORT_SYMBOL(__bforget);
>  
> -static struct buffer_head *__bread_slow(struct buffer_head *bh)
> +static struct buffer_head *__bread_slow(struct buffer_head *bh,
> +					blk_opf_t op_flags,
> +					bool check_write_error)
>  {
>  	lock_buffer(bh);
> -	if (buffer_uptodate(bh)) {
> +	if (buffer_uptodate(bh) ||
> +	    (check_write_error && buffer_uptodate_or_error(bh))) {
>  		unlock_buffer(bh);
>  		return bh;
>  	} else {
>  		get_bh(bh);
>  		bh->b_end_io = end_buffer_read_sync;
> -		submit_bh(REQ_OP_READ, bh);
> +		submit_bh(REQ_OP_READ | op_flags, bh);
>  		wait_on_buffer(bh);
>  		if (buffer_uptodate(bh))
>  			return bh;
> @@ -1445,6 +1448,31 @@ void __breadahead(struct block_device *bdev, sector_t block, unsigned size)
>  }
>  EXPORT_SYMBOL(__breadahead);
>  
> +static struct buffer_head *
> +bread_gfp(struct block_device *bdev, sector_t block, unsigned int size,
> +	  blk_opf_t op_flags, gfp_t gfp, bool check_write_error)
> +{
> +	struct buffer_head *bh;
> +
> +	gfp |= mapping_gfp_constraint(bdev->bd_inode->i_mapping, ~__GFP_FS);
> +
> +	/*
> +	 * Prefer looping in the allocator rather than here, at least that
> +	 * code knows what it's doing.
> +	 */
> +	gfp |= __GFP_NOFAIL;
> +
> +	bh = bdev_getblk(bdev, block, size, gfp);
> +	if (unlikely(!bh))
> +		return NULL;
> +
> +	if (buffer_uptodate(bh) ||
> +	    (check_write_error && buffer_uptodate_or_error(bh)))
> +		return bh;
> +
> +	return __bread_slow(bh, op_flags, check_write_error);
> +}
> +
>  /**
>   *  __bread_gfp() - reads a specified block and returns the bh
>   *  @bdev: the block_device to read from
> @@ -1458,27 +1486,27 @@ EXPORT_SYMBOL(__breadahead);
>   *  It returns NULL if the block was unreadable.
>   */
>  struct buffer_head *
> -__bread_gfp(struct block_device *bdev, sector_t block,
> -		   unsigned size, gfp_t gfp)
> +__bread_gfp(struct block_device *bdev, sector_t block, unsigned int size,
> +	    gfp_t gfp)
>  {
> -	struct buffer_head *bh;
> -
> -	gfp |= mapping_gfp_constraint(bdev->bd_inode->i_mapping, ~__GFP_FS);
> -
> -	/*
> -	 * Prefer looping in the allocator rather than here, at least that
> -	 * code knows what it's doing.
> -	 */
> -	gfp |= __GFP_NOFAIL;
> -
> -	bh = bdev_getblk(bdev, block, size, gfp);
> -
> -	if (likely(bh) && !buffer_uptodate(bh))
> -		bh = __bread_slow(bh);
> -	return bh;
> +	return bread_gfp(bdev, block, size, 0, gfp, false);
>  }
>  EXPORT_SYMBOL(__bread_gfp);
>  
> +/*
> + * This works like __bread_gfp() except:
> + * 1) If buffer write failed before, set buffer uptodate and don't read
> + * block from disk;
> + * 2) Caller can pass in additional op_flags like REQ_META;
> + */
> +struct buffer_head *
> +__bread_gfp2(struct block_device *bdev, sector_t block, unsigned int size,
> +	     blk_opf_t op_flags, gfp_t gfp)
> +{
> +	return bread_gfp(bdev, block, size, op_flags, gfp, true);
> +}
> +EXPORT_SYMBOL(__bread_gfp2);
> +
>  static void __invalidate_bh_lrus(struct bh_lru *b)
>  {
>  	int i;
> diff --git a/include/linux/buffer_head.h b/include/linux/buffer_head.h
> index 5f23ee599889..751b2744b4ae 100644
> --- a/include/linux/buffer_head.h
> +++ b/include/linux/buffer_head.h
> @@ -171,6 +171,18 @@ static __always_inline int buffer_uptodate(const struct buffer_head *bh)
>  	return test_bit_acquire(BH_Uptodate, &bh->b_state);
>  }
>  
> +static __always_inline int buffer_uptodate_or_error(struct buffer_head *bh)
> +{
> +	/*
> +	 * If the buffer has the write error flag, data was failed to write
> +	 * out in the block. In this case, set buffer uptodate to prevent
> +	 * reading old data.
> +	 */
> +	if (buffer_write_io_error(bh))
> +		set_buffer_uptodate(bh);
> +	return buffer_uptodate(bh);
> +}
> +
>  static inline unsigned long bh_offset(const struct buffer_head *bh)
>  {
>  	return (unsigned long)(bh)->b_data & (page_size(bh->b_page) - 1);
> @@ -231,7 +243,11 @@ void __brelse(struct buffer_head *);
>  void __bforget(struct buffer_head *);
>  void __breadahead(struct block_device *, sector_t block, unsigned int size);
>  struct buffer_head *__bread_gfp(struct block_device *,
> -				sector_t block, unsigned size, gfp_t gfp);
> +				sector_t block, unsigned int size, gfp_t gfp);
> +struct buffer_head *__bread_gfp2(struct block_device *bdev, sector_t block,
> +				 unsigned int size, blk_opf_t op_flags,
> +				 gfp_t gfp);
> +
>  struct buffer_head *alloc_buffer_head(gfp_t gfp_flags);
>  void free_buffer_head(struct buffer_head * bh);
>  void unlock_buffer(struct buffer_head *bh);
> -- 
> 2.39.2
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR