linux-kernel - Re: [PATCH RFC v3 for-6.8/block 09/17] btrfs: use bdev apis

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Date: Sat, 23 Dec 2023 17:31:55 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Yu Kuai <yukuai1@...weicloud.com>
Cc: axboe@...nel.dk, roger.pau@...rix.com, colyli@...e.de,
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Subject: Re: [PATCH RFC v3 for-6.8/block 09/17] btrfs: use bdev apis

On Thu, Dec 21, 2023 at 04:57:04PM +0800, Yu Kuai wrote:
> @@ -3674,16 +3670,17 @@ struct btrfs_super_block *btrfs_read_dev_one_super(struct block_device *bdev,
>  		 * Drop the page of the primary superblock, so later read will
>  		 * always read from the device.
>  		 */
> -		invalidate_inode_pages2_range(mapping,
> -				bytenr >> PAGE_SHIFT,
> +		invalidate_bdev_range(bdev, bytenr >> PAGE_SHIFT,
>  				(bytenr + BTRFS_SUPER_INFO_SIZE) >> PAGE_SHIFT);
>  	}
>  
> -	page = read_cache_page_gfp(mapping, bytenr >> PAGE_SHIFT, GFP_NOFS);
> -	if (IS_ERR(page))
> -		return ERR_CAST(page);
> +	nofs_flag = memalloc_nofs_save();
> +	folio = bdev_read_folio(bdev, bytenr);
> +	memalloc_nofs_restore(nofs_flag);

This is the wrong way to use memalloc_nofs_save/restore.  They should be
used at the point that the filesystem takes/releases whatever lock is
also used during reclaim.  I don't know btrfs well enough to suggest
what lock is missing these annotations.