linux-ext4 - Re: [PATCH RFC v3 for-6.8/block 09/17] btrfs: use bdev apis

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Date: Thu, 4 Jan 2024 12:49:58 +0100
From: Jan Kara <jack@...e.cz>
To: Matthew Wilcox <willy@...radead.org>
Cc: Yu Kuai <yukuai1@...weicloud.com>, axboe@...nel.dk,
	roger.pau@...rix.com, colyli@...e.de, kent.overstreet@...il.com,
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Subject: Re: [PATCH RFC v3 for-6.8/block 09/17] btrfs: use bdev apis

On Sat 23-12-23 17:31:55, Matthew Wilcox wrote:
> 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.

In principle I agree with you but in this particular case I agree the ask
is just too big. I suspect it is one of btrfs btree locks or maybe
chunk_mutex but I doubt even btrfs developers know and maybe it is just a
cargo cult. And it is not like this would be the first occurence of this
anti-pattern in btrfs - see e.g. device_list_add(), add_missing_dev(),
btrfs_destroy_delalloc_inodes() (here the wrapping around
invalidate_inode_pages2() looks really weird), and many others...

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR