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

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Date: Sat, 23 Dec 2023 13:39:24 -0500
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: Matthew Wilcox <willy@...radead.org>
Cc: Yu Kuai <yukuai1@...weicloud.com>, axboe@...nel.dk, 
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Subject: Re: [PATCH RFC v3 for-6.8/block 09/17] btrfs: use bdev apis

On Sat, Dec 23, 2023 at 05:31:55PM +0000, 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.

Yes, but considering this is a cross-filesystem cleanup I wouldn't want
to address that in this patchset. And the easier, more incremental
approach for the conversion would be to first convert every GFP_NOFS
usage  to memalloc_nofs_save() like this patch does, as small local
changes, and then let the btrfs people combine them and move them to the
approproate location in a separate patchstet.