Received-SPF: pass (google.com: domain of linux-ext4+bounces-681-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:45e3:2400::1 as permitted sender) client-ip=2604:1380:45e3:2400::1;
Date: Thu, 4 Jan 2024 12:49:58 +0100
From: Jan Kara <jack@suse.cz>
To: Matthew Wilcox <willy@infradead.org>
Cc: Yu Kuai <yukuai1@huaweicloud.com>, axboe@kernel.dk,
	roger.pau@citrix.com, colyli@suse.de, kent.overstreet@gmail.com,
	joern@lazybastard.org, miquel.raynal@bootlin.com, richard@nod.at,
	vigneshr@ti.com, sth@linux.ibm.com, hoeppner@linux.ibm.com,
	hca@linux.ibm.com, gor@linux.ibm.com, agordeev@linux.ibm.com,
	jejb@linux.ibm.com, martin.petersen@oracle.com, clm@fb.com,
	josef@toxicpanda.com, dsterba@suse.com, viro@zeniv.linux.org.uk,
	brauner@kernel.org, nico@fluxnic.net, xiang@kernel.org,
	chao@kernel.org, tytso@mit.edu, adilger.kernel@dilger.ca,
	jack@suse.com, konishi.ryusuke@gmail.com, akpm@linux-foundation.org,
	hare@suse.de, p.raghav@samsung.com, linux-block@vger.kernel.org,
	linux-kernel@vger.kernel.org, xen-devel@lists.xenproject.org,
	linux-bcache@vger.kernel.org, linux-mtd@lists.infradead.org,
	linux-s390@vger.kernel.org, linux-scsi@vger.kernel.org,
	linux-bcachefs@vger.kernel.org, linux-btrfs@vger.kernel.org,
	linux-fsdevel@vger.kernel.org, linux-erofs@lists.ozlabs.org,
	linux-ext4@vger.kernel.org, linux-nilfs@vger.kernel.org,
	yukuai3@huawei.com, yi.zhang@huawei.com, yangerkun@huawei.com
Subject: Re: [PATCH RFC v3 for-6.8/block 09/17] btrfs: use bdev apis
Message-ID: <20240104114958.f3cit5q7syp3tn3a@quack3>
References: <20231221085712.1766333-1-yukuai1@huaweicloud.com>
 <20231221085712.1766333-10-yukuai1@huaweicloud.com>
 <ZYcZi5YYvt5QHrG9@casper.infradead.org>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <ZYcZi5YYvt5QHrG9@casper.infradead.org>

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@suse.com>
SUSE Labs, CR