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

Jan Kara
2024-01-04T11:50:05Z

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