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

[Jan Kara <jack@xxxxxxx>]

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@xxxxxxxx>
SUSE Labs, CR