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

Kent Overstreet
2023-12-23T18:39:34Z

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.