Re: [PATCH 08/10] btrfs: keep the free space cache inode mapping at order 0

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

在 2026/9/7 07:59, Tal Zussman 写道:
> The free space cache inode is a data inode, so the read path enables
> large folios on its mapping. The v1 cache code assumes one page per
> folio. With a large folio, consecutive __filemap_get_folio() calls in
> io_ctl_prepare_pages() would return the same folio and deadlock on the
> second FGP_LOCK.
> 
> The v1 on-disk format is PAGE_SIZE granular, so keep the mapping at
> order 0 rather than teaching the io_ctl code about large folios.
> 
> Signed-off-by: Tal Zussman <tz2294@columbia.edu>

V1 space cache is already disabled by commit 545e560a5b0f ("btrfs: 
disable v1 space cache").

I do not think you should add such check for things that we're already 
planning to fully remove.

> ---
>   fs/btrfs/btrfs_inode.h | 7 +++++++
>   1 file changed, 7 insertions(+)
> 
> diff --git a/fs/btrfs/btrfs_inode.h b/fs/btrfs/btrfs_inode.h
> index 89e5e9c0c904..5d85aa69db13 100644
> --- a/fs/btrfs/btrfs_inode.h
> +++ b/fs/btrfs/btrfs_inode.h
> @@ -502,6 +502,13 @@ static inline void btrfs_set_inode_mapping_order(struct btrfs_inode *inode)
>   	/* Metadata inode should not reach here. */
>   	ASSERT(is_data_inode(inode));
>   
> +	/*
> +	 * The v1 free space cache operates on PAGE_SIZE chunks and assumes
> +	 * one page per folio. Leave it at order 0.
> +	 */
> +	if (btrfs_is_free_space_inode(inode))
> +		return;
> +
>   	mapping_set_folio_order_range(inode->vfs_inode.i_mapping,
>   				      inode->root->fs_info->block_min_order,
>   				      inode->root->fs_info->block_max_order);
>