Re: [PATCH 1/5] zonefs: pass GFP_KERNEL to blkdev_zone_mgmt() call

[Mikulas Patocka <mpatocka@redhat.com>]

On Tue, 23 Jan 2024, Johannes Thumshirn wrote:

> Pass GFP_KERNEL instead of GFP_NOFS to the blkdev_zone_mgmt() call in
> zonefs_zone_mgmt().
> 
> As as zonefs_zone_mgmt() and zonefs_inode_zone_mgmt() are never called
> from a place that can recurse back into the filesystem on memory reclaim,
> it is save to call blkdev_zone_mgmt() with GFP_KERNEL.
> 
> Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
> ---
>  fs/zonefs/super.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/zonefs/super.c b/fs/zonefs/super.c
> index 93971742613a..63fbac018c04 100644
> --- a/fs/zonefs/super.c
> +++ b/fs/zonefs/super.c
> @@ -113,7 +113,7 @@ static int zonefs_zone_mgmt(struct super_block *sb,
>  
>  	trace_zonefs_zone_mgmt(sb, z, op);
>  	ret = blkdev_zone_mgmt(sb->s_bdev, op, z->z_sector,
> -			       z->z_size >> SECTOR_SHIFT, GFP_NOFS);
> +			       z->z_size >> SECTOR_SHIFT, GFP_KERNEL);
>  	if (ret) {
>  		zonefs_err(sb,
>  			   "Zone management operation %s at %llu failed %d\n",
> 
> -- 
> 2.43.0

zonefs_inode_zone_mgmt calls 
lockdep_assert_held(&ZONEFS_I(inode)->i_truncate_mutex); - so, this 
function is called with the mutex held - could it happen that the 
GFP_KERNEL allocation recurses into the filesystem and attempts to take 
i_truncate_mutex as well?

i.e. GFP_KERNEL -> iomap_do_writepage -> zonefs_write_map_blocks -> 
zonefs_write_iomap_begin -> mutex_lock(&zi->i_truncate_mutex)

Mikulas