On 2019/1/15 上午1:11, David Sterba wrote:
> On Mon, Jan 07, 2019 at 01:41:47PM +0800, Qu Wenruo wrote:
>> +/*
>> + * Helper structure to keep record of a file tree whose reloc
>> + * root needs to be cleaned up.
>> + *
>> + * Since reloc_control is used less frequently than btrfs_root, this should
>> + * prevent us to add another structure in btrfs_root.
> 
> The alternative is to keep that in btrfs_root and avoid all allocations
> that also remove one failure possibility. The overall root structure
> size is over 1K already and roots get allocated from the generic slabs,
> so quite possible from the 2K slab anyway. A few more bytes will not be
> that expensive though the number of uses of the data would be very low.
> 
> Memory failures are ok for restartable operations and at the beginning
> of the operation. Imagine that relocation fails with ENOMEM after hours
> of shuffling data just before it is about to finish.
> 
> I think in another patch you add 128 bytes to btrfs_root anyway, so some
> kind of memory consumption increas will happen anyway.

Indeed, I'll just use a list_head in btrfs_root to record this.

Thanks,
Qu