Re: [PATCH 2/2] btrfs: zoned: fix compressed writes

[David Sterba <dsterba@suse.cz>]

On Mon, May 17, 2021 at 11:21:49AM +0000, Johannes Thumshirn wrote:
> On 17/05/2021 09:07, Johannes Thumshirn wrote:
> >>> +	if (use_append) {
> >>> +		struct extent_map *em;
> >>> +		struct map_lookup *map;
> >>> +
> >>> +		em = btrfs_get_chunk_map(fs_info, disk_start, PAGE_SIZE);
> >> The caller already does the em lookup, so this is duplicate, allocating
> >> memory, taking locks and doing a tree lookup. All happening on write out
> >> path so this seems heavy.
> > Right, I did not check this, sorry. Is it OK to add another patch as 
> > preparation swapping some of the parameters to btrfs_submit_compressed_write()
> > from the em? Otherwise btrfs_submit_compressed_write() will have 10 parameters
> > which sounds awefull.
> > 
> 
> Actually I can't do that. The caller does calls create_io_em() while this patch
> needs to call brtfs_get_chunk_map(). The 'em' returned by create_io_em() does not
> have em->map_lookup populated and we need the stripe's block device from 
> em->map_lookup.
> 
> So it looks like we need to live with the additional memory allocation and locks.

Ok then, it's limited to zoned mode so the allocation won't affect
regular mode.