Re: [PATCH] Revert "btrfs: compression: don't try to compress if we don't have enough pages"

[David Sterba <dsterba@suse.cz>]

On Wed, Aug 25, 2021 at 01:41:42PM +0800, Qu Wenruo wrote:
> This reverts commit f2165627319ffd33a6217275e5690b1ab5c45763.

At this point the revert is the simplest way to restore the inline
extent compression so that's what I'll probably do. However.
> 
> [BUG]
> It's no longer possible to create compressed inline extent after commit
> f2165627319f ("btrfs: compression: don't try to compress if we don't
> have enough pages").
> 
> [CAUSE]
> For compression code, there are several possible reasons we have a range
> that needs to be compressed while it's no more than one page.
> 
> - Compressed inline write
>   The data is always smaller than one sector.

The missing logic was for the true inline extent. The patch was supposed
to skip compression for single pages other than inline extents, due to
efficiency. So I wonder if we want to do that or just don't bother as
it's probably a negligible amount of wasted time.
> 
> - Compressed subpage write
>   For the incoming subpage compressed write support, we require page
>   alignment of the delalloc range.
>   And for 64K page size, we can compress just one page into smaller
>   sectors.

Oh so the logic would have to be updated to distinguish sectors and
pages, but to simplify the code for subpage compression support it would
be easier to revert it and start from there.