Re: [PATCH 0/6] Remove highmem allocations, kmap/kunmap

[David Sterba <dsterba@suse.cz>]

On Thu, Jul 08, 2021 at 08:45:12AM -0400, Neal Gompa wrote:
> On Thu, Jul 8, 2021 at 7:48 AM David Sterba <dsterba@suse.com> wrote:
> >
> > The highmem was maybe was a good idea long time ago but with 64bit
> > architectures everywhere I don't think we need to take it into account.
> > This does not mean this 32bit won't work, just that it won't try to use
> > temporary pages in highmem for compression and raid56. The key word is
> > temporary. Combining a very fast device (like hundreds of megabytes
> > throughput) and 32bit machine with reasonable memory (for 32bit, like
> > 8G), it could become a problem once low memory is scarce.
> >
> > David Sterba (6):
> >   btrfs: drop from __GFP_HIGHMEM all allocations
> >   btrfs: compression: drop kmap/kunmap from lzo
> >   btrfs: compression: drop kmap/kunmap from zlib
> >   btrfs: compression: drop kmap/kunmap from zstd
> >   btrfs: compression: drop kmap/kunmap from generic helpers
> >   btrfs: check-integrity: drop kmap/kunmap for block pages
> >
> >  fs/btrfs/check-integrity.c | 11 +++-------
> >  fs/btrfs/compression.c     |  6 ++----
> >  fs/btrfs/inode.c           |  3 +--
> >  fs/btrfs/lzo.c             | 42 +++++++++++---------------------------
> >  fs/btrfs/raid56.c          | 10 ++++-----
> >  fs/btrfs/zlib.c            | 42 +++++++++++++-------------------------
> >  fs/btrfs/zstd.c            | 33 +++++++++++-------------------
> >  7 files changed, 49 insertions(+), 98 deletions(-)
> >
> 
> I'd be concerned about the impact of this on SBC devices. All Fedora
> ARM images have zstd compression applied to them, and it would suck if
> we had a performance regression here because of this.

How much memory do the SBC devices have?