Re: [RFC mm/zswap 1/2] mm/zswap: add the flag can_sleep_mapped

[Vitaly Wool <vitaly.wool@konsulko.com>]

On Thu, Jan 14, 2021 at 7:43 PM Shakeel Butt <shakeelb@google.com> wrote:
>
> On Thu, Jan 14, 2021 at 10:29 AM Minchan Kim <minchan@kernel.org> wrote:
> >
> > On Fri, Dec 25, 2020 at 07:02:50PM +0800, Tian Tao wrote:
> > > add a flag to zpool, named is "can_sleep_mapped", and have it set true
> > > for zbud/z3fold, set false for zsmalloc. Then zswap could go the current
> > > path if the flag is true; and if it's false, copy data from src to a
> > > temporary buffer, then unmap the handle, take the mutex, process the
> > > buffer instead of src to avoid sleeping function called from atomic
> > > context.
> > >
> > > Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
> > > ---
> > >  include/linux/zpool.h |  3 +++
> > >  mm/zpool.c            | 13 +++++++++++++
> > >  mm/zswap.c            | 50 +++++++++++++++++++++++++++++++++++++++++++++-----
> > >  3 files changed, 61 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/include/linux/zpool.h b/include/linux/zpool.h
> > > index 51bf430..e899701 100644
> > > --- a/include/linux/zpool.h
> > > +++ b/include/linux/zpool.h
> > > @@ -73,6 +73,7 @@ u64 zpool_get_total_size(struct zpool *pool);
> > >   * @malloc:  allocate mem from a pool.
> > >   * @free:    free mem from a pool.
> > >   * @shrink:  shrink the pool.
> > > + * @sleep_mapped: whether zpool driver can sleep during map.
> >
> > I don't think it's a good idea. It just breaks zpool abstraction
> > in that it exposes internal implementation to user to avoid issue
> > zswap recently introduced. It also conflicts zpool_map_handle's
> > semantic.
> >
> > Rather than introducing another break in zpool due to the new
> > zswap feature recenlty introduced, zswap could introduce
> > CONFIG_ZSWAP_HW_COMPRESSOR. Once it's configured, zsmalloc could
> > be disabled. And with disabling CONFIG_ZSWAP_HW_COMPRESSOR, zswap
> > doesn't need to make any bounce buffer copy so that no existing
> > zsmalloc user will see performance regression.
> >
>
> I agree with Minchan. There is no reason to add extra overhead for
> configurations where there is no hardware available.

I still don't get what the suggested alternative is. How would
disabling CONFIG_ZSWAP_HW_COMPRESSOR help if zswap is using acomp API?

Best regards,
   Vitaly