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 8:21 PM Shakeel Butt <shakeelb@google.com> wrote:
>
> On Thu, Jan 14, 2021 at 11:05 AM Vitaly Wool <vitaly.wool@konsulko.com> wrote:
> >
> > On Thu, Jan 14, 2021 at 7:56 PM Minchan Kim <minchan@kernel.org> wrote:
> > >
> > > On Thu, Jan 14, 2021 at 07:40:50PM +0100, Vitaly Wool wrote:
> > > > On Thu, Jan 14, 2021 at 7:29 PM 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 believe it won't help that much -- the new compressor API presumes
> > > > that the caller may sleep during compression and that will be an
> > > > accident waiting to happen as long as we use it and keep the handle
> > > > mapped in zsmalloc case.
> > > >
> > > > Or maybe I interpreted you wrong and you are suggesting re-introducing
> > > > calls to the old API under this #ifdef, is that the case?
> > >
> > > Yub. zswap could abstract that part under #ifdef to keep old behavior.
> >
> > We can reconsider this option when zsmalloc implements reclaim
> > callback. So far it's obviously too much a mess for a reason so weak.
> >
>
> Sorry I don't understand the link between zsmalloc implementing shrink
> callback and this patch.

There is none. There is a link between taking all the burden to revive
zsmalloc for zswap at the cost of extra zswap complexity and zsmalloc
not being fully zswap compatible.

The ultimate zswap goal is to cache hottest swapped-out pages in a
compressed format. zsmalloc doesn't implement reclaim callback, and
therefore zswap can *not* fulfill its goal since old pages are there
to stay, and it can't accept new hotter ones. So, let me make it
clear: zswap/zsmalloc combo is a legacy corner case.

> This patch is adding an overhead for all
> zswap+zsmalloc users irrespective of availability of hardware. If we
> want to add support for new hardware, please add without impacting the
> current users.

No, it's not like that. zswap+zsmalloc combination is currently
already broken and this patch implements a way to work that around.
The users are already impacted and that is of course a problem. The
workaround is not perfect but looks good enough for me.

The suggested messy #ifdef-based alternative will turn zswap into
something really tricky to understand and maintain and therefore is
not going to work.

The best possible thing here would be for zsmalloc to stop taking
spinlock in map() callback and releasing in unmap() (it _is_ legit --
I don't argue that -- but it does look weird and goes against the
recent efforts to make Linux kernel more realtime and generally more
responsive; moreover, it may be just a way to conceal some race
conditions which could be easily fixed otherwise).

Best regards,
   Vitaly