RE: [PATCH v7 6/8] mm: zswap: Support mTHP swapout in zswap_store().

[Sridhar, Kanchana P]

> -----Original Message-----
> From: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
> Sent: Thursday, September 26, 2024 11:46 AM
> To: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Sridhar, Kanchana P <kanchana.p.sridhar@xxxxxxxxx>; linux-
> kernel@xxxxxxxxxxxxxxx; linux-mm@xxxxxxxxx; nphamcs@xxxxxxxxx;
> chengming.zhou@xxxxxxxxx; usamaarif642@xxxxxxxxx;
> shakeel.butt@xxxxxxxxx; ryan.roberts@xxxxxxx; Huang, Ying
> <ying.huang@xxxxxxxxx>; 21cnbao@xxxxxxxxx; akpm@xxxxxxxxxxxxxxxxxxxx;
> Zou, Nanhai <nanhai.zou@xxxxxxxxx>; Feghali, Wajdi K
> <wajdi.k.feghali@xxxxxxxxx>; Gopal, Vinodh <vinodh.gopal@xxxxxxxxx>
> Subject: Re: [PATCH v7 6/8] mm: zswap: Support mTHP swapout in
> zswap_store().
> 
> On Thu, Sep 26, 2024 at 11:43 AM Johannes Weiner <hannes@xxxxxxxxxxx>
> wrote:
> >
> > On Thu, Sep 26, 2024 at 05:29:30PM +0000, Sridhar, Kanchana P wrote:
> > > > > 3) Keep the approach in v7 where obj_cgroup_get/put is localized to
> > > > >     zswap_store_page for both success and error conditions, and any
> > > > unwinding
> > > > >     state in zswap_store will take care of dropping references obtained
> from
> > > > >     prior successful writes (from this or prior invocations of
> zswap_store).
> > > >
> > > > I am also fine with doing that and doing the reference batching as a
> follow up.
> > >
> > > I think so too! We could try and improve upon (3) with reference batching
> > > in a follow-up patch.
> >
> > Yeah, I agree. The percpu-refcounts are not that expensive, we should
> > be able to live with per-page ops for now.
> >
> > One thing you *can* do from the start is tryget a pool reference in
> > zswap_store(), to prevent the pools untimely demise while you work on
> > it, and then in zswap_store_page() you can do gets instead of trygets.
> >
> > You'd have to rename zswap_pool_get() to zswap_pool_tryget() (which is
> > probably for the best) and implement the trivial new zswap_pool_get().
> 
> Yeah I was actually planning to send a follow-up patch to do exactly
> that until we figure out proper patching for the refcounts. Even
> better if Kanchana incorporates it in the next version :)

Sure, Yosry, I will incorporate it in the next version!

Thanks again,
Kanchana