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

["Sridhar, Kanchana P" <kanchana.p.sridhar@intel.com>]

> -----Original Message-----
> From: Nhat Pham <nphamcs@gmail.com>
> Sent: Tuesday, September 24, 2024 4:11 PM
> To: Yosry Ahmed <yosryahmed@google.com>
> Cc: Sridhar, Kanchana P <kanchana.p.sridhar@intel.com>; linux-
> kernel@vger.kernel.org; linux-mm@kvack.org; hannes@cmpxchg.org;
> chengming.zhou@linux.dev; usamaarif642@gmail.com;
> shakeel.butt@linux.dev; ryan.roberts@arm.com; Huang, Ying
> <ying.huang@intel.com>; 21cnbao@gmail.com; akpm@linux-foundation.org;
> Zou, Nanhai <nanhai.zou@intel.com>; Feghali, Wajdi K
> <wajdi.k.feghali@intel.com>; Gopal, Vinodh <vinodh.gopal@intel.com>;
> joshua.hahnjy@gmail.com
> Subject: Re: [PATCH v7 6/8] mm: zswap: Support mTHP swapout in
> zswap_store().
> 
> On Tue, Sep 24, 2024 at 2:38 PM Yosry Ahmed <yosryahmed@google.com>
> wrote:
> >
> >
> > We can also do what we discussed before about double charging. The
> > pages that are being reclaimed are already charged, so technically we
> > don't need to charge them again. We can uncharge the difference
> > between compressed and uncompressed sizes after compression and call
> > it a day. This fixes the limit checking and the double charging in one
> > go.
> > I am a little bit nervous though about zswap uncharing the pages from
> > under reclaim, there are likely further accesses of the page memcg
> > after zswap. Maybe we can plumb the info back to reclaim or set a flag
> > on the page to avoid uncharging it when it's freed.
> 
> Hmm this is just for memory usage charging, no? The problem here is
> the zswap usage (zswap.current), and its relation to the limit.
> 
> One thing we can do is check the zswap usage against the limit for
> every subpage, but that's likely expensive...?

This is the approach currently implemented in v7.
Data gathered doesn’t indicate a performance issue with this
specific workload in the two scenarios validated, namely,
zswap-4K vs. zswap-mTHP and SSD-mTHP vs. zswap-mTHP (we only
see performance gains with explainable sys time increase).

Of course, the existing implementation could be a baseline for
validating performance of other approaches, e.g., checking zswap usage
per mTHP. However, these other approaches would also need to be
evaluated for more global multi-instance implications as far as all
processes being able to make progress. 

> 
> With the new atomic counters Joshua is working on, we can
> check-and-charge at the same time, after we have compressed the whole
> large folio, like this:
> 
> for (memcg = original_memcg; !mem_cgroup_is_root(memcg);
>      memcg = parent_mem_cgroup(memcg));
>      old_usage = atomic_read(&memcg->zswap);
> 
>      do {
>         new_usage = old_usage + size;
>         if (new_usage > limit) {
>            /* undo charging of descendants, then return false */
>         }
>       } while (!atomic_try_cmpxchg(&memcg->zswap, old_usage, new_usage))
> }
> 
> But I don't know what we can do in the current design. I gave it some
> more thought, and even if we only check after we know the size, we can
> still potentially overshoot the limit :(

I agree. Moreover, these checks based on estimated ratio or compressed size
could also add overhead in the normal case where we are not near the usage
limits.