linux-kernel - Re: [PATCH v4] mm/zswap: store <PAGE_SIZE compression failed page as-is

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Message-ID: <CAGsJ_4yLDLvOZ2=3iVcQhu2jnbWQ+iTQsqVefJsx4_YT4bnEZg@mail.gmail.com>
Date: Thu, 21 Aug 2025 18:27:52 +0800
From: Barry Song <21cnbao@...il.com>
To: Nhat Pham <nphamcs@...il.com>
Cc: Herbert Xu <herbert@...dor.apana.org.au>, SeongJae Park <sj@...nel.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, Chengming Zhou <chengming.zhou@...ux.dev>, 
	Johannes Weiner <hannes@...xchg.org>, Yosry Ahmed <yosry.ahmed@...ux.dev>, kernel-team@...a.com, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
	Takero Funaki <flintglass@...il.com>, David Hildenbrand <david@...hat.com>, Baoquan He <bhe@...hat.com>, 
	Chris Li <chrisl@...nel.org>, Kairui Song <kasong@...cent.com>
Subject: Re: [PATCH v4] mm/zswap: store <PAGE_SIZE compression failed page as-is

On Thu, Aug 21, 2025 at 1:33 AM Nhat Pham <nphamcs@...il.com> wrote:
>
> On Tue, Aug 19, 2025 at 6:37 PM Herbert Xu <herbert@...dor.apana.org.au> wrote:
> >
> > On Wed, Aug 20, 2025 at 01:34:01PM +1200, Barry Song wrote:
> > >
> > > We might want to revisit the old thread to check whether it is now safe for us
> > > to move to PAGE_SIZE in zswap now.
> >
> > It's perfectly safe as LZO was fixed months ago.
>
> Perfect. Then I'll revive Chengming's patch (see [1]) to reduce the
> compression buffer :)

Nice!

But perhaps we should wait until SeongJae sends a new version that
addresses the counter issue? Also, I noticed the following code may
have problems with the patch:

        if (comp_ret == -ENOSPC || alloc_ret == -ENOSPC)
                zswap_reject_compress_poor++;

Can we still reach the code comp_ret == -ENOSPC since we already
handled comp_ret by ...

+       if (comp_ret || !dlen) {
+               zswap_crypto_compress_fail++;
+               dlen = PAGE_SIZE;
+       }
+       if (dlen >= PAGE_SIZE) {
+               if (!mem_cgroup_zswap_writeback_enabled(
+                                       folio_memcg(page_folio(page)))) {
+                       comp_ret = -EINVAL;
+                       goto unlock;
+               }
+               comp_ret = 0;
+               dlen = PAGE_SIZE;
+               dst = kmap_local_page(page);
+       }

Thanks
Barry