linux-kernel - Re: [PATCH v1] mm: zswap: add per-memcg stat for incompressible pages

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Message-ID: <20260206031509.68313-1-sj@kernel.org>
Date: Thu,  5 Feb 2026 19:15:09 -0800
From: SeongJae Park <sj@...nel.org>
To: "Jiayuan Chen" <jiayuan.chen@...ux.dev>
Cc: SeongJae Park <sj@...nel.org>,
	linux-mm@...ck.org,
	"Jiayuan Chen" <jiayuan.chen@...pee.com>,
	"Johannes Weiner" <hannes@...xchg.org>,
	"Michal Hocko" <mhocko@...nel.org>,
	"Roman Gushchin" <roman.gushchin@...ux.dev>,
	"Shakeel Butt" <shakeel.butt@...ux.dev>,
	"Muchun Song" <muchun.song@...ux.dev>,
	"Yosry Ahmed" <yosry.ahmed@...ux.dev>,
	"Nhat Pham" <nphamcs@...il.com>,
	"Chengming Zhou" <chengming.zhou@...ux.dev>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Nick Terrell" <terrelln@...com>,
	"David Sterba" <dsterba@...e.com>,
	cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] mm: zswap: add per-memcg stat for incompressible pages

On Fri, 06 Feb 2026 02:47:58 +0000 "Jiayuan Chen" <jiayuan.chen@...ux.dev> wrote:

> February 6, 2026 at 10:21, "SeongJae Park" <sj@...nel.org mailto:sj@...nel.org?to=%22SeongJae%20Park%22%20%3Csj%40kernel.org%3E > wrote:
> 
> 
> > 
> > On Thu, 5 Feb 2026 13:30:12 +0800 Jiayuan Chen <jiayuan.chen@...ux.dev> wrote:
[...]
> > >  @@ -941,7 +941,7 @@ static bool zswap_decompress(struct zswap_entry *entry, struct folio *folio)
> > >  zs_obj_read_sg_begin(pool->zs_pool, entry->handle, input, entry->length);
> > >  
> > >  /* zswap entries of length PAGE_SIZE are not compressed. */
> > >  - if (entry->length = PAGE_SIZE) {
> > >  + if (zswap_is_raw(entry->length)) {
> > >  WARN_ON_ONCE(input->length != PAGE_SIZE);
> > >  memcpy_from_sglist(kmap_local_folio(folio, 0), input, 0, PAGE_SIZE);
> > >  dlen = PAGE_SIZE;
> > > 
> > Below this part, I show 'dlen = PAGE_SIZE'. Should it also be converted to
> > use the helper function?
> > 
> 
> The dlen variable represents the decompressed (plaintext) size.
> Since we compress individual pages, the decompressed output should
> always be PAGE_SIZE in normal cases.
> 
> This check validates whether decompression produced the expected result, not whether the entry is incompressible.
> 
> Using zswap_is_incomp() here would be semantically incorrect - the helper is meant to check if an
> entry was stored without compression (i.e., compression failed to reduce size), while dlen = PAGE_SIZE
> verifies the output of decompression is valid.

You are right.  Thank you for kindly correcting me.


Thanks,
SJ