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From:   Hyeonggon Yoo <42.hyeyoo@gmail.com>
Date:   Fri, 21 Jul 2023 06:52:05 +0900
Message-ID: <CAB=+i9Quz9iP2-Lq=oQfKVVnzPDtOaKMm=hUPbnRg5hRxH+qaA@mail.gmail.com>
Subject: Re: [RFC PATCH v2 00/21] mm/zsmalloc: Split zsdesc from struct page
To:     Yosry Ahmed <yosryahmed@google.com>
Cc:     Sergey Senozhatsky <senozhatsky@chromium.org>,
        Minchan Kim <minchan@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Matthew Wilcox <willy@infradead.org>,
        Mike Rapoport <rppt@kernel.org>
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On Fri, Jul 21, 2023 at 6:39=E2=80=AFAM Yosry Ahmed <yosryahmed@google.com>=
 wrote:
>
> <snip>
> >
> > > > > It seems to me though the sizeof(zsdesc) is actually 56 bytes (on
> > > > > 64-bit), so sizeof(zsdesc) + sizeof(memdesc) would be equal to th=
e
> > > > > current size of struct page. If that's true, then there is no los=
s,
> > > >
> > > > Yeah, zsdesc would be 56 bytes on 64 bit CPUs as memcg_data field i=
s
> > > > not used in zsmalloc.
> > > > More fields in the current struct page might not be needed in the
> > > > future, although it's hard to say at the moment.
> > > > but it's not a loss.
> > >
> > > Is page->memcg_data something that we can drop? Aren't there code
> > > paths that will check page->memcg_data even for kernel pages (e.g.
> > > __folio_put() -> __folio_put_small() -> mem_cgroup_uncharge() ) ?
> >
> > zsmalloc pages are not accounted for via __GFP_ACCOUNT,
>
> Yeah, but the code in the free path above will check page->memcg_data
> nonetheless to check if it is charged.

Right.

> I think to drop memcg_data we need to enlighten the code that some pages
> do not even have memcg_data at all

I agree with you. It should be one of the milestones for all of this to wor=
k.
It won't be complicated for the code to be aware of it, because there will =
be
a freeing (and uncharging if need) routine per type of descriptors.