linux-kernel - Re: [RFC PATCH v2 00/21] mm/zsmalloc: Split zsdesc from struct page

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Date:   Thu, 20 Jul 2023 14:57:15 -0700
From:   Yosry Ahmed <yosryahmed@...gle.com>
To:     Hyeonggon Yoo <42.hyeyoo@...il.com>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Minchan Kim <minchan@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Matthew Wilcox <willy@...radead.org>,
        Mike Rapoport <rppt@...nel.org>
Subject: Re: [RFC PATCH v2 00/21] mm/zsmalloc: Split zsdesc from struct page

On Thu, Jul 20, 2023 at 2:52 PM Hyeonggon Yoo <42.hyeyoo@...il.com> wrote:
>
> On Fri, Jul 21, 2023 at 6:39 AM Yosry Ahmed <yosryahmed@...gle.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 the
> > > > > > current size of struct page. If that's true, then there is no loss,
> > > > >
> > > > > Yeah, zsdesc would be 56 bytes on 64 bit CPUs as memcg_data field is
> > > > > 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 work.
> 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.

Right.

For this patch series, do we need to maintain memcg_data in zsdec to
avoid any subtle problems?