Re: [RFC PATCH v3 00/21] mm/zsmalloc: Split zsdesc from struct page

[Hyeonggon Yoo <42.hyeyoo@gmail.com>]

On Sat, Dec 2, 2023 at 1:36 PM Sergey Senozhatsky
<senozhatsky@chromium.org> wrote:
>
> On (23/12/01 11:28), Minchan Kim wrote:
> > On Thu, Nov 30, 2023 at 07:12:21PM +0900, Hyeonggon Yoo wrote:
> > > RFC v2: https://lore.kernel.org/linux-mm/20230713042037.980211-1-42.hyeyoo@gmail.com/
> > >
> > > v2 -> v3:
> > >  - rebased to the latest mm-unstable
> > >  - adjusted comments from Sergey Senozhatsky (Moving zsdesc definition,
> > >    kerneldoc fix) and Yosry Ahmed (adding memcg_data field to zsdesc)
> > >
> > >
> > > V3 update is a bit late, but I still believe this is worth doing.
> > > It would be nice to get comments/reviews/acks from maintainers/people.
> > >
> > > Cover Letter:
> > >
> > > The purpose of this series is to define own memory descriptor for zsmalloc,
> > > instead of re-using various fields of struct page. This is a part of the
> > > effort to reduce the size of struct page to unsigned long and enable
> > > dynamic allocation of memory descriptors.
> > >
> > > While [1] outlines this ultimate objective, the current use of struct page
> > > is highly dependent on its definition, making it challenging to separately
> > > allocate memory descriptors.
> > >
> > > Therefore, this series introduces new descriptor for zsmalloc, called
> > > zsdesc. It overlays struct page for now, but will eventually be allocated
> > > independently in the future. And apart from dynamic allocation of descriptors,
> > > this is a nice cleanup.
> >
> > And the new descriptor doesn't bloat anything for zsmalloc meta size. Right?
> > Please specify it into the description.
>
> Right, that popped up [1] previously but I'm not sure if we had a definitive
> answer.
> https://lore.kernel.org/lkml/20230720071826.GE955071@google.com/

Oh, I thought I did, maybe I missed it.

Let me explain it here, it does not bloat the struct page:
> static_assert(sizeof(struct zsdesc) <= sizeof(struct page));
Otherwise the compiler will complain.

So with this patch series, it does not bloat the size of struct page,
nor allocates
additional memory for zsmalloc. Will add that in the description of
the next revision.

As Matthew pointed, in the future struct page and zsdesc may end up being
shrunken and separately allocated. What this series does is only
separating definition
of the descriptor (but still overlaying struct page)

Thanks a lot!
--
Hyeonggon