linux-kernel - Re: [PATCH] Docs/mm/damon/design: describe the differences for pageout's virtual address and physcial address

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Date: Sat, 24 Feb 2024 14:08:19 -0800
From: SeongJae Park <sj@...nel.org>
To: Barry Song <21cnbao@...il.com>
Cc: corbet@....net,
	linux-mm@...ck.org,
	akpm@...ux-foundation.org,
	damon@...ts.linux.dev,
	linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Barry Song <v-songbaohua@...o.com>,
	SeongJae Park <sj@...nel.org>,
	Minchan Kim <minchan@...nel.org>,
	Michal Hocko <mhocko@...e.com>,
	Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH] Docs/mm/damon/design: describe the differences for pageout's virtual address and physcial address

Hi Barry,

On Sun, 25 Feb 2024 10:50:23 +1300 Barry Song <21cnbao@...il.com> wrote:

> From: Barry Song <v-songbaohua@...o.com>
> 
> The doc needs update. As only in the case of virtual address, we are
> calling madvise() with MADV_PAGEOUT; but in the case of physical
> address, we are actually calling reclaim_pages() directly. And
> madvise() with MADV_PAGEOUT is much more aggresive on reclamation
> compared to reclaim_pages() on paddr.

Thank you for this nice patch :)

> 
> Cc: SeongJae Park <sj@...nel.org>
> Cc: Minchan Kim <minchan@...nel.org>
> Cc: Michal Hocko <mhocko@...e.com>
> Cc: Johannes Weiner <hannes@...xchg.org>
> Signed-off-by: Barry Song <v-songbaohua@...o.com>
> ---
>  Documentation/mm/damon/design.rst | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/mm/damon/design.rst b/Documentation/mm/damon/design.rst
> index 8c89d26f0baa..6c50743552f5 100644
> --- a/Documentation/mm/damon/design.rst
> +++ b/Documentation/mm/damon/design.rst
> @@ -315,7 +315,13 @@ that supports each action are as below.
>     Supported by ``vaddr`` and ``fvaddr`` operations set.
>   - ``cold``: Call ``madvise()`` for the region with ``MADV_COLD``.
>     Supported by ``vaddr`` and ``fvaddr`` operations set.
> - - ``pageout``: Call ``madvise()`` for the region with ``MADV_PAGEOUT``.
> + - ``pageout``: Call ``madvise()`` for the region with ``MADV_PAGEOUT`` if
> +   the region is using virtual address; otherwise, call ``reclaim_pages()``
> +   for the region if physical address is used.
> +   ``madvise()``  with ``MADV_PAGEOUT``  is aggressively reclaiming memory
> +   when pages are not shared by multiple processes. Differently, paddr's
> +   pageout  will do the best-effort reclamation by scanning PTEs to figure
> +   out if pages are young, if not, pages are reclaimed.

I think this might be too much details.  Especially, explanation of
``MADV_PAGEOUT`` might better to be done on it's document.  We wouldn't want to
update two places for future changes to ``MADV_PAGEOUT``.  What do you think
about making it simple and independent from the implementation detail?  For
example,

    - - ``pageout``: Call ``madvise()`` for the region with ``MADV_PAGEOUT``.
    + - ``pageout``: Reclaim the region.

Thanks,
SJ

>     Supported by ``vaddr``, ``fvaddr`` and ``paddr`` operations set.
>   - ``hugepage``: Call ``madvise()`` for the region with ``MADV_HUGEPAGE``.
>     Supported by ``vaddr`` and ``fvaddr`` operations set.
> -- 
> 2.34.1