linux-kernel - Re: [PATCH 1/1] mm/vmscan: avoid split PMD-mapped THP during shrink_folio

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Date: Wed, 17 Apr 2024 16:08:55 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Lance Yang <ioworker0@...il.com>
Cc: akpm@...ux-foundation.org, maskray@...gle.com, ziy@...dia.com,
	ryan.roberts@....com, david@...hat.com, 21cnbao@...il.com,
	mhocko@...e.com, fengwei.yin@...el.com, zokeefe@...gle.com,
	shy828301@...il.com, xiehuan09@...il.com,
	wangkefeng.wang@...wei.com, songmuchun@...edance.com,
	peterx@...hat.com, minchan@...nel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] mm/vmscan: avoid split PMD-mapped THP during
 shrink_folio_list()

On Wed, Apr 17, 2024 at 10:11:11PM +0800, Lance Yang wrote:
> When the user no longer requires the pages, they would use madvise(madv_free)
> to mark the pages as lazy free. IMO, they would not typically rewrite to the
> given range.
> 
> At present, a PMD-mapped THP marked as lazyfree during shrink_folio_list()
> is unconditionally split, which may be unnecessary. If the THP is exclusively
> mapped and clean, and the PMD associated with it is also clean, then we can
> attempt to remove the PMD mapping from it. This change will improve the
> efficiency of memory reclamation in this case.
> 
> On an Intel i5 CPU, reclaiming 1GiB of PMD-mapped THPs using
> mem_cgroup_force_empty() results in the following runtimes in seconds
> (shorter is better):
> 
> --------------------------------------------
> |     Old       |      New       |  Change  |
> --------------------------------------------
> |   0.683426    |    0.049197    |  -92.80% |
> --------------------------------------------
> 
> Signed-off-by: Lance Yang <ioworker0@...il.com>
> ---
>  include/linux/huge_mm.h |  1 +
>  include/linux/rmap.h    |  1 +
>  mm/huge_memory.c        |  2 +-
>  mm/rmap.c               | 81 +++++++++++++++++++++++++++++++++++++++++
>  mm/vmscan.c             |  7 ++++
>  5 files changed, 91 insertions(+), 1 deletion(-)

I'm confused why we need all this extra code.  If we remove a folio
from the pagecache, we can just call truncate_inode_folio() and
unmap_mapping_folio() takes care of all the necessary unmappings.
Why can't you call unmap_mapping_folio() here?