Re: [PATCH v5 9/9] mm: switch deferred split shrinker to list_lru

[SeongJae Park <sj@kernel.org>]

Hi Johannes,

On Wed, 27 May 2026 16:45:16 -0400 Johannes Weiner <hannes@cmpxchg.org> wrote:

> The deferred split queue handles cgroups in a suboptimal fashion. The
> queue is per-NUMA node or per-cgroup, not the intersection. That means
> on a cgrouped system, a node-restricted allocation entering reclaim
> can end up splitting large pages on other nodes:
> 
>         alloc/unmap
>           deferred_split_folio()
>             list_add_tail(memcg->split_queue)
>             set_shrinker_bit(memcg, node, deferred_shrinker_id)
> 
>         for_each_zone_zonelist_nodemask(restricted_nodes)
>           mem_cgroup_iter()
>             shrink_slab(node, memcg)
>               shrink_slab_memcg(node, memcg)
>                 if test_shrinker_bit(memcg, node, deferred_shrinker_id)
>                   deferred_split_scan()
>                     walks memcg->split_queue
> 
> The shrinker bit adds an imperfect guard rail. As soon as the cgroup
> has a single large page on the node of interest, all large pages owned
> by that memcg, including those on other nodes, will be split.
> 
> list_lru properly sets up per-node, per-cgroup lists. As a bonus, it
> streamlines a lot of the list operations and reclaim walks. It's used
> widely by other major shrinkers already. Convert the deferred split
> queue as well.
> 
> The list_lru per-memcg heads are instantiated on demand when the first
> object of interest is allocated for a cgroup, by calling
> folio_memcg_alloc_deferred(). Add calls to where splittable pages are
> created: anon faults, swapin faults, khugepaged collapse.
> 
> These calls create all possible node heads for the cgroup at once, so
> the migration code (between nodes) doesn't need any special care.
> 
> Reported-by: Mikhail Zaslonko <zaslonko@linux.ibm.com>
> Tested-by: Mikhail Zaslonko <zaslonko@linux.ibm.com>
> Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
> Reviewed-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> ---
>  include/linux/huge_mm.h    |   7 +-
>  include/linux/memcontrol.h |   4 -
>  include/linux/mmzone.h     |  12 --
>  mm/huge_memory.c           | 364 +++++++++++++------------------------
>  mm/internal.h              |   2 +-
>  mm/khugepaged.c            |   5 +
>  mm/memcontrol.c            |  12 +-
>  mm/memory.c                |   4 +
>  mm/mm_init.c               |  15 --
>  mm/swap_state.c            |  10 +
>  10 files changed, 150 insertions(+), 285 deletions(-)
> 
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index edece3e26985..f6c2531a27a3 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -423,10 +423,10 @@ static inline int split_huge_page(struct page *page)
>  {
>  	return split_huge_page_to_list_to_order(page, NULL, 0);
>  }
> +
> +int folio_memcg_alloc_deferred(struct folio *folio);
> +
>  void deferred_split_folio(struct folio *folio, bool partially_mapped);
> -#ifdef CONFIG_MEMCG
> -void reparent_deferred_split_queue(struct mem_cgroup *memcg);
> -#endif
>  
>  void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
>  		unsigned long address, bool freeze);
> @@ -664,7 +664,6 @@ static inline int folio_split(struct folio *folio, unsigned int new_order,
>  }
>  
>  static inline void deferred_split_folio(struct folio *folio, bool partially_mapped) {}
> -static inline void reparent_deferred_split_queue(struct mem_cgroup *memcg) {}
>  #define split_huge_pmd(__vma, __pmd, __address)	\
>  	do { } while (0)

I found this patch is now in mm-new and it makes UM mode kunit fails like
below.

    $ ./tools/testing/kunit/kunit.py run --kunitconfig mm/damon/tests/
    [00:00:02] Configuring KUnit Kernel ...
    [00:00:02] Building KUnit Kernel ...
    Populating config with:
    $ make ARCH=um O=.kunit olddefconfig
    Building with:
    $ make all compile_commands.json scripts_gdb ARCH=um O=.kunit --jobs=8
    ERROR:root:../mm/swap_state.c: In function ‘__swap_cache_alloc’:
    ../mm/swap_state.c:468:26: error: implicit declaration of function ‘folio_memcg_alloc_deferred’ [-Wimplicit-function-declaration]
      468 |         if (order > 1 && folio_memcg_alloc_deferred(folio)) {
          |                          ^~~~~~~~~~~~~~~~~~~~~~~~~~
    make[4]: *** [../scripts/Makefile.build:289: mm/swap_state.o] Error 1
    make[4]: *** Waiting for unfinished jobs....
    make[3]: *** [../scripts/Makefile.build:548: mm] Error 2
    make[3]: *** Waiting for unfinished jobs....
    make[2]: *** [/home/lkhack/linux/Makefile:2143: .] Error 2
    make[1]: *** [/home/lkhack/linux/Makefile:248: __sub-make] Error 2
    make: *** [Makefile:248: __sub-make] Error 2

Maybe we can define the function for CONFIG_TRANSPARENT_HUGEPAGE unset case?  I
confirmed the below attaching temporal fix works for at least kunit.


Thanks,
SJ

[...]
=== >8 ===
From 23b5800dd49085707baee5774b74782c3e424f24 Mon Sep 17 00:00:00 2001
From: SeongJae Park <sj@kernel.org>
Date: Wed, 27 May 2026 23:58:07 -0700
Subject: [PATCH] mm/huge_mm: define memcg_alloc_deferred() for
 !CONFIG_TRANSPARENT_HUGEPPAGE
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Without this, UM mode kunit fails like below.

    $ ./tools/testing/kunit/kunit.py run --kunitconfig mm/damon/tests/
    [00:00:02] Configuring KUnit Kernel ...
    [00:00:02] Building KUnit Kernel ...
    Populating config with:
    $ make ARCH=um O=.kunit olddefconfig
    Building with:
    $ make all compile_commands.json scripts_gdb ARCH=um O=.kunit --jobs=8
    ERROR:root:../mm/swap_state.c: In function ‘__swap_cache_alloc’:
    ../mm/swap_state.c:468:26: error: implicit declaration of function ‘folio_memcg_alloc_deferred’ [-Wimplicit-function-declaration]
      468 |         if (order > 1 && folio_memcg_alloc_deferred(folio)) {
          |                          ^~~~~~~~~~~~~~~~~~~~~~~~~~
    make[4]: *** [../scripts/Makefile.build:289: mm/swap_state.o] Error 1
    make[4]: *** Waiting for unfinished jobs....
    make[3]: *** [../scripts/Makefile.build:548: mm] Error 2
    make[3]: *** Waiting for unfinished jobs....
    make[2]: *** [/home/lkhack/linux/Makefile:2143: .] Error 2
    make[1]: *** [/home/lkhack/linux/Makefile:248: __sub-make] Error 2
    make: *** [Makefile:248: __sub-make] Error 2

Fix by implementing the function for CONFIG_TRANSPARENT_HUGEPPAGE unset
case.

Fixes: https://lore.kernel.org/20260527204757.2544958-10-hannes@cmpxchg.org
Signed-off-by: SeongJae Park <sj@kernel.org>
---
 include/linux/huge_mm.h | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index f6c2531a27a35..055de7b8ed487 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -663,6 +663,11 @@ static inline int folio_split(struct folio *folio, unsigned int new_order,
 	return -EINVAL;
 }
 
+static inline int folio_memcg_alloc_deferred(struct folio *folio)
+{
+	return 0;
+}
+
 static inline void deferred_split_folio(struct folio *folio, bool partially_mapped) {}
 #define split_huge_pmd(__vma, __pmd, __address)	\
 	do { } while (0)
-- 
2.47.3