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Message-ID: <a20d605c-79a0-4d89-985b-9512a990b492@redhat.com>
Date: Sat, 16 Aug 2025 09:22:29 +0200
From: David Hildenbrand <david@...hat.com>
To: Huacai Chen <chenhuacai@...ngson.cn>, Huacai Chen
<chenhuacai@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-mm@...ck.org, Zi Yan <ziy@...dia.com>,
Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
Harry Yoo <harry.yoo@...cle.com>, linux-kernel@...r.kernel.org,
Minchan Kim <minchan@...nel.org>,
Sergey Senozhatsky <senozhatsky@...omium.org>,
"Michael S. Tsirkin" <mst@...hat.com>
Subject: Re: [PATCH] mm/migrate: Fix NULL movable_ops if CONFIG_ZSMALLOC=m
On 15.08.25 11:05, Huacai Chen wrote:
Hi,
please CC the appropriate maintainers next time. You missed (some)
balloon and zsmalloc maintainers.
> After commit 84caf98838a3e5f4bdb34 ("mm: stop storing migration_ops in
> page->mapping") we get such an error message if CONFIG_ZSMALLOC=m:
>
> WARNING: CPU: 3 PID: 42 at mm/migrate.c:142 isolate_movable_ops_page+0xa8/0x1c0
> CPU: 3 UID: 0 PID: 42 Comm: kcompactd0 Not tainted 6.16.0-rc5+ #2133 PREEMPT
> pc 9000000000540bd8 ra 9000000000540b84 tp 9000000100420000 sp 9000000100423a60
> a0 9000000100193a80 a1 000000000000000c a2 000000000000001b a3 ffffffffffffffff
> a4 ffffffffffffffff a5 0000000000000267 a6 0000000000000000 a7 9000000100423ae0
> t0 00000000000000f1 t1 00000000000000f6 t2 0000000000000000 t3 0000000000000001
> t4 ffffff00010eb834 t5 0000000000000040 t6 900000010c89d380 t7 90000000023fcc70
> t8 0000000000000018 u0 0000000000000000 s9 ffffff00010eb800 s0 ffffff00010eb800
> s1 000000000000000c s2 0000000000043ae0 s3 0000800000000000 s4 900000000219cc40
> s5 0000000000000000 s6 ffffff00010eb800 s7 0000000000000001 s8 90000000025b4000
> ra: 9000000000540b84 isolate_movable_ops_page+0x54/0x1c0
> ERA: 9000000000540bd8 isolate_movable_ops_page+0xa8/0x1c0
> CRMD: 000000b0 (PLV0 -IE -DA +PG DACF=CC DACM=CC -WE)
> PRMD: 00000004 (PPLV0 +PIE -PWE)
> EUEN: 00000000 (-FPE -SXE -ASXE -BTE)
> ECFG: 00071c1d (LIE=0,2-4,10-12 VS=7)
> ESTAT: 000c0000 [BRK] (IS= ECode=12 EsubCode=0)
> PRID: 0014c010 (Loongson-64bit, Loongson-3A5000)
> CPU: 3 UID: 0 PID: 42 Comm: kcompactd0 Not tainted 6.16.0-rc5+ #2133 PREEMPT
> Stack : 90000000021fd000 0000000000000000 9000000000247720 9000000100420000
> 90000001004236a0 90000001004236a8 0000000000000000 90000001004237e8
> 90000001004237e0 90000001004237e0 9000000100423550 0000000000000001
> 0000000000000001 90000001004236a8 725a84864a19e2d9 90000000023fcc58
> 9000000100420000 90000000024c6848 9000000002416848 0000000000000001
> 0000000000000000 000000000000000a 0000000007fe0000 ffffff00010eb800
> 0000000000000000 90000000021fd000 0000000000000000 900000000205cf30
> 000000000000008e 0000000000000009 ffffff00010eb800 0000000000000001
> 90000000025b4000 0000000000000000 900000000024773c 00007ffff103d748
> 00000000000000b0 0000000000000004 0000000000000000 0000000000071c1d
> ...
> Call Trace:
> [<900000000024773c>] show_stack+0x5c/0x190
> [<90000000002415e0>] dump_stack_lvl+0x70/0x9c
> [<90000000004abe6c>] isolate_migratepages_block+0x3bc/0x16e0
> [<90000000004af408>] compact_zone+0x558/0x1000
> [<90000000004b0068>] compact_node+0xa8/0x1e0
> [<90000000004b0aa4>] kcompactd+0x394/0x410
> [<90000000002b3c98>] kthread+0x128/0x140
> [<9000000001779148>] ret_from_kernel_thread+0x28/0xc0
> [<9000000000245528>] ret_from_kernel_thread_asm+0x10/0x88
>
> The reason is that defined(CONFIG_ZSMALLOC) evaluates to 1 only when
> CONFIG_ZSMALLOC=y, we should use IS_ENABLED(CONFIG_ZSMALLOC) instead.
Ouch, I missed that CONFIG_ZSMALLOC can be configured like that. I
thought it would always be builtin.
> But when I use IS_ENABLED(CONFIG_ZSMALLOC), page_movable_ops() cannot
> access zsmalloc_mops because zsmalloc_mops is in a module.
>
> To solve this problem, we define a movable_ops[] array in mm/migrate.c,
> initialise its elements at mm/balloon_compaction.c & mm/zsmalloc.c, and
> let the page_movable_ops() function return elements from movable_ops[].
Before I took that easy route to just get it working quickly, I
envisioned a proper registration interface. See below.
>
> Fixes: 84caf98838a3e5f ("mm: stop storing migration_ops in page->mapping")
> Signed-off-by: Huacai Chen <chenhuacai@...ngson.cn>
> ---
> include/linux/migrate.h | 6 ++++++
> mm/balloon_compaction.c | 7 +++++++
> mm/migrate.c | 16 ++++++++++------
> mm/zsmalloc.c | 6 ++++++
> 4 files changed, 29 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/migrate.h b/include/linux/migrate.h
> index acadd41e0b5c..58fac171e4d5 100644
> --- a/include/linux/migrate.h
> +++ b/include/linux/migrate.h
> @@ -57,6 +57,12 @@ struct movable_operations {
> void (*putback_page)(struct page *);
> };
>
> +#define MOVABLE_BALLOON 0
> +#define MOVABLE_ZSMALLOC 1
> +#define MOVABLE_MAX 2
> +
> +extern const struct movable_operations *movable_ops[MOVABLE_MAX];
> +
> /* Defined in mm/debug.c: */
> extern const char *migrate_reason_names[MR_TYPES];
>
> diff --git a/mm/balloon_compaction.c b/mm/balloon_compaction.c
> index 2a4a649805c1..a1d2625b5c39 100644
> --- a/mm/balloon_compaction.c
> +++ b/mm/balloon_compaction.c
> @@ -254,4 +254,11 @@ const struct movable_operations balloon_mops = {
> .putback_page = balloon_page_putback,
> };
>
> +static int __init balloon_init(void)
> +{
> + movable_ops[MOVABLE_BALLOON] = &balloon_mops;
> + return 0;
> +}
> +core_initcall(balloon_init);
> +
> #endif /* CONFIG_BALLOON_COMPACTION */
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 425401b2d4e1..92918fab3e1a 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -53,6 +53,10 @@
> #include "internal.h"
> #include "swap.h"
>
> +const struct movable_operations *movable_ops[MOVABLE_MAX];
> +
> +EXPORT_SYMBOL(movable_ops);
> +
> static const struct movable_operations *page_movable_ops(struct page *page)
> {
> VM_WARN_ON_ONCE_PAGE(!page_has_movable_ops(page), page);
> @@ -62,15 +66,15 @@ static const struct movable_operations *page_movable_ops(struct page *page)
> * it as movable, the page type must be sticky until the page gets freed
> * back to the buddy.
> */
> -#ifdef CONFIG_BALLOON_COMPACTION
> +#if IS_ENABLED(CONFIG_BALLOON_COMPACTION)
> if (PageOffline(page))
> /* Only balloon compaction sets PageOffline pages movable. */
> - return &balloon_mops;
> -#endif /* CONFIG_BALLOON_COMPACTION */
> -#if defined(CONFIG_ZSMALLOC) && defined(CONFIG_COMPACTION)
> + return movable_ops[MOVABLE_BALLOON];
> +#endif /* IS_ENABLED(CONFIG_BALLOON_COMPACTION) */
> +#if IS_ENABLED(CONFIG_ZSMALLOC) && IS_ENABLED(CONFIG_COMPACTION)
> if (PageZsmalloc(page))
> - return &zsmalloc_mops;
> -#endif /* defined(CONFIG_ZSMALLOC) && defined(CONFIG_COMPACTION) */
> + return movable_ops[MOVABLE_ZSMALLOC];
> +#endif /* IS_ENABLED(CONFIG_ZSMALLOC) && IS_ENABLED(CONFIG_COMPACTION) */
> return NULL;
> }
>
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index 2c5e56a65354..052fcc0d29e5 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -2246,6 +2246,9 @@ EXPORT_SYMBOL_GPL(zs_destroy_pool);
>
> static int __init zs_init(void)
> {
> +#ifdef CONFIG_MIGRATION
> + movable_ops[MOVABLE_ZSMALLOC] = &zsmalloc_mops;
> +#endif
No, not like this.
Please add a proper registration function, and use the page type as an
input.
int register_movable_ops(const struct movable_operations *, enum
pagetype);
Probably best to refuse any page_type except PGTY_offline and
PGTY_zsmalloc for now, and let the function fail with -EBUSY if already
registered. The function can have a stub that returns 0 without
CONFIG_MIGRATION.
The you can do here
int rc;
if (IS_ENABLED(CONFIG_MIGRATION)) {
rc = register_movable_ops(&zsmalloc_mops, PGTY_zsmalloc);
if (WARN_ON_ONCE(rc))
return rc;
}
--
Cheers
David / dhildenb
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