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From: Guopeng Zhang <guopeng.zhang@linux.dev>
To: longman@redhat.com,
	ridong.chen@linux.dev,
	cgroups@vger.kernel.org
Cc: tj@kernel.org,
	hannes@cmpxchg.org,
	mkoutny@suse.com,
	peterz@infradead.org,
	corbet@lwn.net,
	linux-doc@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	zhangguopeng@kylinos.cn
Subject: [PATCH v2] cgroup/cpuset: Remove obsolete PFA_SPREAD_SLAB task flag
Date: Tue, 11 Aug 2026 16:30:27 +0800
Message-ID: <20260811083027.140577-1-guopeng.zhang@linux.dev>
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From: Guopeng Zhang <zhangguopeng@kylinos.cn>

Commit 16a1d968358a ("mm/slab: remove mm/slab.c and slab_def.h")
removed the SLAB allocator, the only allocator that implemented cpuset
slab spreading. Commit 61a182ab61a6 ("cgroup/cpuset: Remove
cpuset_do_slab_mem_spread()") then removed the last task_spread_slab()
caller. Commit 3ab67a9ce82f ("cgroup/cpuset: Mark memory_spread_slab as
obsolete") marked the legacy control obsolete.

cpuset still updates PFA_SPREAD_SLAB when tasks attach to a legacy
cpuset and walks all tasks in a cpuset when memory_spread_slab changes.
Remove the unused task flag and its helpers, and make spread task
updates depend only on memory_spread_page.

Keep the memory_spread_slab control and CS_SPREAD_SLAB state so legacy
users retain the existing write, readback and inheritance behavior.
Update the comments and documentation to describe only page-cache
spreading as functional.

Assisted-by: LLM
Signed-off-by: Guopeng Zhang <zhangguopeng@kylinos.cn>
Reviewed-by: Waiman Long <longman@redhat.com
---
Changes in v2:
- Restore the default-value description and clarify the write semantics.
- Retain the generic slab allocator fallback note in the
  cpuset_spread_node() comment.

 .../admin-guide/cgroup-v1/cpusets.rst         | 42 +++++++------------
 include/linux/sched.h                         |  5 ---
 kernel/cgroup/cpuset-v1.c                     | 13 ++----
 kernel/cgroup/cpuset.c                        | 19 ++++-----
 4 files changed, 27 insertions(+), 52 deletions(-)

diff --git a/Documentation/admin-guide/cgroup-v1/cpusets.rst b/Documentatio=
n/admin-guide/cgroup-v1/cpusets.rst
index 52a213aff04e..fb3fb6fa88cb 100644
--- a/Documentation/admin-guide/cgroup-v1/cpusets.rst
+++ b/Documentation/admin-guide/cgroup-v1/cpusets.rst
@@ -179,7 +179,7 @@ files describing that cpuset:
  - cpuset.mem_hardwall flag:  is memory allocation hardwalled
  - cpuset.memory_pressure: measure of how much paging pressure in cpuset
  - cpuset.memory_spread_page flag: if set, spread page cache evenly on all=
owed nodes
- - cpuset.memory_spread_slab flag: OBSOLETE. Doesn't have any function.
+ - cpuset.memory_spread_slab flag: OBSOLETE. Has no effect on allocation b=
ehavior.
  - cpuset.sched_load_balance flag: if set, load balance within CPUs on tha=
t cpuset
  - cpuset.sched_relax_domain_level: the searching range when migrating tas=
ks
=20
@@ -318,26 +318,20 @@ times 1000.
=20
 1.6 What is memory spread ?
 ---------------------------
-There are two boolean flag files per cpuset that control where the
-kernel allocates pages for the file system buffers and related in
-kernel data structures.  They are called 'cpuset.memory_spread_page' and
-'cpuset.memory_spread_slab'.
+The 'cpuset.memory_spread_page' boolean flag file controls where the kernel
+allocates page-cache pages.
+The 'cpuset.memory_spread_slab' file is obsolete and has no effect on
+allocation behavior, but is retained for compatibility.
=20
 If the per-cpuset boolean flag file 'cpuset.memory_spread_page' is set, th=
en
 the kernel will spread the file system buffers (page cache) evenly
 over all the nodes that the faulting task is allowed to use, instead
 of preferring to put those pages on the node where the task is running.
=20
-If the per-cpuset boolean flag file 'cpuset.memory_spread_slab' is set,
-then the kernel will spread some file system related slab caches,
-such as for inodes and dentries evenly over all the nodes that the
-faulting task is allowed to use, instead of preferring to put those
-pages on the node where the task is running.
-
-The setting of these flags does not affect anonymous data segment or
+The setting of this flag does not affect anonymous data segment or
 stack segment pages of a task.
=20
-By default, both kinds of memory spreading are off, and memory
+By default, page cache memory spreading is off, and memory
 pages are allocated on the node local to where the task is running,
 except perhaps as modified by the task's NUMA mempolicy or cpuset
 configuration, so long as sufficient free memory pages are available.
@@ -345,18 +339,18 @@ configuration, so long as sufficient free memory page=
s are available.
 When new cpusets are created, they inherit the memory spread settings
 of their parent.
=20
-Setting memory spreading causes allocations for the affected page
-or slab caches to ignore the task's NUMA mempolicy and be spread
-instead.    Tasks using mbind() or set_mempolicy() calls to set NUMA
-mempolicies will not notice any change in these calls as a result of
-their containing task's memory spread settings.  If memory spreading
+Setting page cache memory spreading causes affected allocations to ignore =
the
+task's NUMA mempolicy and be spread instead. Tasks using mbind() or
+set_mempolicy() to set NUMA mempolicies will not notice any change as a
+result of their containing task's memory spread settings.  If memory sprea=
ding
 is turned off, then the currently specified NUMA mempolicy once again
 applies to memory page allocations.
=20
-Both 'cpuset.memory_spread_page' and 'cpuset.memory_spread_slab' are boole=
an flag
-files.  By default they contain "0", meaning that the feature is off
-for that cpuset.  If a "1" is written to that file, then that turns
-the named feature on.
+Both 'cpuset.memory_spread_page' and 'cpuset.memory_spread_slab' are boole=
an
+flag files. In the root cpuset, both files initially contain "0". Writing =
"1"
+or "0" to 'cpuset.memory_spread_page' enables or disables page-cache sprea=
ding,
+respectively. The value of 'cpuset.memory_spread_slab' is retained, can be=
 read
+back and inherited, but it does not affect allocation behavior.
=20
 The implementation is simple.
=20
@@ -367,10 +361,6 @@ is modified to perform an inline check for this PFA_SP=
READ_PAGE task
 flag, and if set, a call to a new routine cpuset_mem_spread_node()
 returns the node to prefer for the allocation.
=20
-Similarly, setting 'cpuset.memory_spread_slab' turns on the flag
-PFA_SPREAD_SLAB, and appropriately marked slab caches will allocate
-pages from the node returned by cpuset_mem_spread_node().
-
 The cpuset_mem_spread_node() routine is also simple.  It uses the
 value of a per-task rotor cpuset_mem_spread_rotor to select the next
 node in the current task's mems_allowed to prefer for the allocation.
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 499943987c1a..8b3d47a325cc 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1869,7 +1869,6 @@ static __always_inline bool is_user_task(struct task_=
struct *task)
 /* Per-process atomic flags. */
 #define PFA_NO_NEW_PRIVS		0	/* May not gain new privileges. */
 #define PFA_SPREAD_PAGE			1	/* Spread page cache over cpuset */
-#define PFA_SPREAD_SLAB			2	/* Spread some slab caches over cpuset */
 #define PFA_SPEC_SSB_DISABLE		3	/* Speculative Store Bypass disabled */
 #define PFA_SPEC_SSB_FORCE_DISABLE	4	/* Speculative Store Bypass force dis=
abled*/
 #define PFA_SPEC_IB_DISABLE		5	/* Indirect branch speculation restricted */
@@ -1895,10 +1894,6 @@ TASK_PFA_TEST(SPREAD_PAGE, spread_page)
 TASK_PFA_SET(SPREAD_PAGE, spread_page)
 TASK_PFA_CLEAR(SPREAD_PAGE, spread_page)
=20
-TASK_PFA_TEST(SPREAD_SLAB, spread_slab)
-TASK_PFA_SET(SPREAD_SLAB, spread_slab)
-TASK_PFA_CLEAR(SPREAD_SLAB, spread_slab)
-
 TASK_PFA_TEST(SPEC_SSB_DISABLE, spec_ssb_disable)
 TASK_PFA_SET(SPEC_SSB_DISABLE, spec_ssb_disable)
 TASK_PFA_CLEAR(SPEC_SSB_DISABLE, spec_ssb_disable)
diff --git a/kernel/cgroup/cpuset-v1.c b/kernel/cgroup/cpuset-v1.c
index 3e9968dd91e9..562ad35f00d0 100644
--- a/kernel/cgroup/cpuset-v1.c
+++ b/kernel/cgroup/cpuset-v1.c
@@ -204,7 +204,7 @@ static s64 cpuset_read_s64(struct cgroup_subsys_state *=
css, struct cftype *cft)
 }
=20
 /*
- * update task's spread flag if cpuset's page/slab spread flag is set
+ * Update a task's spread flag if the cpuset's page spread flag is set.
  *
  * Call with callback_lock or cpuset_mutex held. The check can be skipped
  * if on default hierarchy.
@@ -219,18 +219,13 @@ void cpuset1_update_task_spread_flags(struct cpuset *=
cs,
 		task_set_spread_page(tsk);
 	else
 		task_clear_spread_page(tsk);
-
-	if (is_spread_slab(cs))
-		task_set_spread_slab(tsk);
-	else
-		task_clear_spread_slab(tsk);
 }
=20
 /**
- * cpuset1_update_tasks_flags - update the spread flags of tasks in the cp=
uset.
- * @cs: the cpuset in which each task's spread flags needs to be changed
+ * cpuset1_update_tasks_flags - update the page spread flag of cpuset tasks
+ * @cs: the cpuset whose tasks need their page spread flag updated
  *
- * Iterate through each task of @cs updating its spread flags.  As this
+ * Iterate through each task of @cs updating its page spread flag.  As this
  * function is called with cpuset_mutex held, cpuset membership stays
  * stable.
  */
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 0a0fffb5673c..f1dd0fa56b3b 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -2862,7 +2862,7 @@ int cpuset_update_flag(cpuset_flagbits_t bit, struct =
cpuset *cs,
 {
 	struct cpuset *trialcs;
 	int balance_flag_changed;
-	int spread_flag_changed;
+	int spread_page_changed;
 	int err;
=20
 	trialcs =3D dup_or_alloc_cpuset(cs);
@@ -2881,8 +2881,7 @@ int cpuset_update_flag(cpuset_flagbits_t bit, struct =
cpuset *cs,
 	balance_flag_changed =3D (is_sched_load_balance(cs) !=3D
 				is_sched_load_balance(trialcs));
=20
-	spread_flag_changed =3D ((is_spread_slab(cs) !=3D is_spread_slab(trialcs))
-			|| (is_spread_page(cs) !=3D is_spread_page(trialcs)));
+	spread_page_changed =3D is_spread_page(cs) !=3D is_spread_page(trialcs);
=20
 	spin_lock_irq(&callback_lock);
 	cs->flags =3D trialcs->flags;
@@ -2895,7 +2894,7 @@ int cpuset_update_flag(cpuset_flagbits_t bit, struct =
cpuset *cs,
 			rebuild_sched_domains_locked();
 	}
=20
-	if (spread_flag_changed)
+	if (spread_page_changed)
 		cpuset1_update_tasks_flags(cs);
 out:
 	free_cpuset(trialcs);
@@ -4484,14 +4483,10 @@ void cpuset_nodes_allowed(struct cgroup *cgroup, no=
demask_t *mask)
  * cpuset_spread_node() - On which node to begin search for a page
  * @rotor: round robin rotor
  *
- * If a task is marked PF_SPREAD_PAGE or PF_SPREAD_SLAB (as for
- * tasks in a cpuset with is_spread_page or is_spread_slab set),
- * and if the memory allocation used cpuset_mem_spread_node()
- * to determine on which node to start looking, as it will for
- * certain page cache or slab cache pages such as used for file
- * system buffers and inode caches, then instead of starting on the
- * local node to look for a free page, rather spread the starting
- * node around the tasks mems_allowed nodes.
+ * If a task is marked PFA_SPREAD_PAGE and a page cache allocation uses
+ * cpuset_mem_spread_node() to determine where to start looking, spread the
+ * starting node around the task's mems_allowed nodes instead of starting =
on
+ * the local node.
  *
  * We don't have to worry about the returned node being offline
  * because "it can't happen", and even if it did, it would be ok.
--=20
2.43.0