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Harry Yoo (Meta) af2fb3ee66 mm/slab: do not wake up kswapd in __kfree_rcu_sheaf()
Since kfree_rcu() can be called under pi_lock (a raw spinlock in the
scheduler), kfree_rcu() itself should never allocate memory with
__GFP_KSWAPD_RECLAIM as waking up kswapd ends up acquiring pi_lock,
which leads to a deadlock.

Reproducing the issue even intentionally was not straightforward.
The set_cpus_allowed_force() path that is called under pi_lock is
exercised very rarely, and competing tasks that allocate memory will
most likely wake kswapd up.

Therefore the existence of the deadlock was verified with a modified
kernel that has a lockdep map for waking up kswapd:

  ======================================================
  WARNING: possible circular locking dependency detected
  7.2.0-rc1-slab-for-next+ #10 Not tainted
  ------------------------------------------------------
  git/6660 is trying to acquire lock:
  ffff8e524533c550 (&p->pi_lock){-.-.}-{2:2}, at: _raw_spin_lock_irqsave+0x12/0x20

  but task is already holding lock:
  ffff8e57bffff1a0 (&pgdat->kswapd_wait){....}-{3:3}, at: _raw_spin_lock_irqsave+0x12/0x20

  which lock already depends on the new lock.

  the existing dependency chain (in reverse order) is:

  -> #2 (&pgdat->kswapd_wait){....}-{3:3}:
         __lock_acquire+0x5a4/0xc50
         lock_acquire.part.0+0xb7/0x240
         lock_acquire+0x70/0x170
         __raw_spin_lock_irqsave+0x44/0x80
         _raw_spin_lock_irqsave+0x12/0x20
         __wake_up_common_lock+0x31/0xa0
         __wake_up+0x20/0x40
         kswapd_wakeup_wake+0x7e/0x110
         wakeup_kswapd+0x295/0x320
         wake_all_kswapds+0xac/0x1a0
         __alloc_pages_slowpath.constprop.0+0x282/0xf80
         __alloc_frozen_pages_noprof+0x32a/0x360
         alloc_slab_page+0x2e/0x160
         allocate_slab+0x82/0x420
         new_slab+0x52/0xb0
         refill_objects+0x13d/0x190
         refill_sheaf+0x5d/0xd0
         __pcs_replace_empty_main+0x230/0xb10
         [...]

  -> #1 (kswapd_wakeup){-.-.}-{0:0}:
         __lock_acquire+0x5a4/0xc50
         lock_sync.part.0+0x75/0x100
         lock_sync+0x36/0x70
         might_wakeup_kswapd+0x61/0xa0
         __kfree_rcu_sheaf+0x33/0xd90
         kvfree_call_rcu+0x1d4/0x3b0
         set_cpus_allowed_force+0x163/0x220
         cpuset_cpus_allowed_fallback+0x18b/0x240
         select_fallback_rq+0x1e6/0x250
	 [...]

  -> #0 (&p->pi_lock){-.-.}-{2:2}:
         check_prev_add+0xe6/0xe00
         validate_chain+0x51e/0x6e0
         __lock_acquire+0x5a4/0xc50
         lock_acquire.part.0+0xb7/0x240
         lock_acquire+0x70/0x170
         __raw_spin_lock_irqsave+0x44/0x80
         _raw_spin_lock_irqsave+0x12/0x20
         try_to_wake_up+0x77/0xa90
         default_wake_function+0x27/0x60
         autoremove_wake_function+0x23/0xb0
         __wake_up_common+0xb8/0x170
         __wake_up_common_lock+0x51/0xa0
         __wake_up+0x20/0x40
         kswapd_wakeup_wake+0x7e/0x110
         wakeup_kswapd+0x295/0x320
         wake_all_kswapds+0xac/0x1a0
         __alloc_pages_slowpath.constprop.0+0x282/0xf80
         __alloc_frozen_pages_noprof+0x32a/0x360
         alloc_slab_page+0x2e/0x160
         allocate_slab+0x82/0x420
         new_slab+0x52/0xb0
         refill_objects+0x13d/0x190
         refill_sheaf+0x5d/0xd0
         __pcs_replace_empty_main+0x230/0xb10
         [...]

  other info that might help us debug this:

  Chain exists of:
    &p->pi_lock --> kswapd_wakeup --> &pgdat->kswapd_wait

   Possible unsafe locking scenario:

         CPU0                    CPU1
         ----                    ----
    lock(&pgdat->kswapd_wait);
                                 lock(kswapd_wakeup);
                                 lock(&pgdat->kswapd_wait);
    lock(&p->pi_lock);

   *** DEADLOCK ***

  3 locks held by git/6660:
   #0: ffff8e519f0e9e20 (&type->i_mutex_dir_key#6){++++}-{4:4}, at: lookup_slow+0x2d/0x60
   #1: ffffffff8a31c460 (kswapd_wakeup){-.-.}-{0:0}, at: kswapd_wakeup_wake+0x4d/0x110
   #2: ffff8e57bffff1a0 (&pgdat->kswapd_wait){....}-{3:3}, at: _raw_spin_lock_irqsave+0x12/0x20

  [...]

Fix this by always avoiding waking up kswapd in __kfree_rcu_sheaf().
Note that there are two paths that might wake up kswapd:

  1) __kfree_rcu_sheaf()
     // __GFP_KSWAPD_RECLAIM might wake up kswapd
     -> alloc_empty_sheaf(GFP_NOWAIT)

  2) __kfree_rcu_sheaf()
     // Let's say __kfree_rcu_sheaf() doesn't pass GFP_NOWAIT
     -> alloc_empty_sheaf(__GFP_NOWARN)
     -> kmalloc_flags()
     -> slab_alloc_node()
     -> alloc_from_pcs()
     -> __pcs_replace_empty_main()
     // Free a sheaf in an allocation path when the sheaf becomes empty
     // and refilling the sheaf fails
     -> free_empty_sheaf()
     -> slab_free()
     -> free_to_pcs()
     -> __pcs_replace_full_main()
     // However free path always assumes it's safe to wake up kswapd
     -> alloc_empty_sheaf(GFP_NOWAIT)

Drop __GFP_KSWAPD_RECLAIM in both cases. Note that the kfree_rcu() is
not the only user of free_to_pcs() path, but it should be fixed as it
can be invoked under pi_lock.

Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260831-b4-kfree_rcu_hotfix-v1-1-4f0fb882638b%40kernel.org
Fixes: ec66e0d599 ("slab: add sheaf support for batching kfree_rcu() operations")
Link: https://lore.kernel.org/linux-mm/20260831143500.x-saxdAs@linutronix.de
Assisted-by: LLM # dicovery and verification
Signed-off-by: Harry Yoo (Meta) <harry@kernel.org>
Link: https://patch.msgid.link/20260922-kfree-rcu-dont-wakeup-kswapd-v1-1-42d7e2636f9e@kernel.org
Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
2026-09-24 10:14:02 +02:00
arch x86 fixes: 2026-09-20 09:49:10 -07:00
block block: save page offset gaps in cloned bio 2026-09-01 08:42:34 -06:00
certs treewide: fix indentation and whitespace in Kconfig files 2026-05-28 21:24:42 -07:00
crypto This push fixes a memory allocation overrun in crypto acomp. 2026-08-28 09:28:40 -07:00
Documentation Input updates for v7.3-rc3 2026-09-20 07:02:34 -07:00
drivers dmaengine fixes for v7.3 2026-09-20 10:45:22 -07:00
fs Timer race fixes: 2026-09-20 09:41:00 -07:00
include Input updates for v7.3-rc3 2026-09-20 07:02:34 -07:00
init Bootconfig fixes for v7.3-rc3 2026-09-13 09:16:36 -07:00
io_uring io_uring-7.3-20260911 2026-09-11 12:36:13 -07:00
ipc mm.git review status for master..mm-nonmm-stable 2026-08-23 08:07:11 -07:00
kernel Timer race fixes: 2026-09-20 09:41:00 -07:00
lib 14 hotfixes. 10 are cc:stable. 11 are for MM. 2026-09-14 08:59:09 -07:00
LICENSES LICENSES: Add modern form of the LGPL-2.1 tags to the usage guide section 2025-10-22 07:58:19 +02:00
mm mm/slab: do not wake up kswapd in __kfree_rcu_sheaf() 2026-09-24 10:14:02 +02:00
net net: qrtr: resend HELLO on MHI resume 2026-09-20 11:21:10 -07:00
rust Including fixes from Netfilter, Bluetooth, IPSec and WiFi. 2026-09-17 10:40:48 -07:00
samples rust: samples: add missing newlines in rust_print_main 2026-09-01 02:20:22 +02:00
scripts x86 fixes: 2026-09-20 09:49:10 -07:00
security selinux/stable-7.3 PR 20260919 2026-09-20 06:50:31 -07:00
sound ASoC: Fixes for v7.3 2026-09-17 08:15:32 +02:00
tools x86 fixes: 2026-09-20 09:49:10 -07:00
usr usr: Correct a spelling by changing a letter 2026-08-05 14:51:58 +02:00
virt Arm: 2026-08-25 11:48:04 -07:00
.clang-format drm/bridge: rename drm_for_each_bridge_in_chain_scoped() to drm_for_each_bridge_in_chain() 2026-07-14 17:55:51 +02:00
.clippy.toml rust: bump Clippy's MSRV and clean incompatible_msrv allows 2026-04-07 09:51:39 +02:00
.cocciconfig
.editorconfig editorconfig: add rst extension 2026-01-26 19:07:09 -08:00
.get_maintainer.ignore .get_maintainer.ignore: add myself 2026-09-07 10:01:33 +02:00
.gitattributes .gitattributes: set diff driver for Rust source code files 2023-05-31 17:48:25 +02:00
.gitignore SPDX patches for 7.2-rc1 2026-06-22 12:06:22 -07:00
.mailmap 14 hotfixes. 10 are cc:stable. 11 are for MM. 2026-09-14 08:59:09 -07:00
.pylintrc docs: Move the python libraries to tools/lib/python 2025-11-18 09:22:40 -07:00
.rustfmt.toml rust: add .rustfmt.toml 2022-09-28 09:02:20 +02:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS Driver core fixes for 7.3-rc2 2026-09-05 11:59:05 -07:00
Kbuild checksyscalls: move instance functionality into generic code 2026-04-05 09:21:32 +02:00
Kconfig io_uring: Rename KConfig to Kconfig 2025-02-19 14:53:27 -07:00
MAINTAINERS MAINTAINERS: update slab.git URL 2026-09-21 11:45:53 +01:00
Makefile Linux 7.3-rc4 2026-09-20 13:48:15 -07:00
README README: remove out of place emdashes 2026-07-15 10:51:29 -06:00

Linux kernel
============

The Linux kernel is the core of any Linux operating system. It manages hardware,
system resources, and provides the fundamental services for all other software.

Quick Start
-----------

* Report a bug: See Documentation/admin-guide/reporting-issues.rst
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-----------------------

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============

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==================

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--------------------

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-------------------

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-----------------------------

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--------------------

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----------

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-----------------------

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