Update:  The hang happens only when the kernel parameter is on the kernel command line (damon_reclaim.enabled=Y)

Enabling DAMON_RECLAIM and then enabling by writing Y to /sys/module/damon_reclaim/parameters/enabled after boot does not cause a hang.

That was poorly written.

Enabling DAMON_RECLAIM in the kernel config, but not enabling it on the kernel command line, doesn't hang - even when DAMON_RECLAIM is subsequently enabled after boot.

(switched to email.  Please respond via emailed reply-to-all, not via the
bugzilla web interface).

On Sat, 04 Jun 2022 15:49:50 +0000 bugzilla-daemon@kernel.org wrote:

> https://bugzilla.kernel.org/show_bug.cgi?id=216072
> 
>             Bug ID: 216072
>            Summary: regression:
>                     ccccccgcdkgekhjervgbdfbhdjugcjkfdhiegeuugugtHang at
>                     boot when DAMON is enabled
>            Product: Memory Management
>            Version: 2.5
>     Kernel Version: 5.19 pre-rc1
>           Hardware: All
>                 OS: Linux
>               Tree: Mainline
>             Status: NEW
>           Severity: normal
>           Priority: P1
>          Component: Other
>           Assignee: akpm@linux-foundation.org
>           Reporter: gwhite@kupulau.com
>         Regression: No
> 
> I see a hang on boot whenever DAMON is enabled.  The specific commit that
> causes this is listed below.  There is no printk / dmesg output, only the
> message about an initrd being loaded by EFIStup.  Then a hard hang.  Removing
> the commit below - or disabling DAMON entirely - fixes the issue.
> 
> commit 059342d1dd4e01d634184793fa3f8437e62afaa1
> Author: Hailong Tu <tuhailong@gmail.com>
> Date:   Fri Apr 29 14:37:00 2022 -0700
> 
>     mm/damon/reclaim: fix the timer always stays active
> 
>     The timer stays active even if the reclaim mechanism is never enabled. 
>     It
>     is unnecessary overhead can be completely avoided by using
>     module_param_cb() for enabled flag.
> 
>     Link:
> https://lkml.kernel.org/r/20220421125910.1052459-1-tuhailong@gmail.com
>     Signed-off-by: Hailong Tu <tuhailong@gmail.com>
>     Reviewed-by: SeongJae Park <sj@kernel.org>
>     Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> 
> ver_linux:
> 
> If some fields are empty or look unusual you may have an old version.
> Compare to the current minimal requirements in Documentation/Changes.
> 
> Linux surprise 5.18.0-GTW1 #1285 SMP PREEMPT_DYNAMIC Sat Jun 4 11:40:01 EDT
> 2022 x86_64 GNU/Linux
> 
> GNU C                   12.1.0
> GNU Make                4.3
> Binutils                2.38
> Util-linux              2.38
> Mount                   2.38
> Module-init-tools       29
> E2fsprogs               1.46.5
> Jfsutils                1.1.15
> Reiserfsprogs           3.6.27
> Xfsprogs                5.18.0
> PPP                     2.4.9
> Nfs-utils               2.6.1
> Bison                   3.8.2
> Flex                    2.6.4
> Linux C++ Library       6.0.30
> Dynamic linker (ldd)    2.35
> Procps                  3.3.17
> Net-tools               2.10
> Kbd                     2.5.0
> Console-tools           2.5.0
> Sh-utils                9.1
> Udev                    251
> Wireless-tools          30
> 
> lspci:
> 
>  ~/linux  ⌂ v5.18 …  lspci
> 00:00.0 Host bridge: Advanced Micro Devices, Inc. [AMD] Starship/Matisse Root
> Complex
> 00:00.2 IOMMU: Advanced Micro Devices, Inc. [AMD] Starship/Matisse IOMMU
> 00:01.0 Host bridge: Advanced Micro Devices, Inc. [AMD] Starship/Matisse PCIe
> Dummy Host Bridge
> 00:01.1 PCI bridge: Advanced Micro Devices, Inc. [AMD] Starship/Matisse GPP
> Bridge
> 00:01.2 PCI bridge: Advanced Micro Devices, Inc. [AMD] Starship/Matisse GPP
> Bridge
> 00:02.0 Host bridge: Advanced Micro Devices, Inc. [AMD] Starship/Matisse PCIe
> Dummy Host Bridge
> 00:03.0 Host bridge: Advanced Micro Devices, Inc. [AMD] Starship/Matisse PCIe
> Dummy Host Bridge
> 00:03.1 PCI bridge: Advanced Micro Devices, Inc. [AMD] Starship/Matisse GPP
> Bridge
> 00:03.2 PCI bridge: Advanced Micro Devices, Inc. [AMD] Starship/Matisse GPP
> Bridge
> 00:04.0 Host bridge: Advanced Micro Devices, Inc. [AMD] Starship/Matisse PCIe
> Dummy Host Bridge
> 00:05.0 Host bridge: Advanced Micro Devices, Inc. [AMD] Starship/Matisse PCIe
> Dummy Host Bridge
> 00:07.0 Host bridge: Advanced Micro Devices, Inc. [AMD] Starship/Matisse PCIe
> Dummy Host Bridge
> 00:07.1 PCI bridge: Advanced Micro Devices, Inc. [AMD] Starship/Matisse
> Internal PCIe GPP Bridge 0 to bus[E:B]
> 00:08.0 Host bridge: Advanced Micro Devices, Inc. [AMD] Starship/Matisse PCIe
> Dummy Host Bridge
> 00:08.1 PCI bridge: Advanced Micro Devices, Inc. [AMD] Starship/Matisse
> Internal PCIe GPP Bridge 0 to bus[E:B]
> 00:14.0 SMBus: Advanced Micro Devices, Inc. [AMD] FCH SMBus Controller (rev
> 61)
> 00:14.3 ISA bridge: Advanced Micro Devices, Inc. [AMD] FCH LPC Bridge (rev
> 51)
> 00:18.0 Host bridge: Advanced Micro Devices, Inc. [AMD] Matisse/Vermeer Data
> Fabric: Device 18h; Function 0
> 00:18.1 Host bridge: Advanced Micro Devices, Inc. [AMD] Matisse/Vermeer Data
> Fabric: Device 18h; Function 1
> 00:18.2 Host bridge: Advanced Micro Devices, Inc. [AMD] Matisse/Vermeer Data
> Fabric: Device 18h; Function 2
> 00:18.3 Host bridge: Advanced Micro Devices, Inc. [AMD] Matisse/Vermeer Data
> Fabric: Device 18h; Function 3
> 00:18.4 Host bridge: Advanced Micro Devices, Inc. [AMD] Matisse/Vermeer Data
> Fabric: Device 18h; Function 4
> 00:18.5 Host bridge: Advanced Micro Devices, Inc. [AMD] Matisse/Vermeer Data
> Fabric: Device 18h; Function 5
> 00:18.6 Host bridge: Advanced Micro Devices, Inc. [AMD] Matisse/Vermeer Data
> Fabric: Device 18h; Function 6
> 00:18.7 Host bridge: Advanced Micro Devices, Inc. [AMD] Matisse/Vermeer Data
> Fabric: Device 18h; Function 7
> 01:00.0 Non-Volatile memory controller: Sandisk Corp WD PC SN810 / Black
> SN850
> NVMe SSD (rev 01)
> 02:00.0 PCI bridge: Advanced Micro Devices, Inc. [AMD] Matisse Switch
> Upstream
> 03:00.0 PCI bridge: Advanced Micro Devices, Inc. [AMD] Matisse PCIe GPP
> Bridge
> 03:01.0 PCI bridge: Advanced Micro Devices, Inc. [AMD] Matisse PCIe GPP
> Bridge
> 03:03.0 PCI bridge: Advanced Micro Devices, Inc. [AMD] Matisse PCIe GPP
> Bridge
> 03:04.0 PCI bridge: Advanced Micro Devices, Inc. [AMD] Matisse PCIe GPP
> Bridge
> 03:05.0 PCI bridge: Advanced Micro Devices, Inc. [AMD] Matisse PCIe GPP
> Bridge
> 03:08.0 PCI bridge: Advanced Micro Devices, Inc. [AMD] Matisse PCIe GPP
> Bridge
> 03:09.0 PCI bridge: Advanced Micro Devices, Inc. [AMD] Matisse PCIe GPP
> Bridge
> 03:0a.0 PCI bridge: Advanced Micro Devices, Inc. [AMD] Matisse PCIe GPP
> Bridge
> 04:00.0 Non-Volatile memory controller: Samsung Electronics Co Ltd NVMe SSD
> Controller SM981/PM981/PM983
> 05:00.0 Non-Volatile memory controller: Sandisk Corp WD PC SN810 / Black
> SN850
> NVMe SSD (rev 01)
> 06:00.0 Network controller: Intel Corporation Wi-Fi 6 AX200 (rev 1a)
> 07:00.0 Ethernet controller: Intel Corporation I211 Gigabit Network
> Connection
> (rev 03)
> 08:00.0 Ethernet controller: Realtek Semiconductor Co., Ltd. RTL8125 2.5GbE
> Controller (rev 01)
> 09:00.0 Non-Essential Instrumentation [1300]: Advanced Micro Devices, Inc.
> [AMD] Starship/Matisse Reserved SPP
> 09:00.1 USB controller: Advanced Micro Devices, Inc. [AMD] Matisse USB 3.0
> Host
> Controller
> 09:00.3 USB controller: Advanced Micro Devices, Inc. [AMD] Matisse USB 3.0
> Host
> Controller
> 0a:00.0 SATA controller: Advanced Micro Devices, Inc. [AMD] FCH SATA
> Controller
> [AHCI mode] (rev 51)
> 0b:00.0 SATA controller: Advanced Micro Devices, Inc. [AMD] FCH SATA
> Controller
> [AHCI mode] (rev 51)
> 0c:00.0 Non-Volatile memory controller: Intel Corporation Optane SSD 900P
> Series
> 0d:00.0 PCI bridge: Advanced Micro Devices, Inc. [AMD/ATI] Navi 10 XL
> Upstream
> Port of PCI Express Switch (rev c7)
> 0e:00.0 PCI bridge: Advanced Micro Devices, Inc. [AMD/ATI] Navi 10 XL
> Downstream Port of PCI Express Switch
> 0f:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI]
> Navi
> 24 [Radeon RX 6400 / 6500 XT] (rev c7)
> 0f:00.1 Audio device: Advanced Micro Devices, Inc. [AMD/ATI] Navi 21/23
> HDMI/DP
> Audio Controller
> 10:00.0 Non-Essential Instrumentation [1300]: Advanced Micro Devices, Inc.
> [AMD] Starship/Matisse PCIe Dummy Function
> 11:00.0 Non-Essential Instrumentation [1300]: Advanced Micro Devices, Inc.
> [AMD] Starship/Matisse Reserved SPP
> 11:00.1 Encryption controller: Advanced Micro Devices, Inc. [AMD]
> Starship/Matisse Cryptographic Coprocessor PSPCPP
> 11:00.3 USB controller: Advanced Micro Devices, Inc. [AMD] Matisse USB 3.0
> Host
> Controller
> 11:00.4 Audio device: Advanced Micro Devices, Inc. [AMD] Starship/Matisse HD
> Audio Controller
> 
> -- 
> You may reply to this email to add a comment.
> 
> You are receiving this mail because:
> You are the assignee for the bug.

Cc-ing damon@lists.linux.dev

Thank you for reporting this, Greg!  And thank you for forwarding this, Andrew!

On Sat, 4 Jun 2022 11:27:06 -0700 Andrew Morton <akpm@linux-foundation.org> wrote:

> (switched to email.  Please respond via emailed reply-to-all, not via the
> bugzilla web interface).
> 
> On Sat, 04 Jun 2022 15:49:50 +0000 bugzilla-daemon@kernel.org wrote:
> 
> > https://bugzilla.kernel.org/show_bug.cgi?id=216072
> > 
> >             Bug ID: 216072
> >            Summary: regression:
> >                     ccccccgcdkgekhjervgbdfbhdjugcjkfdhiegeuugugtHang at
> >                     boot when DAMON is enabled
> >            Product: Memory Management
> >            Version: 2.5
> >     Kernel Version: 5.19 pre-rc1
> >           Hardware: All
> >                 OS: Linux
> >               Tree: Mainline
> >             Status: NEW
> >           Severity: normal
> >           Priority: P1
> >          Component: Other
> >           Assignee: akpm@linux-foundation.org
> >           Reporter: gwhite@kupulau.com
> >         Regression: No
> > 
> > I see a hang on boot whenever DAMON is enabled.  The specific commit that
> > causes this is listed below.  There is no printk / dmesg output, only the
> > message about an initrd being loaded by EFIStup.  Then a hard hang. 
> Removing
> > the commit below - or disabling DAMON entirely - fixes the issue.
> > 
> > commit 059342d1dd4e01d634184793fa3f8437e62afaa1
> > Author: Hailong Tu <tuhailong@gmail.com>
> > Date:   Fri Apr 29 14:37:00 2022 -0700
> > 
> >     mm/damon/reclaim: fix the timer always stays active
> > 
> >     The timer stays active even if the reclaim mechanism is never enabled. 
> It
> >     is unnecessary overhead can be completely avoided by using
> >     module_param_cb() for enabled flag.
> > 
> >     Link:
> > https://lkml.kernel.org/r/20220421125910.1052459-1-tuhailong@gmail.com
> >     Signed-off-by: Hailong Tu <tuhailong@gmail.com>
> >     Reviewed-by: SeongJae Park <sj@kernel.org>
> >     Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Greg has further mentioned that the issue can be reproduced when
the kernel is booting with damon_reclaim.enabled=Y parameter, and I was also
reproducible on my test machine.

DAMON_RECLAIM calls 'schedule_delayed_work()', which uses 'system_wq', from a
parameter store callback ('enabled_store()'), which is called from
'parse_args()', which is again called from 'start_kernel()'.

And 'system_wq' is initialized from 'workqueue_init_early()', which is called
from 'start_kernel()' after 'parse_args()'.

Therefore the 'schedule_delayed_work()' touches the uninitialized 'system_wq',
and the init process gets kernel NULL pointer dereference, and the system
hangs.

I further confirmed below simple change fixes this issue.  I will format it as
a patch and send soon.

diff --git a/mm/damon/reclaim.c b/mm/damon/reclaim.c
index 53c0c084f046..78984c8d1047 100644
--- a/mm/damon/reclaim.c
+++ b/mm/damon/reclaim.c
@@ -374,6 +374,8 @@ static void damon_reclaim_timer_fn(struct work_struct *work)
 }
 static DECLARE_DELAYED_WORK(damon_reclaim_timer, damon_reclaim_timer_fn);

+static bool damon_reclaim_initialized;
+
 static int enabled_store(const char *val,
                const struct kernel_param *kp)
 {
@@ -382,6 +384,9 @@ static int enabled_store(const char *val,
        if (rc < 0)
                return rc;

+       if (!damon_reclaim_initialized)
+               return rc;
+
        if (enabled)
                schedule_delayed_work(&damon_reclaim_timer, 0);

@@ -450,6 +455,8 @@ static int __init damon_reclaim_init(void)
        damon_add_target(ctx, target);

        schedule_delayed_work(&damon_reclaim_timer, 0);
+
+       damon_reclaim_initialized = true;
        return 0;
 }



Thanks,
SJ

[...]