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Date:   Thu, 14 Oct 2021 09:55:48 +0800
From:   Ming Lei <>
To:     Luis Chamberlain <>
Subject: Re: [PATCH v8 11/12] zram: fix crashes with cpu hotplug multistate

On Mon, Sep 27, 2021 at 09:38:04AM -0700, Luis Chamberlain wrote:
> Provide a simple state machine to fix races with driver exit where we
> remove the CPU multistate callbacks and re-initialization / creation of
> new per CPU instances which should be managed by these callbacks.
> The zram driver makes use of cpu hotplug multistate support, whereby it
> associates a struct zcomp per CPU. Each struct zcomp represents a
> compression algorithm in charge of managing compression streams per
> CPU. Although a compiled zram driver only supports a fixed set of
> compression algorithms, each zram device gets a struct zcomp allocated
> per CPU. The "multi" in CPU hotplug multstate refers to these per
> cpu struct zcomp instances. Each of these will have the CPU hotplug
> callback called for it on CPU plug / unplug. The kernel's CPU hotplug
> multistate keeps a linked list of these different structures so that
> it will iterate over them on CPU transitions.
> By default at driver initialization we will create just one zram device
> (num_devices=1) and a zcomp structure then set for the now default
> lzo-rle comrpession algorithm. At driver removal we first remove each
> zram device, and so we destroy the associated struct zcomp per CPU. But
> since we expose sysfs attributes to create new devices or reset /
> initialize existing zram devices, we can easily end up re-initializing
> a struct zcomp for a zram device before the exit routine of the module
> removes the cpu hotplug callback. When this happens the kernel's CPU
> hotplug will detect that at least one instance (struct zcomp for us)
> exists. This can happen in the following situation:
> CPU 1                            CPU 2
>                                 disksize_store(...);
> class_unregister(...);
> idr_for_each(...);
> zram_debugfs_destroy();
> idr_destroy(...);
> unregister_blkdev(...);
> cpuhp_remove_multi_state(...);
> The warning comes up on cpuhp_remove_multi_state() when it sees that the
> state for CPUHP_ZCOMP_PREPARE does not have an empty instance linked list.
> In this case, that a struct zcom still exists, the driver allowed its
> creation per CPU even though we could have just freed them per CPU
> though a call on another CPU, and we are then later trying to remove the
> hotplug callback.
> Fix all this by providing a zram initialization boolean
> protected the shared in the driver zram_index_mutex, which we
> can use to annotate when sysfs attributes are safe to use or
> not -- once the driver is properly initialized. When the driver
> is going down we also are sure to not let userspace muck with
> attributes which may affect each per cpu struct zcomp.
> This also fixes a series of possible memory leaks. The
> crashes and memory leaks can easily be caused by issuing
> the script from the LTP project [0] in a loop
> in two separate windows:
>   cd testcases/kernel/device-drivers/zram
>   while true; do PATH=$PATH:$PWD:$PWD/../../../lib/ ./; done
> You end up with a splat as follows:
> kernel: zram: Removed device: zram0
> kernel: zram: Added device: zram0
> kernel: zram0: detected capacity change from 0 to 209715200
> kernel: Adding 104857596k swap on /dev/zram0.  <etc>
> kernel: zram0: detected capacitky change from 209715200 to 0
> kernel: zram0: detected capacity change from 0 to 209715200
> kernel: ------------[ cut here ]------------
> kernel: Error: Removing state 63 which has instances left.
> kernel: WARNING: CPU: 7 PID: 70457 at \
> 	kernel/cpu.c:2069 __cpuhp_remove_state_cpuslocked+0xf9/0x100
> kernel: Modules linked in: zram(E-) zsmalloc(E) <etc>
> kernel: CPU: 7 PID: 70457 Comm: rmmod Tainted: G            \
> 	E     5.12.0-rc1-next-20210304 #3
> kernel: Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), \
> 	BIOS 1.14.0-2 04/01/2014
> kernel: RIP: 0010:__cpuhp_remove_state_cpuslocked+0xf9/0x100
> kernel: Code: <etc>
> kernel: RSP: 0018:ffffa800c139be98 EFLAGS: 00010282
> kernel: RAX: 0000000000000000 RBX: ffffffff9083db58 RCX: ffff9609f7dd86d8
> kernel: RDX: 00000000ffffffd8 RSI: 0000000000000027 RDI: ffff9609f7dd86d0
> kernel: RBP: 0000000000000000i R08: 0000000000000000 R09: ffffa800c139bcb8
> kernel: R10: ffffa800c139bcb0 R11: ffffffff908bea40 R12: 000000000000003f
> kernel: R13: 00000000000009d8 R14: 0000000000000000 R15: 0000000000000000
> kernel: FS: 00007f1b075a7540(0000) GS:ffff9609f7dc0000(0000) knlGS:<etc>
> kernel: CS:  0010 DS: 0000 ES 0000 CR0: 0000000080050033
> kernel: CR2: 00007f1b07610490 CR3: 00000001bd04e000 CR4: 0000000000350ee0
> kernel: Call Trace:
> kernel: __cpuhp_remove_state+0x2e/0x80
> kernel: __do_sys_delete_module+0x190/0x2a0
> kernel:  do_syscall_64+0x33/0x80
> kernel: entry_SYSCALL_64_after_hwframe+0x44/0xae
> The "Error: Removing state 63 which has instances left" refers
> to the zram per CPU struct zcomp instances left.
> [0]
> Acked-by: Minchan Kim <>
> Signed-off-by: Luis Chamberlain <>
> ---

Hello Luis,

Can you test the following patch and see if the issue can be addressed?

Please see the idea from the inline comment.

Also zram_index_mutex isn't needed in zram disk's store() compared with
your patch, then the deadlock issue you are addressing in this series can
be avoided.

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index fcaf2750f68f..3c17927d23a7 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -1985,11 +1985,17 @@ static int zram_remove(struct zram *zram)
 	/* Make sure all the pending I/O are finished */
-	zram_reset_device(zram);
 	pr_info("Removed device: %s\n", zram->disk->disk_name);
+	/*
+	 * reset device after gendisk is removed, so any change from sysfs
+	 * store won't come in, then we can really reset device here
+	 */
+	zram_reset_device(zram);
 	return 0;
@@ -2073,7 +2079,12 @@ static int zram_remove_cb(int id, void *ptr, void *data)
 static void destroy_devices(void)
+	/* hold the global lock so new device can't be added */
+	mutex_lock(&zram_index_mutex);
 	idr_for_each(&zram_index_idr, &zram_remove_cb, NULL);
+	mutex_unlock(&zram_index_mutex);
 	unregister_blkdev(zram_major, "zram");


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