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Subject: Re: [PATCH -next 2/3] mm/zswap: delay the initializaton of zswap
 until the first enablement
To:     Nathan Chancellor <nathan@kernel.org>
References: <20220825142037.3214152-1-liushixin2@huawei.com>
 <20220825142037.3214152-3-liushixin2@huawei.com>
 <YwksRZPIfXmlOmHR@dev-arch.thelio-3990X>
CC:     Seth Jennings <sjenning@redhat.com>,
        Dan Streetman <ddstreet@ieee.org>,
        Vitaly Wool <vitaly.wool@konsulko.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        <linux-mm@kvack.org>, <linux-kernel@vger.kernel.org>
From:   Liu Shixin <liushixin2@huawei.com>
Message-ID: <fe86df53-6bed-11ff-83cf-f4aab3249f73@huawei.com>
Date:   Sat, 27 Aug 2022 18:21:15 +0800
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On 2022/8/27 4:25, Nathan Chancellor wrote:
> Hi Liu,
>
> On Thu, Aug 25, 2022 at 10:20:36PM +0800, Liu Shixin wrote:
>> In the initialization of zswap, about 18MB memory will be allocated for
>> zswap_pool in my machine. Since not all users use zswap, the memory may be
>> wasted. Save the memory for these users by delaying the initialization of
>> zswap to first enablement.
>>
>> Signed-off-by: Liu Shixin <liushixin2@huawei.com>
>> ---
>>  mm/zswap.c | 48 +++++++++++++++++++++++++++++++++++++++---------
>>  1 file changed, 39 insertions(+), 9 deletions(-)
>>
>> diff --git a/mm/zswap.c b/mm/zswap.c
>> index 84e38300f571..90df72aceb08 100644
>> --- a/mm/zswap.c
>> +++ b/mm/zswap.c
>> @@ -81,6 +81,8 @@ static bool zswap_pool_reached_full;
>>  
>>  #define ZSWAP_PARAM_UNSET ""
>>  
>> +static int zswap_setup(void);
>> +
>>  /* Enable/disable zswap */
>>  static bool zswap_enabled = IS_ENABLED(CONFIG_ZSWAP_DEFAULT_ON);
>>  static int zswap_enabled_param_set(const char *,
>> @@ -220,6 +222,8 @@ static atomic_t zswap_pools_count = ATOMIC_INIT(0);
>>  
>>  /* init state */
>>  static int zswap_init_state;
>> +/* used to ensure the integrity of initialization */
>> +static DEFINE_MUTEX(zswap_init_lock);
>>  
>>  /* init completed, but couldn't create the initial pool */
>>  static bool zswap_has_pool;
>> @@ -273,13 +277,13 @@ static void zswap_update_total_size(void)
>>  **********************************/
>>  static struct kmem_cache *zswap_entry_cache;
>>  
>> -static int __init zswap_entry_cache_create(void)
>> +static int zswap_entry_cache_create(void)
>>  {
>>  	zswap_entry_cache = KMEM_CACHE(zswap_entry, 0);
>>  	return zswap_entry_cache == NULL;
>>  }
>>  
>> -static void __init zswap_entry_cache_destroy(void)
>> +static void zswap_entry_cache_destroy(void)
>>  {
>>  	kmem_cache_destroy(zswap_entry_cache);
>>  }
>> @@ -664,7 +668,7 @@ static struct zswap_pool *zswap_pool_create(char *type, char *compressor)
>>  	return NULL;
>>  }
>>  
>> -static __init struct zswap_pool *__zswap_pool_create_fallback(void)
>> +static struct zswap_pool *__zswap_pool_create_fallback(void)
>>  {
>>  	bool has_comp, has_zpool;
>>  
>> @@ -782,11 +786,17 @@ static int __zswap_param_set(const char *val, const struct kernel_param *kp,
>>  	if (!strcmp(s, *(char **)kp->arg) && zswap_has_pool)
>>  		return 0;
>>  
>> -	/* if this is load-time (pre-init) param setting,
>> +	/*
>> +	 * if zswap has not been initialized,
>>  	 * don't create a pool; that's done during init.
>>  	 */
>> -	if (zswap_init_state == ZSWAP_UNINIT)
>> -		return param_set_charp(s, kp);
>> +	mutex_lock(&zswap_init_lock);
>> +	if (zswap_init_state == ZSWAP_UNINIT) {
>> +		ret = param_set_charp(s, kp);
>> +		mutex_unlock(&zswap_init_lock);
>> +		return ret;
>> +	}
>> +	mutex_unlock(&zswap_init_lock);
>>  
>>  	if (!type) {
>>  		if (!zpool_has_pool(s)) {
>> @@ -876,6 +886,14 @@ static int zswap_zpool_param_set(const char *val,
>>  static int zswap_enabled_param_set(const char *val,
>>  				   const struct kernel_param *kp)
>>  {
>> +	if (system_state == SYSTEM_RUNNING) {
>> +		mutex_lock(&zswap_init_lock);
>> +		if (zswap_setup()) {
>> +			mutex_unlock(&zswap_init_lock);
>> +			return -ENODEV;
>> +		}
>> +		mutex_unlock(&zswap_init_lock);
>> +	}
>>  	if (zswap_init_state == ZSWAP_INIT_FAILED) {
>>  		pr_err("can't enable, initialization failed\n");
>>  		return -ENODEV;
>> @@ -1432,7 +1450,7 @@ static const struct frontswap_ops zswap_frontswap_ops = {
>>  
>>  static struct dentry *zswap_debugfs_root;
>>  
>> -static int __init zswap_debugfs_init(void)
>> +static int zswap_debugfs_init(void)
>>  {
>>  	if (!debugfs_initialized())
>>  		return -ENODEV;
>> @@ -1463,7 +1481,7 @@ static int __init zswap_debugfs_init(void)
>>  	return 0;
>>  }
>>  #else
>> -static int __init zswap_debugfs_init(void)
>> +static int zswap_debugfs_init(void)
>>  {
>>  	return 0;
>>  }
>> @@ -1472,11 +1490,14 @@ static int __init zswap_debugfs_init(void)
>>  /*********************************
>>  * module init and exit
>>  **********************************/
>> -static int __init init_zswap(void)
>> +static int zswap_setup(void)
>>  {
>>  	struct zswap_pool *pool;
>>  	int ret;
>>  
>> +	if (zswap_init_state != ZSWAP_UNINIT)
>> +		return 0;
>> +
>>  	if (zswap_entry_cache_create()) {
>>  		pr_err("entry cache creation failed\n");
>>  		goto cache_fail;
>> @@ -1534,6 +1555,15 @@ static int __init init_zswap(void)
>>  	zswap_enabled = false;
>>  	return -ENOMEM;
>>  }
>> +
>> +static int __init init_zswap(void)
>> +{
>> +	/* skip init if zswap is disabled when system startup */
>> +	if (!zswap_enabled)
>> +		return 0;
>> +	return zswap_setup();
>> +}
>> +
>>  /* must be late so crypto has time to come up */
>>  late_initcall(init_zswap);
>>  
>> -- 
>> 2.25.1
>>
>>
> This change is in -next as commit 22100432cf14 ("mm/zswap: delay the
> initializaton of zswap until the first enablement"). I just bisected my
> arm64 test system running Fedora failing to boot due to that commit,
> with the following stack trace:
>
>   Unable to handle kernel access to user memory outside uaccess routines at virtual address 0000000000000000
>   Mem abort info:
>     ESR = 0x0000000096000004
>     EC = 0x25: DABT (current EL), IL = 32 bits
>     SET = 0, FnV = 0
>     EA = 0, S1PTW = 0
>     FSC = 0x04: level 0 translation fault
>   Data abort info:
>     ISV = 0, ISS = 0x00000004
>     CM = 0, WnR = 0
>   user pgtable: 4k pages, 48-bit VAs, pgdp=00000020a4fab000
>   [0000000000000000] pgd=0000000000000000, p4d=0000000000000000
>   Internal error: Oops: 96000004 [#1] SMP
>   Modules linked in: zram fsl_dpaa2_eth pcs_lynx phylink ahci_qoriq crct10dif_ce ghash_ce sbsa_gwdt fsl_mc_dpio nvme lm90 nvme_core at803x xhci_plat_hcd rtc_fsl_ftm_alarm xgmac_mdio ahci_platform i2c_imx ip6_tables ip_tables fuse
>   Unloaded tainted modules: cppc_cpufreq():1
>   CPU: 10 PID: 761 Comm: swapon Not tainted 6.0.0-rc2-00454-g22100432cf14 #1
>   Hardware name: SolidRun Ltd. SolidRun CEX7 Platform, BIOS EDK II Jun 21 2022
>   pstate: 00400005 (nzcv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
>   pc : frontswap_init+0x38/0x60
>   lr : __do_sys_swapon+0x8a8/0x9f4
>   sp : ffff80000969bcf0
>   x29: ffff80000969bcf0 x28: ffff37bee0d8fc00 x27: ffff80000a7f5000
>   x26: fffffcdefb971e80 x25: ffffaba797453b90 x24: 0000000000000064
>   x23: ffff37c1f209d1a8 x22: ffff37bee880e000 x21: ffffaba797748560
>   x20: ffff37bee0d8fce4 x19: ffffaba797748488 x18: 0000000000000014
>   x17: 0000000030ec029a x16: ffffaba795a479b0 x15: 0000000000000000
>   x14: 0000000000000000 x13: 0000000000000030 x12: 0000000000000001
>   x11: ffff37c63c0aba18 x10: 0000000000000000 x9 : ffffaba7956b8c88
>   x8 : ffff80000969bcd0 x7 : 0000000000000000 x6 : 0000000000000000
>   x5 : 0000000000000001 x4 : 0000000000000000 x3 : ffffaba79730f000
>   x2 : ffff37bee0d8fc00 x1 : 0000000000000000 x0 : 0000000000000000
>   Call trace:
>   frontswap_init+0x38/0x60
>   __do_sys_swapon+0x8a8/0x9f4
>   __arm64_sys_swapon+0x28/0x3c
>   invoke_syscall+0x78/0x100
>   el0_svc_common.constprop.0+0xd4/0xf4
>   do_el0_svc+0x38/0x4c
>   el0_svc+0x34/0x10c
>   el0t_64_sync_handler+0x11c/0x150
>   el0t_64_sync+0x190/0x194
>   Code: d000e283 910003fd f9006c41 f946d461 (f9400021)
>   ---[ end trace 0000000000000000 ]---
>
> This is with Fedora's configuration:
>
> https://src.fedoraproject.org/rpms/kernel/raw/rawhide/f/kernel-aarch64-fedora.config
>
> If there is any more information I can provide or patches I can test, I
> am more than happy to do so!
>
> Cheers,
> Nathan
> .
Thanks for your reporting, it looks likt that this problem will occur when zswap is not initial succeed.
zswap is the only backend for frontswap for now, but frontswap does not check if zswap is valid.

I fix this problem in v3 by skipping frontswap_ops->init in frontswap_init if zswap is not ready.

Link: https://lore.kernel.org/all/20220827104600.1813214-1-liushixin2@huawei.com/

Thanks,