Re: [PATCH] zram: Using GFP_ATOMIC instead of GFP_KERNEL to allocate bitmap memory in backing_dev_store

[Minchan Kim]

On Thu, Nov 30, 2023 at 11:20:47PM +0800, Dongyun Liu wrote:
> We encountered a deadlock issue when using zram on kernel version 5.10.
> The reason is that the backing_dev_store will first hold the
> zram->init_lock, and then it will allocate memory with kvmalloc_node. At
> this moment, the system may be in a state of memory shortage, so it will
> enter the memory swapping process. In zram_bvec_write, we will trigger
> our own thread to move data from zram to the backing device to free up
> more available memory, which is the work done in the
> try_wakeup_zram_wbd. In this function, zram->init_lock will be acquired
> again to read zram->bd_wb_limit, which causes a deadlock as follow call
> trace.

Isn't it your custom feature instead of upstream?

> 
> The latest version of Linux does not have the bug, but a potential risk
> in future.If we try to acquire zram->init_lock again in the zram process
> (for example, when we need to read zram->bd_wb_limit), there is a risk
> of deadlock. The bug is quite elusive, and it only appears with low

It would be very helpful if you show how the zram in the upstream could
cause the deadlock instead of your custom feature.

> probability after conducting a week-long stress test using 50 devices.
> Therefore, I suggest passing the GFP_ATOMIC flag to allocate memory in
> the backing_dev_store, to avoid similar issues we encountered. Please
> consider it, Thank you.
> 
> INFO: task init:331 blocked for more than 120 seconds.  "echo 0 >
> /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> task:init   state:D stack:    0 pid:    1 ppid:     0 flags:0x04000000
> Call trace:
>   __switch_to+0x244/0x4e4
>   __schedule+0x5bc/0xc48
>   schedule+0x80/0x164
>   rwsem_down_read_slowpath+0x4fc/0xf9c
>   __down_read+0x140/0x188
>   down_read+0x14/0x24
>   try_wakeup_wbd_thread+0x78/0x1ec [zram]
>   __zram_bvec_write+0x720/0x878 [zram]
>   zram_bvec_rw+0xa8/0x234 [zram]
>   zram_submit_bio+0x16c/0x268 [zram]
>   submit_bio_noacct+0x128/0x3c8
>   submit_bio+0x1cc/0x3d0
>   __swap_writepage+0x5c4/0xd4c
>   swap_writepage+0x130/0x158
>   pageout+0x1f4/0x478
>   shrink_page_list+0x9b4/0x1eb8
>   shrink_inactive_list+0x2f4/0xaa8
>   shrink_lruvec+0x184/0x340
>   shrink_node_memcgs+0x84/0x3a0
>   shrink_node+0x2c4/0x6c4
>   shrink_zones+0x16c/0x29c
>   do_try_to_free_pages+0xe4/0x2b4
>   try_to_free_pages+0x388/0x7b4
>   __alloc_pages_direct_reclaim+0x88/0x278
>   __alloc_pages_slowpath+0x4ec/0xf6c
>   __alloc_pages_nodemask+0x1f4/0x3dc
>   kmalloc_order+0x54/0x338
>   kmalloc_order_trace+0x34/0x1bc
>   __kmalloc+0x5e8/0x9c0
>   kvmalloc_node+0xa8/0x264
>   backing_dev_store+0x1a4/0x818 [zram]
>   dev_attr_store+0x38/0x8c
>   sysfs_kf_write+0x64/0xc4
>   kernfs_fop_write_iter+0x168/0x2ac
>   vfs_write+0x300/0x37c
>   ksys_write+0x7c/0xf0
>   __arm64_sys_write+0x20/0x30
>   el0_svc_common+0xd4/0x270
>   el0_svc+0x28/0x98
>   el0_sync_handler+0x8c/0xf0
>   el0_sync+0x1b4/0x1c0
> 
> Signed-off-by: Dongyun Liu <dongyun.liu@xxxxxxxxxxxxx>
> ---
>  drivers/block/zram/zram_drv.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> index d77d3664ca08..ee6c22c50e09 100644
> --- a/drivers/block/zram/zram_drv.c
> +++ b/drivers/block/zram/zram_drv.c
> @@ -514,7 +514,7 @@ static ssize_t backing_dev_store(struct device *dev,
>  
>  	nr_pages = i_size_read(inode) >> PAGE_SHIFT;
>  	bitmap_sz = BITS_TO_LONGS(nr_pages) * sizeof(long);
> -	bitmap = kvzalloc(bitmap_sz, GFP_KERNEL);
> +	bitmap = kmalloc(bitmap_sz, GFP_ATOMIC);
>  	if (!bitmap) {
>  		err = -ENOMEM;
>  		goto out;
> -- 
> 2.25.1
>