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Subject: Re: [PATCH] ext4: mballoc: Disable preemption before getting per-CPU
 pointer
To:     YueHaibing <yuehaibing@huawei.com>, linux-ext4@vger.kernel.org,
        linux-kernel@vger.kernel.org, tytso@mit.edu,
        adilger.kernel@dilger.ca
References: <20200610134919.73688-1-yuehaibing@huawei.com>
From:   Ritesh Harjani <riteshh@linux.ibm.com>
Date:   Thu, 11 Jun 2020 06:51:14 +0530
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Hello,

Fix for this is already submitted @

https://patchwork.ozlabs.org/project/linux-ext4/patch/534f275016296996f54ecf65168bb3392b6f653d.1591699601.git.riteshh@linux.ibm.com/

-ritesh

On 6/10/20 7:19 PM, YueHaibing wrote:
> BUG: using smp_processor_id() in preemptible [00000000] code: kworker/u16:3/2181
> caller is ext4_mb_new_blocks+0x388/0xed0
> CPU: 2 PID: 2181 Comm: kworker/u16:3 Not tainted 5.7.0+ #182
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
> Workqueue: writeback wb_workfn (flush-8:0)
> Call Trace:
>   dump_stack+0xb9/0xfc
>   debug_smp_processor_id+0xc8/0xd0
>   ext4_mb_new_blocks+0x388/0xed0
>   ext4_ext_map_blocks+0xa92/0xff0
>   ext4_map_blocks+0x34e/0x580
>   ext4_writepages+0xa28/0x11b0
>   do_writepages+0x46/0xe0
>   __writeback_single_inode+0x5f/0x6b0
>   writeback_sb_inodes+0x290/0x620
>   __writeback_inodes_wb+0x62/0xb0
>   wb_writeback+0x36c/0x520
>   wb_workfn+0x319/0x680
>   process_one_work+0x271/0x640
>   worker_thread+0x3a/0x3a0
>   kthread+0x14e/0x170
>   ret_from_fork+0x27/0x40
> 
> Disable preemption before accessing discard_pa_seq.
> 
> Fixes: 07b5b8e1ac40 ("ext4: mballoc: introduce pcpu seqcnt for freeing PA to improve ENOSPC handling")
> Signed-off-by: YueHaibing <yuehaibing@huawei.com>
> ---
>   fs/ext4/mballoc.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index a9083113a8c0..30b3bfb1e06a 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -4708,7 +4708,8 @@ ext4_fsblk_t ext4_mb_new_blocks(handle_t *handle,
>   	}
>   
>   	ac->ac_op = EXT4_MB_HISTORY_PREALLOC;
> -	seq = *this_cpu_ptr(&discard_pa_seq);
> +	seq = *get_cpu_ptr(&discard_pa_seq);
> +	put_cpu_ptr(&discard_pa_seq);
>   	if (!ext4_mb_use_preallocated(ac)) {
>   		ac->ac_op = EXT4_MB_HISTORY_ALLOC;
>   		ext4_mb_normalize_request(ac, ar);
>