Subject: Re: [PATCHv5 1/1] ext4: mballoc: Use raw_cpu_ptr instead of this_cpu_ptr

From: Marek Szyprowski <[email-masked]>
Date: Wed, 3 Jun 2020 12:24:22 +0200

Hi Ritesh,

On 02.06.2020 15:47, Ritesh Harjani wrote:
> It doesn't really matter in ext4_mb_new_blocks() about whether the code
> is rescheduled on any other cpu due to preemption. Because we care
> about discard_pa_seq only when the block allocation fails and then too
> we add the seq counter of all the cpus against the initial sampled one
> to check if anyone has freed any blocks while we were doing allocation.
>
> So just use raw_cpu_ptr instead of this_cpu_ptr to avoid this BUG.
>
> BUG: using smp_processor_id() in preemptible [00000000] code: syz-fuzzer/6927
> caller is ext4_mb_new_blocks+0xa4d/0x3b70 fs/ext4/mballoc.c:4711
> CPU: 1 PID: 6927 Comm: syz-fuzzer Not tainted 5.7.0-next-20200602-syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> Call Trace:
>   __dump_stack lib/dump_stack.c:77 [inline]
>   dump_stack+0x18f/0x20d lib/dump_stack.c:118
>   check_preemption_disabled+0x20d/0x220 lib/smp_processor_id.c:48
>   ext4_mb_new_blocks+0xa4d/0x3b70 fs/ext4/mballoc.c:4711
>   ext4_ext_map_blocks+0x201b/0x33e0 fs/ext4/extents.c:4244
>   ext4_map_blocks+0x4cb/0x1640 fs/ext4/inode.c:626
>   ext4_getblk+0xad/0x520 fs/ext4/inode.c:833
>   ext4_bread+0x7c/0x380 fs/ext4/inode.c:883
>   ext4_append+0x153/0x360 fs/ext4/namei.c:67
>   ext4_init_new_dir fs/ext4/namei.c:2757 [inline]
>   ext4_mkdir+0x5e0/0xdf0 fs/ext4/namei.c:2802
>   vfs_mkdir+0x419/0x690 fs/namei.c:3632
>   do_mkdirat+0x21e/0x280 fs/namei.c:3655
>   do_syscall_64+0x60/0xe0 arch/x86/entry/common.c:359
>   entry_SYSCALL_64_after_hwframe+0x44/0xa9
>
> Signed-off-by: Ritesh Harjani <[email-masked]>
> Reported-by: [email-masked]

This fixes the warning observed on various Samsung Exynos SoC based 
boards with linux-next 20200602.

Tested-by: Marek Szyprowski <[email-masked]>

> ---
>   fs/ext4/mballoc.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index a9083113a8c0..b79b32dbe3ea 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -4708,7 +4708,7 @@ 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 = *raw_cpu_ptr(&discard_pa_seq);
>   	if (!ext4_mb_use_preallocated(ac)) {
>   		ac->ac_op = EXT4_MB_HISTORY_ALLOC;
>   		ext4_mb_normalize_request(ac, ar);

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland