Re: [f2fs-dev] [PATCH v3] f2fs: avoid race condition for shinker count

[Jaegeuk Kim]

On 11/11, Jaegeuk Kim wrote:
> Light reported sometimes shinker gets nat_cnt < dirty_nat_cnt resulting in
> wrong do_shinker work. Let's avoid to get stale data by using nat_tree_lock.
> 
> Reported-by: Light Hsieh <Light.Hsieh@xxxxxxxxxxxx>
> Signed-off-by: Jaegeuk Kim <jaegeuk@xxxxxxxxxx>
> ---
> v3:
>  - fix to use NM_I(sbi)
> 
>  fs/f2fs/shrinker.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/f2fs/shrinker.c b/fs/f2fs/shrinker.c
> index d66de5999a26..555712ba06d8 100644
> --- a/fs/f2fs/shrinker.c
> +++ b/fs/f2fs/shrinker.c
> @@ -18,7 +18,11 @@ static unsigned int shrinker_run_no;
>  
>  static unsigned long __count_nat_entries(struct f2fs_sb_info *sbi)
>  {
> -	long count = NM_I(sbi)->nat_cnt - NM_I(sbi)->dirty_nat_cnt;
> +	long count;
> +
> +	down_read(&NM_I(sbi)->nat_tree_lock);
> +	count = NM_I(sbi)->nat_cnt - NM_I(sbi)->dirty_nat_cnt;
> +	up_read(&NM_I(sbi)->nat_tree_lock);

I just fosund this can give kernel hang due to the following backtrace.
f2fs_shrink_count
shrink_slab
shrink_node
do_try_to_free_pages
try_to_free_pages
__alloc_pages_nodemask
alloc_pages_current
allocate_slab
__slab_alloc
__slab_alloc
kmem_cache_alloc
add_free_nid
f2fs_flush_nat_entries
f2fs_write_checkpoint

Let me just check like this.