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Date:   Wed, 2 Dec 2020 22:07:03 -0800
From:   Jaegeuk Kim <jaegeuk@kernel.org>
To:     linux-kernel@vger.kernel.org,
        linux-f2fs-devel@lists.sourceforge.net, kernel-team@android.com
Cc:     Light Hsieh <Light.Hsieh@mediatek.com>
Subject: Re: [f2fs-dev] [PATCH v3] f2fs: avoid race condition for shinker
 count
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References: <20201109170012.2129411-1-jaegeuk@kernel.org>
 <20201112053414.GB3826485@google.com>
 <20201112054051.GA4092972@google.com>
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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@mediatek.com>
> Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
> ---
> 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.

From 971163330224449d90aac90957ea38f77d494f0f Mon Sep 17 00:00:00 2001
From: Jaegeuk Kim <jaegeuk@kernel.org>
Date: Fri, 6 Nov 2020 13:22:05 -0800
Subject: [PATCH] f2fs: avoid race condition for shrinker count

Light reported sometimes shinker gets nat_cnt < dirty_nat_cnt resulting in
wrong do_shinker work. Let's avoid to return insane overflowed value.

Reported-by: Light Hsieh <Light.Hsieh@mediatek.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
---
 fs/f2fs/shrinker.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/fs/f2fs/shrinker.c b/fs/f2fs/shrinker.c
index d66de5999a26..75b5b4aaed99 100644
--- a/fs/f2fs/shrinker.c
+++ b/fs/f2fs/shrinker.c
@@ -18,9 +18,9 @@ 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;
-
-	return count > 0 ? count : 0;
+	if (NM_I(sbi)->nat_cnt > NM_I(sbi)->dirty_nat_cnt)
+		return NM_I(sbi)->nat_cnt - NM_I(sbi)->dirty_nat_cnt;
+	return 0;
 }
 
 static unsigned long __count_free_nids(struct f2fs_sb_info *sbi)
-- 
2.29.2.454.gaff20da3a2-goog


>  
>  	return count > 0 ? count : 0;
>  }
> -- 
> 2.29.2.299.gdc1121823c-goog
> 
> 
> 
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