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

[Chao Yu]

On 2020/12/3 14:07, Jaegeuk Kim wrote:

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.hs...@mediatek.com>
Signed-off-by: Jaegeuk Kim <jaeg...@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

Oh, I missed that case.

Let me just check like this.

From 971163330224449d90aac90957ea38f77d494f0f Mon Sep 17 00:00:00 2001

From: Jaegeuk Kim <jaeg...@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.hs...@mediatek.com>
Signed-off-by: Jaegeuk Kim <jaeg...@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;

Hmm.. in the case that we are not in checkpoint progress, nat_cnt and 
dirty_nat_cnt
is mutable, how can we make sure the calculation is non-negative after the check
condition? :(

Thanks

+       return 0;
  }

static unsigned long __count_free_nids(struct f2fs_sb_info *sbi)