linux-kernel - Re: [PATCH RFC] f2fs: fix to use spinlock to avoid page.private update race

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Date:   Mon, 10 Apr 2023 16:25:58 -0700
From:   Jaegeuk Kim <jaegeuk@...nel.org>
To:     Chao Yu <chao@...nel.org>
Cc:     linux-f2fs-devel@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] f2fs: fix to use spinlock to avoid page.private
 update race

On 04/10, Chao Yu wrote:
> There may be subtle race condition, make PagePrivate and page_private
> being inconsistent, result in decreasing page count incorrectly,
> introduce a per-inode spinlock to avoid such condition.

No...what have you found? The set/clear.. were supposed to be done in page_lock,
and checking the flag should not corrupt any memory.

> 
> Signed-off-by: Chao Yu <chao@...nel.org>
> ---
>  fs/f2fs/f2fs.h  | 19 ++++++++++++++++++-
>  fs/f2fs/super.c |  2 ++
>  2 files changed, 20 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
> index c378aedcadea..6b31bef5853e 100644
> --- a/fs/f2fs/f2fs.h
> +++ b/fs/f2fs/f2fs.h
> @@ -856,6 +856,8 @@ struct f2fs_inode_info {
>  
>  	unsigned int atomic_write_cnt;
>  	loff_t original_i_size;		/* original i_size before atomic write */
> +
> +	spinlock_t private_lock;	/* protect page->private */
>  };
>  
>  static inline void get_read_extent_info(struct extent_info *ext,
> @@ -1413,21 +1415,28 @@ static inline bool page_private_##name(struct page *page) \
>  		test_bit(PAGE_PRIVATE_##flagname, &page_private(page)); \
>  }
>  
> +static inline struct f2fs_inode_info *F2FS_I(struct inode *inode);
>  #define PAGE_PRIVATE_SET_FUNC(name, flagname) \
>  static inline void set_page_private_##name(struct page *page) \
>  { \
> +	unsigned long flags; \
> +	spin_lock_irqsave(&F2FS_I(page->mapping->host)->private_lock, flags); \
>  	if (!PagePrivate(page)) \
>  		attach_page_private(page, (void *)page->private); \
>  	set_bit(PAGE_PRIVATE_NOT_POINTER, &page_private(page)); \
>  	set_bit(PAGE_PRIVATE_##flagname, &page_private(page)); \
> +	spin_unlock_irqrestore(&F2FS_I(page->mapping->host)->private_lock, flags); \
>  }
>  
>  #define PAGE_PRIVATE_CLEAR_FUNC(name, flagname) \
>  static inline void clear_page_private_##name(struct page *page) \
>  { \
> +	unsigned long flags; \
> +	spin_lock_irqsave(&F2FS_I(page->mapping->host)->private_lock, flags); \
>  	clear_bit(PAGE_PRIVATE_##flagname, &page_private(page)); \
> -	if (page_private(page) == BIT(PAGE_PRIVATE_NOT_POINTER)) \
> +	if (page_private(page) == (BIT(PAGE_PRIVATE_NOT_POINTER))) \
>  		detach_page_private(page); \
> +	spin_unlock_irqrestore(&F2FS_I(page->mapping->host)->private_lock, flags); \
>  }
>  
>  PAGE_PRIVATE_GET_FUNC(nonpointer, NOT_POINTER);
> @@ -1456,17 +1465,25 @@ static inline unsigned long get_page_private_data(struct page *page)
>  
>  static inline void set_page_private_data(struct page *page, unsigned long data)
>  {
> +	unsigned long flags;
> +
> +	spin_lock_irqsave(&F2FS_I(page->mapping->host)->private_lock, flags);
>  	if (!PagePrivate(page))
>  		attach_page_private(page, 0);
>  	set_bit(PAGE_PRIVATE_NOT_POINTER, &page_private(page));
>  	page_private(page) |= data << PAGE_PRIVATE_MAX;
> +	spin_unlock_irqrestore(&F2FS_I(page->mapping->host)->private_lock, flags);
>  }
>  
>  static inline void clear_page_private_data(struct page *page)
>  {
> +	unsigned long flags;
> +
> +	spin_lock_irqsave(&F2FS_I(page->mapping->host)->private_lock, flags);
>  	page_private(page) &= GENMASK(PAGE_PRIVATE_MAX - 1, 0);
>  	if (page_private(page) == BIT(PAGE_PRIVATE_NOT_POINTER))
>  		detach_page_private(page);
> +	spin_unlock_irqrestore(&F2FS_I(page->mapping->host)->private_lock, flags);
>  }
>  
>  /* For compression */
> diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
> index a1b570a5e50f..555424dd85fd 100644
> --- a/fs/f2fs/super.c
> +++ b/fs/f2fs/super.c
> @@ -1419,6 +1419,8 @@ static struct inode *f2fs_alloc_inode(struct super_block *sb)
>  	init_f2fs_rwsem(&fi->i_gc_rwsem[WRITE]);
>  	init_f2fs_rwsem(&fi->i_xattr_sem);
>  
> +	spin_lock_init(&fi->private_lock);
> +
>  	/* Will be used by directory only */
>  	fi->i_dir_level = F2FS_SB(sb)->dir_level;
>  
> -- 
> 2.25.1