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Date:   Wed, 6 Jan 2021 14:28:52 -0800
From:   Jaegeuk Kim <jaegeuk@kernel.org>
To:     Chao Yu <yuchao0@huawei.com>
Cc:     linux-f2fs-devel@lists.sourceforge.net,
        linux-kernel@vger.kernel.org, chao@kernel.org
Subject: Re: [PATCH v2] f2fs: fix to keep isolation of atomic write
Message-ID: <X/Y5pJr4Aaf0bBqj@google.com>
References: <20201230075557.108818-1-yuchao0@huawei.com>
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In-Reply-To: <20201230075557.108818-1-yuchao0@huawei.com>
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Hi Chao,

With a quick test, this patch causes down_write failure resulting in blocking
process. I didn't dig in the bug so, please check the code again. :P

On 12/30, Chao Yu wrote:
> ThreadA					ThreadB
> - f2fs_ioc_start_atomic_write
> - write
> - f2fs_ioc_commit_atomic_write
>  - f2fs_commit_inmem_pages
>  - f2fs_drop_inmem_pages
>  - f2fs_drop_inmem_pages
>   - __revoke_inmem_pages
> 					- f2fs_vm_page_mkwrite
> 					 - set_page_dirty
> 					  - tag ATOMIC_WRITTEN_PAGE and add page
> 					    to inmem_pages list
>   - clear_inode_flag(FI_ATOMIC_FILE)
> 					- f2fs_vm_page_mkwrite
> 					  - set_page_dirty
> 					   - f2fs_update_dirty_page
> 					    - f2fs_trace_pid
> 					     - tag inmem page private to pid
> 					- truncate
> 					 - f2fs_invalidate_page
> 					 - set page->mapping to NULL
> 					  then it will cause panic once we
> 					  access page->mapping
> 
> The root cause is we missed to keep isolation of atomic write in the case
> of commit_atomic_write vs mkwrite, let commit_atomic_write helds i_mmap_sem
> lock to avoid this issue.
> 
> Signed-off-by: Chao Yu <yuchao0@huawei.com>
> ---
> v2:
> - use i_mmap_sem to avoid mkwrite racing with below flows:
>  * f2fs_ioc_start_atomic_write
>  * f2fs_drop_inmem_pages
>  * f2fs_commit_inmem_pages
> 
>  fs/f2fs/file.c    | 3 +++
>  fs/f2fs/segment.c | 7 +++++++
>  2 files changed, 10 insertions(+)
> 
> diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
> index 4e6d4b9120a8..a48ec650d691 100644
> --- a/fs/f2fs/file.c
> +++ b/fs/f2fs/file.c
> @@ -2050,6 +2050,7 @@ static int f2fs_ioc_start_atomic_write(struct file *filp)
>  		goto out;
>  
>  	down_write(&F2FS_I(inode)->i_gc_rwsem[WRITE]);
> +	down_write(&F2FS_I(inode)->i_mmap_sem);
>  
>  	/*
>  	 * Should wait end_io to count F2FS_WB_CP_DATA correctly by
> @@ -2060,6 +2061,7 @@ static int f2fs_ioc_start_atomic_write(struct file *filp)
>  			  inode->i_ino, get_dirty_pages(inode));
>  	ret = filemap_write_and_wait_range(inode->i_mapping, 0, LLONG_MAX);
>  	if (ret) {
> +		up_write(&F2FS_I(inode)->i_mmap_sem);
>  		up_write(&F2FS_I(inode)->i_gc_rwsem[WRITE]);
>  		goto out;
>  	}
> @@ -2073,6 +2075,7 @@ static int f2fs_ioc_start_atomic_write(struct file *filp)
>  	/* add inode in inmem_list first and set atomic_file */
>  	set_inode_flag(inode, FI_ATOMIC_FILE);
>  	clear_inode_flag(inode, FI_ATOMIC_REVOKE_REQUEST);
> +	up_write(&F2FS_I(inode)->i_mmap_sem);
>  	up_write(&F2FS_I(inode)->i_gc_rwsem[WRITE]);
>  
>  	f2fs_update_time(F2FS_I_SB(inode), REQ_TIME);
> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
> index d8570b0359f5..dab870d9faf6 100644
> --- a/fs/f2fs/segment.c
> +++ b/fs/f2fs/segment.c
> @@ -327,6 +327,8 @@ void f2fs_drop_inmem_pages(struct inode *inode)
>  	struct f2fs_sb_info *sbi = F2FS_I_SB(inode);
>  	struct f2fs_inode_info *fi = F2FS_I(inode);
>  
> +	down_write(&F2FS_I(inode)->i_mmap_sem);
> +
>  	while (!list_empty(&fi->inmem_pages)) {
>  		mutex_lock(&fi->inmem_lock);
>  		__revoke_inmem_pages(inode, &fi->inmem_pages,
> @@ -344,6 +346,8 @@ void f2fs_drop_inmem_pages(struct inode *inode)
>  		sbi->atomic_files--;
>  	}
>  	spin_unlock(&sbi->inode_lock[ATOMIC_FILE]);
> +
> +	up_write(&F2FS_I(inode)->i_mmap_sem);
>  }
>  
>  void f2fs_drop_inmem_page(struct inode *inode, struct page *page)
> @@ -467,6 +471,7 @@ int f2fs_commit_inmem_pages(struct inode *inode)
>  	f2fs_balance_fs(sbi, true);
>  
>  	down_write(&fi->i_gc_rwsem[WRITE]);
> +	down_write(&F2FS_I(inode)->i_mmap_sem);
>  
>  	f2fs_lock_op(sbi);
>  	set_inode_flag(inode, FI_ATOMIC_COMMIT);
> @@ -478,6 +483,8 @@ int f2fs_commit_inmem_pages(struct inode *inode)
>  	clear_inode_flag(inode, FI_ATOMIC_COMMIT);
>  
>  	f2fs_unlock_op(sbi);
> +
> +	up_write(&F2FS_I(inode)->i_mmap_sem);
>  	up_write(&fi->i_gc_rwsem[WRITE]);
>  
>  	return err;
> -- 
> 2.29.2