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Date:   Thu, 28 Jan 2021 08:21:33 -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
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References: <20201230075557.108818-1-yuchao0@huawei.com>
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 <3923906d-f208-f6c2-f121-5e77e8fb6b28@huawei.com>
 <YActuF2es8IGJfGj@google.com>
 <98ddd61e-1429-5152-10b1-ac267ec4493d@huawei.com>
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On 01/20, Chao Yu wrote:
> On 2021/1/20 3:06, Jaegeuk Kim wrote:
> > On 01/15, Chao Yu wrote:
> > > On 2021/1/15 5:53, Jaegeuk Kim wrote:
> > > > 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
> > > > 
> > > > Hmm, how about removing fs/f2fs/trace.c to make private more complicated
> > > > like this? I think we can get IO traces from tracepoints.
> > > 
> > > Hmm, actually, there is are issues, one is the trace IO, the other is the
> > > race issue (atomic_start,commit,drop vs mkwrite) which can make isolation
> > > semantics of transaction be broken.
> > > 
> > > Or can we avoid atomic file racing with file mmap?
> 
> Otherwise I think we should add i_mmap_sem to avoid the race.
> 
> > 
> > No, we can't. We may need to find other way to check the race. :)
> 
> Well, any thoughts about this issue?
> 
> Thanks,
> 
> > 
> > > 
> > > - atomic_start			- file_mmap
> > > 				 - inode_lock
> > > 				 - if (FI_ATOMIC_FILE) return
> > >   - inode_lock
> > >   - if (FI_MMAP_FILE) return
> > > 
> > > Thanks,
> > > 
> > > > 
> > > > > 					- truncate
> > > > > 					 - f2fs_invalidate_page
> > > > > 					 - set page->mapping to NULL
> > > > > 					  then it will cause panic once we
> > > > > 					  access page->mapping

Are we hitting this, since page was referenced by in-mem list?

> > > > > 
> > > > > 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
> > > > .
> > > > 
> > .
> >