Re: [PATCH] f2fs: fix the panic in do_checkpoint()

[Sahitya Tummala]

Hi Chao,

On Fri, Feb 14, 2020 at 10:26:18AM +0800, Chao Yu wrote:
> Hi Sahitya,
> 
> On 2020/2/12 18:34, Sahitya Tummala wrote:
> > There could be a scenario where f2fs_sync_meta_pages() will not
> > ensure that all F2FS_DIRTY_META pages are submitted for IO. Thus,
> > resulting in the below panic in do_checkpoint() -
> > 
> > f2fs_bug_on(sbi, get_pages(sbi, F2FS_DIRTY_META) &&
> > 				!f2fs_cp_error(sbi));
> > 
> > This can happen in a low-memory condition, where shrinker could
> > also be doing the writepage operation (stack shown below)
> > at the same time when checkpoint is running on another core.
> > 
> > schedule
> > down_write
> > f2fs_submit_page_write -> by this time, this page in page cache is tagged
> > 			as PAGECACHE_TAG_WRITEBACK and PAGECACHE_TAG_DIRTY
> > 			is cleared, due to which f2fs_sync_meta_pages()
> > 			cannot sync this page in do_checkpoint() path.
> > f2fs_do_write_meta_page
> > __f2fs_write_meta_page
> > f2fs_write_meta_page
> > shrink_page_list
> > shrink_inactive_list
> > shrink_node_memcg
> > shrink_node
> > kswapd
> 
> IMO, there may be one more simple fix here:
> 
> -	f2fs_do_write_meta_page(sbi, page, io_type);
> 	dec_page_count(sbi, F2FS_DIRTY_META);
> 
> +	f2fs_do_write_meta_page(sbi, page, io_type);
> 
> If we can remove F2FS_DIRTY_META reference count before we clear
> PAGECACHE_TAG_DIRTY, we can avoid this race condition.
> 
> - dec_page_count(sbi, F2FS_DIRTY_META);
> - f2fs_do_write_meta_page
>  - set_page_writeback
>   - __test_set_page_writeback
>    - xas_clear_mark(&xas, PAGECACHE_TAG_DIRTY);
> 
> Thoughts?

I believe it will be a problem because let's say the shrinker path is 
still in the below stack -

f2fs_submit_page_write();
->down_write(&io->io_rwsem);

Then, the current f2fs_wait_on_all_pages_writeback() will not wait for
that page as it is not yet tagged as F2FS_WB_CP_DATA. Hence, the checkpoint
may proceed without waiting for this meta page to be written to disk.

> 
> > 
> > Signed-off-by: Sahitya Tummala <stummala@xxxxxxxxxxxxxx>
> > ---
> >  fs/f2fs/checkpoint.c | 16 ++++++++--------
> >  fs/f2fs/f2fs.h       |  2 +-
> >  fs/f2fs/super.c      |  2 +-
> >  3 files changed, 10 insertions(+), 10 deletions(-)
> > 
> > diff --git a/fs/f2fs/checkpoint.c b/fs/f2fs/checkpoint.c
> > index ffdaba0..2b651a3 100644
> > --- a/fs/f2fs/checkpoint.c
> > +++ b/fs/f2fs/checkpoint.c
> > @@ -1250,14 +1250,14 @@ static void unblock_operations(struct f2fs_sb_info *sbi)
> >  	f2fs_unlock_all(sbi);
> >  }
> >  
> > -void f2fs_wait_on_all_pages_writeback(struct f2fs_sb_info *sbi)
> > +void f2fs_wait_on_all_pages(struct f2fs_sb_info *sbi, int type)
> >  {
> >  	DEFINE_WAIT(wait);
> >  
> >  	for (;;) {
> >  		prepare_to_wait(&sbi->cp_wait, &wait, TASK_UNINTERRUPTIBLE);
> >  
> > -		if (!get_pages(sbi, F2FS_WB_CP_DATA))
> > +		if (!get_pages(sbi, type))
> >  			break;
> >  
> >  		if (unlikely(f2fs_cp_error(sbi)))
> > @@ -1384,8 +1384,8 @@ static int do_checkpoint(struct f2fs_sb_info *sbi, struct cp_control *cpc)
> >  
> >  	/* Flush all the NAT/SIT pages */
> >  	f2fs_sync_meta_pages(sbi, META, LONG_MAX, FS_CP_META_IO);
> > -	f2fs_bug_on(sbi, get_pages(sbi, F2FS_DIRTY_META) &&
> > -					!f2fs_cp_error(sbi));
> > +	/* Wait for all dirty meta pages to be submitted for IO */
> > +	f2fs_wait_on_all_pages(sbi, F2FS_DIRTY_META);
> 
> I'm afraid calling f2fs_wait_on_all_pages() after we call
> f2fs_sync_meta_pages() is low efficient, as we only want to write out
> dirty meta pages instead of wait for writebacking them to device cache.
> 

I have modified the existing function f2fs_wait_on_all_pages_writeback() to
a generic one f2fs_wait_on_all_pages(), where it will wait according to the
requested type. In this case, it will only wait for dirty F2FS_DIRTY_META pages
but not for the writeback to device cache.

Thanks,

> Thanks,
> 
> >  
> >  	/*
> >  	 * modify checkpoint
> > @@ -1493,11 +1493,11 @@ static int do_checkpoint(struct f2fs_sb_info *sbi, struct cp_control *cpc)
> >  
> >  	/* Here, we have one bio having CP pack except cp pack 2 page */
> >  	f2fs_sync_meta_pages(sbi, META, LONG_MAX, FS_CP_META_IO);
> > -	f2fs_bug_on(sbi, get_pages(sbi, F2FS_DIRTY_META) &&
> > -					!f2fs_cp_error(sbi));
> > +	/* Wait for all dirty meta pages to be submitted for IO */
> > +	f2fs_wait_on_all_pages(sbi, F2FS_DIRTY_META);
> >  
> >  	/* wait for previous submitted meta pages writeback */
> > -	f2fs_wait_on_all_pages_writeback(sbi);
> > +	f2fs_wait_on_all_pages(sbi, F2FS_WB_CP_DATA);
> >  
> >  	/* flush all device cache */
> >  	err = f2fs_flush_device_cache(sbi);
> > @@ -1506,7 +1506,7 @@ static int do_checkpoint(struct f2fs_sb_info *sbi, struct cp_control *cpc)
> >  
> >  	/* barrier and flush checkpoint cp pack 2 page if it can */
> >  	commit_checkpoint(sbi, ckpt, start_blk);
> > -	f2fs_wait_on_all_pages_writeback(sbi);
> > +	f2fs_wait_on_all_pages(sbi, F2FS_WB_CP_DATA);
> >  
> >  	/*
> >  	 * invalidate intermediate page cache borrowed from meta inode
> > diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
> > index 5a888a0..b0e0535 100644
> > --- a/fs/f2fs/f2fs.h
> > +++ b/fs/f2fs/f2fs.h
> > @@ -3196,7 +3196,7 @@ bool f2fs_is_dirty_device(struct f2fs_sb_info *sbi, nid_t ino,
> >  void f2fs_update_dirty_page(struct inode *inode, struct page *page);
> >  void f2fs_remove_dirty_inode(struct inode *inode);
> >  int f2fs_sync_dirty_inodes(struct f2fs_sb_info *sbi, enum inode_type type);
> > -void f2fs_wait_on_all_pages_writeback(struct f2fs_sb_info *sbi);
> > +void f2fs_wait_on_all_pages(struct f2fs_sb_info *sbi, int type);
> >  int f2fs_write_checkpoint(struct f2fs_sb_info *sbi, struct cp_control *cpc);
> >  void f2fs_init_ino_entry_info(struct f2fs_sb_info *sbi);
> >  int __init f2fs_create_checkpoint_caches(void);
> > diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
> > index 5111e1f..084633b 100644
> > --- a/fs/f2fs/super.c
> > +++ b/fs/f2fs/super.c
> > @@ -1105,7 +1105,7 @@ static void f2fs_put_super(struct super_block *sb)
> >  	/* our cp_error case, we can wait for any writeback page */
> >  	f2fs_flush_merged_writes(sbi);
> >  
> > -	f2fs_wait_on_all_pages_writeback(sbi);
> > +	f2fs_wait_on_all_pages(sbi, F2FS_WB_CP_DATA);
> >  
> >  	f2fs_bug_on(sbi, sbi->fsync_node_num);
> >  
> > 

-- 
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