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

>
> Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>
> ---
> 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.

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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