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SubjectRe: [PATCH] f2fs: fix the panic in do_checkpoint()
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@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.
>

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);
> >
> >

--
--
Sent by a consultant of the Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.

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