Re: [PATCH V4] f2fs: Avoid double lock for cp_rwsem during checkpoint

[Jaegeuk Kim]

On 05/09, Chao Yu wrote:
> On 2020/5/9 0:10, Jaegeuk Kim wrote:
> > Hi Sayali,
> > 
> > In order to address the perf regression, how about this?
> > 
> >>From 48418af635884803ffb35972df7958a2e6649322 Mon Sep 17 00:00:00 2001
> > From: Jaegeuk Kim <jaegeuk@xxxxxxxxxx>
> > Date: Fri, 8 May 2020 09:08:37 -0700
> > Subject: [PATCH] f2fs: avoid double lock for cp_rwsem during checkpoint
> > 
> > There could be a scenario where f2fs_sync_node_pages gets
> > called during checkpoint, which in turn tries to flush
> > inline data and calls iput(). This results in deadlock as
> > iput() tries to hold cp_rwsem, which is already held at the
> > beginning by checkpoint->block_operations().
> > 
> > Call stack :
> > 
> > Thread A		Thread B
> > f2fs_write_checkpoint()
> > - block_operations(sbi)
> >  - f2fs_lock_all(sbi);
> >   - down_write(&sbi->cp_rwsem);
> > 
> >                         - open()
> >                          - igrab()
> >                         - write() write inline data
> >                         - unlink()
> > - f2fs_sync_node_pages()
> >  - if (is_inline_node(page))
> >   - flush_inline_data()
> >    - ilookup()
> >      page = f2fs_pagecache_get_page()
> >      if (!page)
> >       goto iput_out;
> >      iput_out:
> > 			-close()
> > 			-iput()
> >        iput(inode);
> >        - f2fs_evict_inode()
> >         - f2fs_truncate_blocks()
> >          - f2fs_lock_op()
> >            - down_read(&sbi->cp_rwsem);
> > 
> > Fixes: 2049d4fcb057 ("f2fs: avoid multiple node page writes due to inline_data")
> > Signed-off-by: Sayali Lokhande <sayalil@xxxxxxxxxxxxxx>
> > Signed-off-by: Jaegeuk Kim <jaegeuk@xxxxxxxxxx>
> > ---
> >  fs/f2fs/node.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/fs/f2fs/node.c b/fs/f2fs/node.c
> > index 1db8cabf727ef..626d7daca09de 100644
> > --- a/fs/f2fs/node.c
> > +++ b/fs/f2fs/node.c
> > @@ -1870,8 +1870,8 @@ int f2fs_sync_node_pages(struct f2fs_sb_info *sbi,
> >  				goto continue_unlock;
> >  			}
> >  
> > -			/* flush inline_data */
> > -			if (is_inline_node(page)) {
> > +			/* flush inline_data, if it's not sync path. */
> > +			if (do_balance && is_inline_node(page)) {
> 
> IIRC, this flow was designed to avoid running out of free space issue
> during checkpoint:
> 
> 2049d4fcb057 ("f2fs: avoid multiple node page writes due to inline_data")
> 
> The sceanrio is:
> 1. create fully node blocks
> 2. flush node blocks
> 3. write inline_data for all the node blocks again
> 4. flush node blocks redundantly
> 
> I guess this may cause failing one case of fstest.

Yeah, actually I was hitting 204 failure, and thus, revised like this.
Now, I don't see any regression in fstest.