Re: [PATCH] f2fs: Fix deadlock under storage almost full/dirty condition

[Sahitya Tummala]

Hi Chao,

On Mon, Nov 11, 2019 at 10:51:10AM +0800, Chao Yu wrote:
> On 2019/11/8 19:03, Sahitya Tummala wrote:
> > There could be a potential deadlock when the storage capacity
> > is almost full and theren't enough free segments available, due
> > to which FG_GC is needed in the atomic commit ioctl as shown in
> > the below callstack -
> > 
> > schedule_timeout
> > io_schedule_timeout
> > congestion_wait
> > f2fs_drop_inmem_pages_all
> > f2fs_gc
> > f2fs_balance_fs
> > __write_node_page
> > f2fs_fsync_node_pages
> > f2fs_do_sync_file
> > f2fs_ioctl
> > 
> > If this inode doesn't have i_gc_failures[GC_FAILURE_ATOMIC] set,
> > then it waits forever in f2fs_drop_inmem_pages_all(), for this
> > atomic inode to be dropped. And the rest of the system is stuck
> > waiting for sbi->gc_mutex lock, which is acquired by f2fs_balance_fs()
> > in the stack above.
> 
> I think the root cause of this issue is there is potential infinite loop in
> f2fs_drop_inmem_pages_all() for the case of gc_failure is true, because once the
> first inode in inode_list[ATOMIC_FILE] list didn't suffer gc failure, we will
> skip dropping its in-memory cache and calling iput(), and traverse the list
> again, most possibly there is the same inode in the head of that list.
> 

I thought we are expecting for those atomic updates (without any gc failures) to be
committed by doing congestion_wait() and thus retrying again. Hence, I just
fixed only if we are ending up waiting for commit to happen in the atomic
commit path itself, which will be a deadlock.

> Could you please check below fix:
> 
> diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
> index 7bf7b0194944..8a3a35b42a37 100644
> --- a/fs/f2fs/f2fs.h
> +++ b/fs/f2fs/f2fs.h
> @@ -1395,6 +1395,7 @@ struct f2fs_sb_info {
>  	unsigned int gc_mode;			/* current GC state */
>  	unsigned int next_victim_seg[2];	/* next segment in victim section */
>  	/* for skip statistic */
> +	unsigned int atomic_files;		/* # of opened atomic file */
>  	unsigned long long skipped_atomic_files[2];	/* FG_GC and BG_GC */
>  	unsigned long long skipped_gc_rwsem;		/* FG_GC only */
> 
> diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
> index ecd063239642..79f4b348951a 100644
> --- a/fs/f2fs/file.c
> +++ b/fs/f2fs/file.c
> @@ -2047,6 +2047,7 @@ static int f2fs_ioc_start_atomic_write(struct file *filp)
>  	spin_lock(&sbi->inode_lock[ATOMIC_FILE]);
>  	if (list_empty(&fi->inmem_ilist))
>  		list_add_tail(&fi->inmem_ilist, &sbi->inode_list[ATOMIC_FILE]);
> +	sbi->atomic_files++;
>  	spin_unlock(&sbi->inode_lock[ATOMIC_FILE]);
> 
>  	/* add inode in inmem_list first and set atomic_file */
> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
> index 8b977bbd6822..6aa0bb693697 100644
> --- a/fs/f2fs/segment.c
> +++ b/fs/f2fs/segment.c
> @@ -288,6 +288,8 @@ void f2fs_drop_inmem_pages_all(struct f2fs_sb_info *sbi,
> bool gc_failure)
>  	struct list_head *head = &sbi->inode_list[ATOMIC_FILE];
>  	struct inode *inode;
>  	struct f2fs_inode_info *fi;
> +	unsigned int count = sbi->atomic_files;

If the sbi->atomic_files decrements just after this, then the below exit condition
may not work. In that case, looped will never be >= count.

> +	unsigned int looped = 0;
>  next:
>  	spin_lock(&sbi->inode_lock[ATOMIC_FILE]);
>  	if (list_empty(head)) {
> @@ -296,22 +298,29 @@ void f2fs_drop_inmem_pages_all(struct f2fs_sb_info *sbi,
> bool gc_failure)
>  	}
>  	fi = list_first_entry(head, struct f2fs_inode_info, inmem_ilist);
>  	inode = igrab(&fi->vfs_inode);
> +	if (inode)
> +		list_move_tail(&fi->inmem_ilist, head);
>  	spin_unlock(&sbi->inode_lock[ATOMIC_FILE]);
> 
>  	if (inode) {
>  		if (gc_failure) {
> -			if (fi->i_gc_failures[GC_FAILURE_ATOMIC])
> -				goto drop;
> -			goto skip;
> +			if (!fi->i_gc_failures[GC_FAILURE_ATOMIC])
> +				goto skip;
>  		}
> -drop:
>  		set_inode_flag(inode, FI_ATOMIC_REVOKE_REQUEST);
>  		f2fs_drop_inmem_pages(inode);
> +skip:
>  		iput(inode);

Does this result into f2fs_evict_inode() in this context for this inode?

thanks,

>  	}
> -skip:
> +
>  	congestion_wait(BLK_RW_ASYNC, HZ/50);
>  	cond_resched();
> +
> +	if (gc_failure) {
> +		if (++looped >= count)
> +			return;
> +	}
> +
>  	goto next;
>  }
> 
> @@ -334,6 +343,7 @@ void f2fs_drop_inmem_pages(struct inode *inode)
>  	spin_lock(&sbi->inode_lock[ATOMIC_FILE]);
>  	if (!list_empty(&fi->inmem_ilist))
>  		list_del_init(&fi->inmem_ilist);
> +	sbi->atomic_files--;
>  	spin_unlock(&sbi->inode_lock[ATOMIC_FILE]);
>  }
> 
> Thanks,

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