Re: [PATCH] fuse: avoid deadlock when write fuse inode

[Huang Jianan <huangjianan@oppo.com>]

Hi all,


This patch works well in our product, but I am not sure this is the correct

way to solve this problem. I think that the inode->i_count shouldn't be

zero after iput is executed in dentry_unlink_inode, then the inode won't

be writeback. But i haven't found where iget is missing.


Thanks,

Jianan

On 2021/2/2 12:08, Huang Jianan wrote:
> We found the following deadlock situations in low memory scenarios:
> Thread A                         Thread B
> - __writeback_single_inode
>   - fuse_write_inode
>    - fuse_simple_request
>     - __fuse_request_send
>      - request_wait_answer
>                                   - fuse_dev_splice_read
>                                    - fuse_copy_fill
>                                     - __alloc_pages_direct_reclaim
>                                      - do_shrink_slab
>                                       - super_cache_scan
>                                        - shrink_dentry_list
>                                         - dentry_unlink_inode
>                                          - iput_final
>                                           - inode_wait_for_writeback
>
> The request and inode processed by Thread A and B are the same, which
> causes a deadlock. To avoid this, we remove the __GFP_FS flag when
> allocating memory in fuse_copy_fill, so there will be no memory
> reclaimation in super_cache_scan.
>
> Signed-off-by: Huang Jianan <huangjianan@oppo.com>
> Signed-off-by: Guo Weichao <guoweichao@oppo.com>
> ---
>   fs/fuse/dev.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
> index 588f8d1240aa..e580b9d04c25 100644
> --- a/fs/fuse/dev.c
> +++ b/fs/fuse/dev.c
> @@ -721,7 +721,7 @@ static int fuse_copy_fill(struct fuse_copy_state *cs)
>   			if (cs->nr_segs >= cs->pipe->max_usage)
>   				return -EIO;
>   
> -			page = alloc_page(GFP_HIGHUSER);
> +			page = alloc_page(GFP_HIGHUSER & ~__GFP_FS);
>   			if (!page)
>   				return -ENOMEM;
>