Re: deadlock during writeback when using f2fs filesystem

[Michal Hocko]

On Fri 01-06-18 15:02:35, Sahitya Tummala wrote:
> Hi,
> 
> We are observing a deadlock scenario during FS writeback under low-memory
> condition with F2FS filesystem.
> 
> Here is the callstack of this scenario -
> 
> shrink_inactive_list()
> shrink_node_memcg.isra.74()
> shrink_node()
> shrink_zones(inline)
> do_try_to_free_pages(inline)
> try_to_free_pages()
> __perform_reclaim(inline)
> __alloc_pages_direct_reclaim(inline)
> __alloc_pages_slowpath(inline)
> no_zone()
> __alloc_pages(inline)
> __alloc_pages_node(inline)
> alloc_pages_node(inline)
> __page_cache_alloc(inline)
> pagecache_get_page()
> find_or_create_page(inline)
> grab_cache_page(inline)
> f2fs_grab_cache_page(inline)
> __get_node_page.part.32()
> __get_node_page(inline)
> get_node_page()
> update_inode_page()
> f2fs_write_inode()
> write_inode(inline)
> __writeback_single_inode()
> writeback_sb_inodes()
> __writeback_inodes_wb()
> wb_writeback()
> wb_do_writeback(inline)
> wb_workfn()
> 
> The writeback thread is entering into the direct reclaim path due to low-memory and is
> getting stuck in shrink_inactive_list(), as shrink_inactive_list() is inturn waiting for
> writeback to happen for the dirty pages present in the inactive list.

shrink_page_list waits only for writeback pages when we are in the memcg
reclaim. The above seems to be the global reclaim though. Moreover
GFP_F2FS_ZERO is GFP_NOFS so we are not waiting for writeback pages at
all. Are you sure the above is really a deadlock?

> Do you think we can use GFP_NOWAIT for node mapping gfp_mask so that we can avoid direct
> reclaim path in the writeback context? As we may now see allocation failures with this flag,
> do you see any risk or issue in using it w.r.t F2FS FS and writeback?
> Appreciate your suggestions on this.
> 
> diff --git a/fs/f2fs/inode.c b/fs/f2fs/inode.c
> index 89c838b..d3daf3b 100644
> --- a/fs/f2fs/inode.c
> +++ b/fs/f2fs/inode.c
> @@ -316,7 +316,7 @@ struct inode *f2fs_iget(struct super_block *sb, unsigned long ino)
>  make_now:
>         if (ino == F2FS_NODE_INO(sbi)) {
>                 inode->i_mapping->a_ops = &f2fs_node_aops;
> -               mapping_set_gfp_mask(inode->i_mapping, GFP_F2FS_ZERO);
> +               mapping_set_gfp_mask(inode->i_mapping, GFP_F2FS_NODE_MAPPING);
>         } else if (ino == F2FS_META_INO(sbi)) {
>                 inode->i_mapping->a_ops = &f2fs_meta_aops;
>                 mapping_set_gfp_mask(inode->i_mapping, GFP_F2FS_ZERO);
> diff --git a/include/linux/f2fs_fs.h b/include/linux/f2fs_fs.h
> index 58aecb6..bb985cd 100644
> --- a/include/linux/f2fs_fs.h
> +++ b/include/linux/f2fs_fs.h
> @@ -47,6 +47,7 @@
>  /* This flag is used by node and meta inodes, and by recovery */
>  #define GFP_F2FS_ZERO          (GFP_NOFS | __GFP_ZERO)
>  #define GFP_F2FS_HIGH_ZERO     (GFP_NOFS | __GFP_ZERO | __GFP_HIGHMEM)
> +#define GFP_F2FS_NODE_MAPPING  (GFP_NOWAIT | __GFP_IO | __GFP_ZERO)
> 
> Thanks,
> Sahitya.
> -- 
> --
> Sent by a consultant of the Qualcomm Innovation Center, Inc.
> The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.

-- 
Michal Hocko
SUSE Labs