Re: deadlock during writeback when using f2fs filesystem

[Sahitya Tummala]

On Fri, Jun 01, 2018 at 12:26:09PM +0200, Michal Hocko wrote:
> 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?
> 

Let me correct my statement. It could be more of a livelock scenario.

The direct reclaim path is not doing any writeback here, so the GFP_NOFS doesn't
make any difference. In this case, the direct reclaim has to reclaim ~32 pages,
which it picks up from the tail of the list. All of those tail pages are dirty
and since direct reclaim path can't do any writeback, it just loops picking and
skipping them.

> > 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

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