Re: [PATCH] f2fs: avoid deadlock in gc thread under low memory

[Wu Yan]

On 4/13/22 17:46, Michal Hocko wrote:
> On Wed 13-04-22 16:44:32, Rokudo Yan wrote:
> > There is a potential deadlock in gc thread may happen
> > under low memory as below:
> >
> > gc_thread_func
> >   -f2fs_gc
> >    -do_garbage_collect
> >     -gc_data_segment
> >      -move_data_block
> >       -set_page_writeback(fio.encrypted_page);
> >       -f2fs_submit_page_write
> > as f2fs_submit_page_write try to do io merge when possible, so the
> > encrypted_page is marked PG_writeback but may not submit to block
> > layer immediately, if system enter low memory when gc thread try
> > to move next data block, it may do direct reclaim and enter fs layer
> > as below:
> >     -move_data_block
> >      -f2fs_grab_cache_page(index=?, for_write=false)
> >       -grab_cache_page
> >        -find_or_create_page
> >         -pagecache_get_page
> >          -__page_cache_alloc --  __GFP_FS is set
> >           -alloc_pages_node
> >            -__alloc_pages
> >             -__alloc_pages_slowpath
> >              -__alloc_pages_direct_reclaim
> >               -__perform_reclaim
> >                -try_to_free_pages
> >                 -do_try_to_free_pages
> >                  -shrink_zones
> >                   -mem_cgroup_soft_limit_reclaim
> >                    -mem_cgroup_soft_reclaim
> >                     -mem_cgroup_shrink_node
> >                      -shrink_node_memcg
> >                       -shrink_list
> >                        -shrink_inactive_list
> >                         -shrink_page_list
> >                          -wait_on_page_writeback -- the page is marked
> >                         writeback during previous move_data_block call
>
> This is a memcg reclaim path and you would have to have __GFP_ACCOUNT in
> the gfp mask to hit it from the page allocator. I am not really familiar
> with f2fs but I doubt it is using this flag.
>
> On the other hand the memory is charged to a memcg when the newly
> allocated page is added to the page cache. That wouldn't trigger the
> soft reclaim path but that is not really necessary because even the
> regular memcg reclaim would trigger wait_on_page_writeback for cgroup
> v1.
>
> Also are you sure that the mapping's gfp mask has __GFP_FS set for this
> allocation? f2fs_iget uses GFP_NOFS like mask for some inode types.
>
> All that being said, you will need to change the above call chain but it
> would be worth double checking the dead lock is real.

Hi, Michal

1. The issue is occur when do monkey test in Android Device with 4GB RAM 
+ 3GB zram, and memory cgroup v1 enabled.

2. full memory dump has caught when the issue occur and the dead lock 
has confirmed from dump. We can see the mapping->gfp_mask is 0x14200ca,
so both __GFP_ACCOUNT(0x1000000) and __GFP_FS(0x80) set

crash-arm64> struct inode.i_mapping 0xFFFFFFDFD578EEA0
  i_mapping = 0xffffffdfd578f028,
crash-arm64> struct address_space.host,gfp_mask -x 0xffffffdfd578f028
  host = 0xffffffdfd578eea0,
  gfp_mask = 0x14200ca,

Thanks
yanwu