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

[Michal Hocko]

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.
-- 
Michal Hocko
SUSE Labs