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

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