Messages in this thread Patch in this message | | | From | Rokudo Yan <> | Subject | [PATCH] f2fs: avoid deadlock in gc thread under low memory | Date | Wed, 13 Apr 2022 16:44:32 +0800 |
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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
the gc thread wait for the encrypted_page writeback complete, but as gc thread held sbi->gc_lock, the writeback & sync thread may blocked waiting for sbi->gc_lock, so the bio contain the encrypted_page may nerver submit to block layer and complete the writeback, which cause deadlock. To avoid this deadlock condition, we mark the gc thread with PF_MEMALLOC_NOFS flag, then it will nerver enter fs layer when try to alloc cache page during move_data_block.
Signed-off-by: Rokudo Yan <wu-yan@tcl.com> --- fs/f2fs/gc.c | 6 ++++++ 1 file changed, 6 insertions(+)
diff --git a/fs/f2fs/gc.c b/fs/f2fs/gc.c index e020804f7b07..cc71f77b98c8 100644 --- a/fs/f2fs/gc.c +++ b/fs/f2fs/gc.c @@ -38,6 +38,12 @@ static int gc_thread_func(void *data) wait_ms = gc_th->min_sleep_time; + /* + * Make sure that no allocations from gc thread will ever + * recurse to the fs layer to avoid deadlock as it will + * hold sbi->gc_lock during garbage collection + */ + memalloc_nofs_save(); set_freezable(); do { bool sync_mode, foreground = false; -- 2.25.1
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