Messages in this thread | | | Date | Wed, 13 Apr 2022 19:18:04 -0700 | From | Jaegeuk Kim <> | Subject | Re: [PATCH] f2fs: avoid deadlock in gc thread under low memory |
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On 04/14, Wu Yan wrote: > On 4/14/22 01:00, Jaegeuk Kim wrote: > > On 04/13, 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 > > > > > > 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(); > > > > I think this cannot cover all the f2fs_gc() call cases. Can we just avoid by: > > > > --- a/fs/f2fs/gc.c > > +++ b/fs/f2fs/gc.c > > @@ -1233,7 +1233,7 @@ static int move_data_block(struct inode *inode, block_t bidx, > > CURSEG_ALL_DATA_ATGC : CURSEG_COLD_DATA; > > > > /* do not read out */ > > - page = f2fs_grab_cache_page(inode->i_mapping, bidx, false); > > + page = f2fs_grab_cache_page(inode->i_mapping, bidx, true); > > if (!page) > > return -ENOMEM; > > > > Thanks, > > > > > set_freezable(); > > > do { > > > bool sync_mode, foreground = false; > > > -- > > > 2.25.1 > > Hi, Jaegeuk > > I'm not sure if any other case may trigger the issue, but the stack traces I > have caught so far are all the same as below: > > f2fs_gc-253:12 D 226966.808196 572 302561 150976 0x1200840 0x0 572 > 237207473347056 > <ffffff889d88668c> __switch_to+0x134/0x150 > <ffffff889e764b6c> __schedule+0xd5c/0x1100 > <ffffff889e76554c> io_schedule+0x90/0xc0 > <ffffff889d9fb880> wait_on_page_bit+0x194/0x208 > <ffffff889da167b4> shrink_page_list+0x62c/0xe74 > <ffffff889da1d354> shrink_inactive_list+0x2c0/0x698 > <ffffff889da181f4> shrink_node_memcg+0x3dc/0x97c > <ffffff889da17d44> mem_cgroup_shrink_node+0x144/0x218 > <ffffff889da6610c> mem_cgroup_soft_limit_reclaim+0x188/0x47c > <ffffff889da17a40> do_try_to_free_pages+0x204/0x3a0 > <ffffff889da176c8> try_to_free_pages+0x35c/0x4d0 > <ffffff889da05d60> __alloc_pages_nodemask+0x7a4/0x10d0 > <ffffff889d9fc82c> pagecache_get_page+0x184/0x2ec
Is this deadlock trying to grab a lock, instead of waiting for writeback? Could you share all the backtraces of the tasks?
For writeback above, looking at the code, f2fs_gc uses three mappings, meta, node, and data, and meta/node inodes are masking GFP_NOFS in f2fs_iget(), while data inode does not. So, the above f2fs_grab_cache_page() in move_data_block() is actually calling w/o NOFS.
> <ffffff889dbf8860> do_garbage_collect+0xfe0/0x2828 > <ffffff889dbf7434> f2fs_gc+0x4a0/0x8ec > <ffffff889dbf6bf4> gc_thread_func+0x240/0x4d4 > <ffffff889d8de9b0> kthread+0x17c/0x18c > <ffffff889d88567c> ret_from_fork+0x10/0x18 > > Thanks > yanwu
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