Hi, Miklos
The similar issue occurs in our Android device(4G RAM + 3G zram + 8 arm cores + kernel-4.14) too.
Under the monkey test, kswapd and fuse daemon thread deadlocked when free pages is extreme low
(less than 1/2 of the min watermark), the backtrace of the 2 threads is as follows. kswapd
try to evict inode to free some memory(blocked at inode_wait_for_writeback), and fuse daemon thread
handle the fuse inode write request, which is throttled when do direct reclaim in page allocation
slow path(blocked at throttle_direct_reclaim). As the __GFP_FS is set, the thread is throttled until
kswapd free enough pages until watermark ok(check allow_direct_reclaim), which cause the deadlock.
Although the kernel version is 4.14, the same issue exists in the upstream kernel too.
kswapd0 D 26485194.538158 157 1287917 23577482 0x1a20840 0x0 157 438599862461462
<ffffff8beec866b4> __switch_to+0x134/0x150
<ffffff8befb838cc> __schedule+0xd5c/0x1100
<ffffff8befb83ce0> schedule+0x70/0x90
<ffffff8befb849b4> bit_wait+0x14/0x54
<ffffff8befb84350> __wait_on_bit+0x74/0xe0
<ffffff8beeeae0b4> inode_wait_for_writeback+0xa0/0xe4
<ffffff8beee9b95c> evict+0xa4/0x284
<ffffff8beee99b58> iput+0x25c/0x2ac
<ffffff8beee9602c> dentry_unlink_inode+0xd8/0xe4
<ffffff8beee93274> __dentry_kill+0xe8/0x22c
<ffffff8beee9374c> shrink_dentry_list+0x19c/0x3b0
<ffffff8beee9340c> prune_dcache_sb+0x54/0x80
<ffffff8beee79c50> super_cache_scan+0x114/0x164
<ffffff8beee16504> shrink_slab+0x454/0x528
<ffffff8beee1b81c> shrink_node+0x144/0x318
<ffffff8beee1a100> kswapd+0x830/0x9e0
<ffffff8beecde9f0> kthread+0x17c/0x18c
<ffffff8beec856a4> ret_from_fork+0x10/0x18
<ffffffffffffffff> 0xffffffffffffffff
Thread-19 D 7542.719029 2888 24823 5064 0x1404840 0x1000008 24235 438599754021693
<ffffff8beec866b4> __switch_to+0x134/0x150
<ffffff8befb838cc> __schedule+0xd5c/0x1100
<ffffff8befb83ce0> schedule+0x70/0x90
<ffffff8beee18258> try_to_free_pages+0x264/0x4b0
<ffffff8beee06978> __alloc_pages_nodemask+0x7a4/0x10d0
<ffffff8beefac784> fuse_copy_fill+0x15c/0x210
<ffffff8beefabbcc> fuse_dev_do_read+0x434/0xc24
<ffffff8beefab56c> fuse_dev_splice_read+0x84/0x1d8
<ffffff8beeeb5788> SyS_splice+0x67c/0x8bc
<ffffff8beec83fc0> el0_svc_naked+0x34/0x38
<ffffffffffffffff> 0xffffffffffffffff
code snippet:
static bool throttle_direct_reclaim(...)
{
...
/*
* If the caller cannot enter the filesystem, it's possible that it
* is due to the caller holding an FS lock or performing a journal
* transaction in the case of a filesystem like ext[3|4]. In this case,
* it is not safe to block on pfmemalloc_wait as kswapd could be
* blocked waiting on the same lock. Instead, throttle for up to a
* second before continuing.
*/
if (!(gfp_mask & __GFP_FS)) {
wait_event_interruptible_timeout(pgdat->pfmemalloc_wait,
allow_direct_reclaim(pgdat), HZ);
goto check_pending;
}
/* Throttle until kswapd wakes the process */
wait_event_killable(zone->zone_pgdat->pfmemalloc_wait,
allow_direct_reclaim(pgdat));
...
}
Thanks,
yanwu
On Wed, 24 Mar 2021 16:28:35 +0100 Miklos Szeredi via <miklos@xxxxxxxxxx> wrote:
> On what kernel are you seeing this?
> I don't see how it can happen on upstream kernels, since there's a
>"write_inode_now(inode, 1)" call in fuse_release() and nothing can
> dirty the inode after the file has been released.
> Thanks,
> Miklos
>On Tue, Feb 2, 2021 at 5:41 AM Huang Jianan via fuse-devel
><fuse-devel@xxxxxxxxxxxxxxxxxxxxx> wrote:
>>
>> We found the following deadlock situations in low memory scenarios:
>> Thread A Thread B
>> - __writeback_single_inode
>> - fuse_write_inode
>> - fuse_simple_request
>> - __fuse_request_send
>> - request_wait_answer
>> - fuse_dev_splice_read
>> - fuse_copy_fill
>> - __alloc_pages_direct_reclaim
>> - do_shrink_slab
>> - super_cache_scan
>> - shrink_dentry_list
>> - dentry_unlink_inode
>> - iput_final
>> - inode_wait_for_writeback
