Re: [stable] ext4 fscrypt_get_encryption_info() circular locking dependency

Sergey Senozhatsky
2020-12-11T04:10:03Z

On (20/12/10 19:48), Eric Biggers wrote:
> >
> > [ 133.454836] Chain exists of:
> > jbd2_handle --> fscrypt_init_mutex --> fs_reclaim
> >
> > [ 133.454840] Possible unsafe locking scenario:
> >
> > [ 133.454841] CPU0 CPU1
> > [ 133.454843] ---- ----
> > [ 133.454844] lock(fs_reclaim);
> > [ 133.454846] lock(fscrypt_init_mutex);
> > [ 133.454848] lock(fs_reclaim);
> > [ 133.454850] lock(jbd2_handle);
> > [ 133.454851]
>
> This actually got fixed by the patch series
> https://lkml.kernel.org/linux-fscrypt/20200913083620.170627-1-ebiggers@kernel.org/
> which went into 5.10. The more recent patch to remove ext4_dir_open() isn't
> related.
>
> It's a hard patch series to backport. Backporting it to 5.4 would be somewhat
> feasible, while 4.19 would be very difficult as there have been a lot of other
> fscrypt commits which would heavily conflict with cherry-picks.
>
> How interested are you in having this fixed? Did you encounter an actual
> deadlock or just the lockdep report?

Difficult to say. On one hand 'yes' I see lockups on my devices (4.19
kernel); I can't tell at the moment what's the root cause. So on the
other hand 'no' I can't say that it's because of ext4_dir_open().

What I saw so far involved ext4, kswapd, khugepaged and lots of other things.

[ 1598.655901] INFO: task khugepaged:66 blocked for more than 122 seconds.
[ 1598.655914] Call Trace:
[ 1598.655920] __schedule+0x506/0x1240
[ 1598.655924] ? kvm_zap_rmapp+0x52/0x69
[ 1598.655927] schedule+0x3f/0x78
[ 1598.655929] __rwsem_down_read_failed_common+0x186/0x201
[ 1598.655933] call_rwsem_down_read_failed+0x14/0x30
[ 1598.655936] down_read+0x2e/0x45
[ 1598.655939] rmap_walk_file+0x73/0x1ce
[ 1598.655941] page_referenced+0x10d/0x154
[ 1598.655948] shrink_active_list+0x1d4/0x475

[..]

[ 1598.655986] INFO: task kswapd0:79 blocked for more than 122 seconds.
[ 1598.655993] Call Trace:
[ 1598.655995] __schedule+0x506/0x1240
[ 1598.655998] schedule+0x3f/0x78
[ 1598.656000] __rwsem_down_read_failed_common+0x186/0x201
[ 1598.656003] call_rwsem_down_read_failed+0x14/0x30
[ 1598.656006] down_read+0x2e/0x45
[ 1598.656008] rmap_walk_file+0x73/0x1ce
[ 1598.656010] page_referenced+0x10d/0x154
[ 1598.656015] shrink_active_list+0x1d4/0x475

[..]

[ 1598.658233] __rwsem_down_read_failed_common+0x186/0x201
[ 1598.658235] call_rwsem_down_read_failed+0x14/0x30
[ 1598.658238] down_read+0x2e/0x45
[ 1598.658240] rmap_walk_file+0x73/0x1ce
[ 1598.658242] page_referenced+0x10d/0x154
[ 1598.658247] shrink_active_list+0x1d4/0x475
[ 1598.658250] shrink_node+0x27e/0x661
[ 1598.658254] try_to_free_pages+0x425/0x7ec
[ 1598.658258] __alloc_pages_nodemask+0x80b/0x1514
[ 1598.658279] __do_page_cache_readahead+0xd4/0x1a9
[ 1598.658282] filemap_fault+0x346/0x573
[ 1598.658287] ext4_filemap_fault+0x31/0x44

-ss