Re: [PATCH] vfs: stop shrinker while fs is freezed

[Junxiao Bi]

Hi Dave,

On 12/15/19 8:09 PM, Dave Chinner wrote:
> On Fri, Dec 13, 2019 at 02:24:40PM -0800, Junxiao Bi wrote:
> > Shrinker could be blocked by freeze while dropping the last reference of
> > some inode that had been removed. As "s_umount" lock was acquired by the
> > Shrinker before blocked, the thaw will hung by this lock. This caused a
> > deadlock.
> >
> >   crash7latest> set 132
> >       PID: 132
> >   COMMAND: "kswapd0:0"
> >      TASK: ffff9cdc9dfb5f00  [THREAD_INFO: ffff9cdc9dfb5f00]
> >       CPU: 6
> >     STATE: TASK_UNINTERRUPTIBLE
> >   crash7latest> bt
> >   PID: 132    TASK: ffff9cdc9dfb5f00  CPU: 6   COMMAND: "kswapd0:0"
> >    #0 [ffffaa5d075bf900] __schedule at ffffffff8186487c
> >    #1 [ffffaa5d075bf998] schedule at ffffffff81864e96
> >    #2 [ffffaa5d075bf9b0] rwsem_down_read_failed at ffffffff818689ee
> >    #3 [ffffaa5d075bfa40] call_rwsem_down_read_failed at ffffffff81859308
> >    #4 [ffffaa5d075bfa90] __percpu_down_read at ffffffff810ebd38
> >    #5 [ffffaa5d075bfab0] __sb_start_write at ffffffff812859ef
> >    #6 [ffffaa5d075bfad0] xfs_trans_alloc at ffffffffc07ebe9c [xfs]
> >    #7 [ffffaa5d075bfb18] xfs_free_eofblocks at ffffffffc07c39d1 [xfs]
> >    #8 [ffffaa5d075bfb80] xfs_inactive at ffffffffc07de878 [xfs]
> >    #9 [ffffaa5d075bfba0] __dta_xfs_fs_destroy_inode_3543 at ffffffffc07e885e [xfs]
> >   #10 [ffffaa5d075bfbd0] destroy_inode at ffffffff812a25de
> >   #11 [ffffaa5d075bfbe8] evict at ffffffff812a2b73
> >   #12 [ffffaa5d075bfc10] dispose_list at ffffffff812a2c1d
> >   #13 [ffffaa5d075bfc38] prune_icache_sb at ffffffff812a421a
> >   #14 [ffffaa5d075bfc70] super_cache_scan at ffffffff812870a1
> >   #15 [ffffaa5d075bfcc8] shrink_slab at ffffffff811eebb3
> >   #16 [ffffaa5d075bfdb0] shrink_node at ffffffff811f4788
> >   #17 [ffffaa5d075bfe38] kswapd at ffffffff811f58c3
> >   #18 [ffffaa5d075bff08] kthread at ffffffff810b75d5
> >   #19 [ffffaa5d075bff50] ret_from_fork at ffffffff81a0035e
> How did you get a file that needed EOF block trimming to be disposed
> of when the filesystem is frozen?
>
> Part of freezing the filesystem is tossing all the reclaimable
> inodes out of the cache, which means all the inodes that might
> require EOF block trimming should have already been removed from the
> cache before the freeze goes into effect....

This issue happened only once, don't know how it was triggered.

I am not xfs developer, don't know what EOF block mean. But this seemed 
a generic issue, destroy inode will engage the transaction on non-EOF 
blocks?

> >   crash7latest> set 31060
> >       PID: 31060
> >   COMMAND: "safefreeze"
> >      TASK: ffff9cd292868000  [THREAD_INFO: ffff9cd292868000]
> >       CPU: 2
> >     STATE: TASK_UNINTERRUPTIBLE
> >   crash7latest> bt
> >   PID: 31060  TASK: ffff9cd292868000  CPU: 2   COMMAND: "safefreeze"
> >    #0 [ffffaa5d10047c90] __schedule at ffffffff8186487c
> >    #1 [ffffaa5d10047d28] schedule at ffffffff81864e96
> >    #2 [ffffaa5d10047d40] rwsem_down_write_failed at ffffffff81868f18
> >    #3 [ffffaa5d10047dd8] call_rwsem_down_write_failed at ffffffff81859367
> >    #4 [ffffaa5d10047e20] down_write at ffffffff81867cfd
> >    #5 [ffffaa5d10047e38] thaw_super at ffffffff81285d2d
> >    #6 [ffffaa5d10047e60] do_vfs_ioctl at ffffffff81299566
> >    #7 [ffffaa5d10047ee8] sys_ioctl at ffffffff81299709
> >    #8 [ffffaa5d10047f28] do_syscall_64 at ffffffff81003949
> >    #9 [ffffaa5d10047f50] entry_SYSCALL_64_after_hwframe at ffffffff81a001ad
> >       RIP: 0000000000453d67  RSP: 00007ffff9c1ce78  RFLAGS: 00000206
> >       RAX: ffffffffffffffda  RBX: 0000000001cbe92c  RCX: 0000000000453d67
> >       RDX: 0000000000000000  RSI: 00000000c0045878  RDI: 0000000000000014
> >       RBP: 00007ffff9c1cf80   R8: 0000000000000000   R9: 0000000000000012
> >       R10: 0000000000000008  R11: 0000000000000206  R12: 0000000000401fb0
> >       R13: 0000000000402040  R14: 0000000000000000  R15: 0000000000000000
> >       ORIG_RAX: 0000000000000010  CS: 0033  SS: 002b
> >
> > Signed-off-by: Junxiao Bi <junxiao.bi@xxxxxxxxxx>
> > ---
> >   fs/super.c | 5 +++++
> >   1 file changed, 5 insertions(+)
> >
> > diff --git a/fs/super.c b/fs/super.c
> > index cfadab2cbf35..adc18652302b 100644
> > --- a/fs/super.c
> > +++ b/fs/super.c
> > @@ -80,6 +80,11 @@ static unsigned long super_cache_scan(struct shrinker *shrink,
> >   	if (!trylock_super(sb))
> >   		return SHRINK_STOP;
> >   
> > +	if (sb->s_writers.frozen != SB_UNFROZEN) {
> > +		up_read(&sb->s_umount);
> > +		return SHRINK_STOP;
> > +	}
> Ah, no. Now go run a filesystem traversal over a filesystem with a
> few tens of million files in it while the filesystem is frozen, and
> what the dentry and inode cache grow and grow until you run out of
> memory....
Yea, this is an issue.
> THe shrinker *needs* to run while the filesystem is frozen, but it
> should not be tripping over files that need modification on
> eviction. Working out how we got a file that required truncation
> during eviction is the first thing to do here so we can then
> determine if a) we should have caught it at freeze time, b) whether
> it can be caught at freeze time, c) whether it can safely be skipped
> during a freeze, or d) something else....

will think about it, thank you.

Thanks,

Junxiao.

> Cheers,
>
> Dave.