Re: [Bug 205135] System hang up when memory swapping (kswapd deadlock)

[Dave Chinner <david@fromorbit.com>]

On Wed, Oct 23, 2019 at 05:49:05PM +1100, Dave Chinner wrote:
> On Tue, Oct 22, 2019 at 06:22:28PM -0700, Darrick J. Wong wrote:
> > On Tue, Oct 22, 2019 at 03:24:22PM -0700, Andrew Morton wrote:
> > > 
> > > (switched to email.  Please respond via emailed reply-to-all, not via the
> > > bugzilla web interface).
> > > 
> > > On Tue, 22 Oct 2019 09:02:22 +0000 bugzilla-daemon@bugzilla.kernel.org wrote:
> > > 
> > > > https://bugzilla.kernel.org/show_bug.cgi?id=205135
> > > > 
> > > > --- Comment #7 from goodmirek@goodmirek.com ---
> > > > Everyone who uses a swapfile on XFS filesystem seem affected by this hang up.
> > > > Not sure about other filesystems, I did not have a chance to test it elsewhere.
> > > > 
> > > > This unreproduced bot crash could be related:
> > > > https://lore.kernel.org/linux-mm/20190910071804.2944-1-hdanton@sina.com/
> > > 
> > > Thanks.  Might be core MM, might be XFS, might be Fedora.
> > > 
> > > Hilf, does your patch look related?  That seems to have gone quiet?
> > > 
> > > Should we progress Tetsuo's patch?
> > 
> > Hmm...
> > 
> > Oct 09 15:44:52 kernel: Linux version 5.4.0-0.rc1.git1.1.fc32.x86_64 (mockbuild@bkernel03.phx2.fedoraproject.org) (gcc version 9.2.1 20190827 (Red Hat 9.2.1-1) (GCC)) #1 SMP Fri Oct 4 14:57:23 UTC 2019
> > 
> > ...istr 5.4-rc1 had some writeback bugs in it...
> > 
> >                         -> #1 (fs_reclaim){+.+.}:
> > Oct 09 13:47:08 kernel:        fs_reclaim_acquire.part.0+0x25/0x30
> > Oct 09 13:47:08 kernel:        __kmalloc+0x4f/0x330
> > Oct 09 13:47:08 kernel:        kmem_alloc+0x83/0x1a0 [xfs]
> > Oct 09 13:47:08 kernel:        kmem_alloc_large+0x3c/0x100 [xfs]
> > Oct 09 13:47:08 kernel:        xfs_attr_copy_value+0x5d/0xa0 [xfs]
> > Oct 09 13:47:08 kernel:        xfs_attr_get+0xe7/0x1d0 [xfs]
> > Oct 09 13:47:08 kernel:        xfs_get_acl+0xad/0x1e0 [xfs]
> > Oct 09 13:47:08 kernel:        get_acl+0x81/0x110
> > Oct 09 13:47:08 kernel:        posix_acl_create+0x58/0x160
> > Oct 09 13:47:08 kernel:        xfs_generic_create+0x7e/0x2f0 [xfs]
> > Oct 09 13:47:08 kernel:        lookup_open+0x5bd/0x820
> > Oct 09 13:47:08 kernel:        path_openat+0x340/0xcb0
> > Oct 09 13:47:08 kernel:        do_filp_open+0x91/0x100
> > Oct 09 13:47:08 kernel:        do_sys_open+0x184/0x220
> > Oct 09 13:47:08 kernel:        do_syscall_64+0x5c/0xa0
> > Oct 09 13:47:08 kernel:        entry_SYSCALL_64_after_hwframe+0x49/0xbe
> > 
> > That's XFS trying to allocate memory to load an acl off disk, only it
> > looks this thread does a MAYFAIL allocation.  It's a GFP_FS (since we
> > don't set KM_NOFS) allocation so we recurse into fs reclaim, and the
> > ACL-getter has locked the inode (which is probably why lockdep
> > triggers).  I wonder if that's really a deadlock vs. just super-slow
> > behavior, but otoh I don't think we're supposed to allow reclaim to jump
> > into the filesystems when the fs has locks held.
> > 
> > That kmem_alloc_large should probably be changed to KM_NOFS.  Dave?
> 
> I suspect it's a false positive, but without the rest of the lockdep
> trace I don't have any context to determine if there is actually a
> deadlock vector there.

Ok, I've looked at the bz now, and the rest of the trace is kswapd
locking an inode from the superblock shrinker. That means I'm pretty
certain this is a false positive and has nothing to do with whatever
hang is occuring on the user's machine.

These:

Oct 09 14:00:18 kernel: DMA-API: cacheline tracking ENOMEM, dma-debug disabled

occur when a radix_tree_insert() call fails, but I don't see a
radix_tree_preload() call anywhere around that code to ensure the
radix tree insert has memory available before locks are taken and
the insert is attempted. Ahhhh:

static RADIX_TREE(dma_active_cacheline, GFP_NOWAIT);

Seems like that is guaranteed to fail under mempry pressure as it
won't allow memory reclaim to block waiting for progress to be made.

Hence I see nothing in the bug to back up the assertions that
"Everyone who uses a swapfile on XFS filesystem seem affected by
this hang up." There's no evidence at all that even points the
subsystem that has hung. sysrq-w, sysrq-l and sysrq-t output are the
first things we need from that machine to see if/where it is
actually hung...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com