shrink_active_list/try_to_release_page bug? (was Re: xfs trace in 4.4.2 / also in 4.3.3 WARNING fs/xfs/xfs_aops.c:1232 xfs_vm_releasepage)

[Dave Chinner <david@fromorbit.com>]

[adding lkml and linux-mm to the cc list]

On Mon, May 30, 2016 at 09:23:48AM +0200, Stefan Priebe - Profihost AG wrote:
> Hi Dave,
>   Hi Brian,
> 
> below are the results with a vanilla 4.4.11 kernel.

Thanks for persisting with the testing, Stefan.

....

> i've now used a vanilla 4.4.11 Kernel and the issue remains. After a
> fresh reboot it has happened again on the root FS for a debian apt file:
> 
> XFS (md127p3): ino 0x41221d1 delalloc 1 unwritten 0 pgoff 0x0 size 0x12b990
> ------------[ cut here ]------------
> WARNING: CPU: 1 PID: 111 at fs/xfs/xfs_aops.c:1239
> xfs_vm_releasepage+0x10f/0x140()
> Modules linked in: netconsole ipt_REJECT nf_reject_ipv4 xt_multiport
> iptable_filter ip_tables x_tables bonding coretemp 8021q garp fuse
> sb_edac edac_core i2c_i801 i40e(O) xhci_pci xhci_hcd shpchp vxlan
> ip6_udp_tunnel udp_tunnel ipmi_si ipmi_msghandler button btrfs xor
> raid6_pq dm_mod raid1 md_mod usbhid usb_storage ohci_hcd sg sd_mod
> ehci_pci ehci_hcd usbcore usb_common igb ahci i2c_algo_bit libahci
> i2c_core mpt3sas ptp pps_core raid_class scsi_transport_sas
> CPU: 1 PID: 111 Comm: kswapd0 Tainted: G           O    4.4.11 #1
> Hardware name: Supermicro Super Server/X10SRH-CF, BIOS 1.0b 05/18/2015
>  0000000000000000 ffff880c4dacfa88 ffffffffa23c5b8f 0000000000000000
>  ffffffffa2a51ab4 ffff880c4dacfac8 ffffffffa20837a7 ffff880c4dacfae8
>  0000000000000001 ffffea00010c3640 ffff8802176b49d0 ffffea00010c3660
> Call Trace:
>  [<ffffffffa23c5b8f>] dump_stack+0x63/0x84
>  [<ffffffffa20837a7>] warn_slowpath_common+0x97/0xe0
>  [<ffffffffa208380a>] warn_slowpath_null+0x1a/0x20
>  [<ffffffffa2326caf>] xfs_vm_releasepage+0x10f/0x140
>  [<ffffffffa218c680>] ? page_mkclean_one+0xd0/0xd0
>  [<ffffffffa218d3a0>] ? anon_vma_prepare+0x150/0x150
>  [<ffffffffa21521c2>] try_to_release_page+0x32/0x50
>  [<ffffffffa2166b2e>] shrink_active_list+0x3ce/0x3e0
>  [<ffffffffa21671c7>] shrink_lruvec+0x687/0x7d0
>  [<ffffffffa21673ec>] shrink_zone+0xdc/0x2c0
>  [<ffffffffa2168539>] kswapd+0x4f9/0x970
>  [<ffffffffa2168040>] ? mem_cgroup_shrink_node_zone+0x1a0/0x1a0
>  [<ffffffffa20a0d99>] kthread+0xc9/0xe0
>  [<ffffffffa20a0cd0>] ? kthread_stop+0x100/0x100
>  [<ffffffffa26b404f>] ret_from_fork+0x3f/0x70
>  [<ffffffffa20a0cd0>] ? kthread_stop+0x100/0x100
> ---[ end trace c9d679f8ed4d7610 ]---
> XFS (md127p3): ino 0x41221d1 delalloc 1 unwritten 0 pgoff 0x1000 size
> 0x12b990
> XFS (md127p3): ino 0x41221d1 delalloc 1 unwritten 0 pgoff 0x2000 size
.....

Ok, I suspect this may be a VM bug. I've been looking at the 4.6
code (so please try to reproduce on that kernel!) but it looks to me
like the only way we can get from shrink_active_list() direct to
try_to_release_page() is if we are over the maximum bufferhead
threshold (i.e buffer_heads_over_limit = true) and we are trying to
reclaim pages direct from the active list.

Because we are called from kswapd()->balance_pgdat(), we have:

        struct scan_control sc = {
                .gfp_mask = GFP_KERNEL,
                .order = order,
                .priority = DEF_PRIORITY,
                .may_writepage = !laptop_mode,
                .may_unmap = 1,
                .may_swap = 1,
        };

The key point here is reclaim is being run with .may_writepage =
true for default configuration kernels. when we get to
shrink_active_list():

	if (!sc->may_writepage)
		isolate_mode |= ISOLATE_CLEAN;

But sc->may_writepage = true and this allows isolate_lru_pages() to
isolate dirty pages from the active list. Normally this isn't a
problem, because the isolated active list pages are rotated to the
inactive list, and nothing else happens to them. *Except when
buffer_heads_over_limit = true*. This special condition would
explain why I have never seen apt/dpkg cause this problem on any of
my (many) Debian systems that all use XFS....

In that case, shrink_active_list() runs:

	if (unlikely(buffer_heads_over_limit)) {
		if (page_has_private(page) && trylock_page(page)) {
			if (page_has_private(page))
				try_to_release_page(page, 0);
			unlock_page(page);
		}
	}

i.e. it locks the page, and if it has buffer heads it trys to get
the bufferheads freed from the page.

But this is a dirty page, which means it may have delalloc or
unwritten state on it's buffers, both of which indicate that there
is dirty data in teh page that hasn't been written. XFS issues a
warning on this because neither shrink_active_list nor
try_to_release_page() check for whether the page is dirty or not.

Hence it seems to me that shrink_active_list() is calling
try_to_release_page() inappropriately, and XFS is just the
messenger. If you turn laptop mode on, it is likely the problem will
go away as kswapd will run with .may_writepage = false, but that
will also cause other behavioural changes relating to writeback and
memory reclaim. It might be worth trying as a workaround for now.

MM-folk - is this analysis correct? If so, why is
shrink_active_list() calling try_to_release_page() on dirty pages?
Is this just an oversight or is there some problem that this is
trying to work around? It seems trivial to fix to me (add a
!PageDirty check), but I don't know why the check is there in the
first place...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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