Re: fs: panic in mnt_set_mountpoint

[Thierry Reding]

On Mon, Aug 18, 2014 at 01:09:24PM +0200, Jonas Jensen wrote:
> Hi,
> 
> I've run into a kernel panic [1] in next-20140818 you might find interesting.
> 
> It surprised me because there was never an error in next-20140815, I
> did a search for related panics but it came up empty.
> 
> The board boots fine after applying the following:
> 
> diff --git a/fs/namespace.c b/fs/namespace.c
> index 1a2bbef..b408a9d 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -746,7 +746,7 @@ static void put_mountpoint(struct mountpoint *mp)
>  {
>         if (!--mp->m_count) {
>                 struct dentry *dentry = mp->m_dentry;
> -               BUG_ON(!hlist_empty(&mp->m_list));
> +/*             BUG_ON(!hlist_empty(&mp->m_list));*/
>                 spin_lock(&dentry->d_lock);
>                 dentry->d_flags &= ~DCACHE_MOUNTED;
>                 spin_unlock(&dentry->d_lock);
> @@ -810,7 +810,7 @@ void mnt_set_mountpoint(struct mount *mnt,
>         child_mnt->mnt_mountpoint = dget(mp->m_dentry);
>         child_mnt->mnt_parent = mnt;
>         child_mnt->mnt_mp = mp;
> -       hlist_add_head(&child_mnt->mnt_mp_list, &mp->m_list);
> +/*     hlist_add_head(&child_mnt->mnt_mp_list, &mp->m_list);*/
>  }
> 
> 
> [1]
> Uncompressing Linux... done, booting the kernel.
> [ 0.000000] Booting Linux on physical CPU 0x0
> [ 0.000000] Linux version 3.17.0-rc1-next-20140818+ (i@Ildjarn) (gcc
> version 4.9.0 (crosstool-NG 1.19.0) ) #2691 PREEMPT Mon Aug 18
> 10:58:16 CEST 2014
> [ 0.000000] CPU: FA526 [66015261] revision 1 (ARMv4), cr=0000397f
> [ 0.000000] CPU: VIVT data cache, VIVT instruction cache
> [ 0.000000] Machine model: MOXA UC-7112-LX
> [ 0.000000] bootconsole [earlycon0] enabled
> [ 0.000000] Memory policy: Data cache writeback
> [ 0.000000] On node 0 totalpages: 8192
> [ 0.000000] free_area_init_node: node 0, pgdat c035cfb4, node_mem_map c1fba000
> [ 0.000000] Normal zone: 64 pages used for memmap
> [ 0.000000] Normal zone: 0 pages reserved
> [ 0.000000] Normal zone: 8192 pages, LIFO batch:0
> [ 0.000000] pcpu-alloc: s0 r0 d32768 u32768 alloc=1*32768
> [ 0.000000] pcpu-alloc: [0] 0
> [ 0.000000] Built 1 zonelists in Zone order, mobility grouping on.
> Total pages: 8128
> [ 0.000000] Kernel command line: console=ttyS0,115200n8 earlyprintk
> root=/dev/mmcblk0p1 rw rootwait debug
> [ 0.000000] PID hash table entries: 128 (order: -3, 512 bytes)
> [ 0.000000] Dentry cache hash table entries: 4096 (order: 2, 16384 bytes)
> [ 0.000000] Inode-cache hash table entries: 2048 (order: 1, 8192 bytes)
> [ 0.000000] Memory: 28852K/32768K available (2718K kernel code, 93K
> rwdata, 484K rodata, 115K init, 126K bss, 3916K reserved)
> [ 0.000000] Virtual kernel memory layout:
> [ 0.000000] vector : 0xffff0000 - 0xffff1000 ( 4 kB)
> [ 0.000000] fixmap : 0xffc00000 - 0xffe00000 (2048 kB)
> [ 0.000000] vmalloc : 0xc2800000 - 0xff000000 ( 968 MB)
> [ 0.000000] lowmem : 0xc0000000 - 0xc2000000 ( 32 MB)
> [ 0.000000] .text : 0xc0008000 - 0xc0328aec (3203 kB)
> [ 0.000000] .init : 0xc0329000 - 0xc0345ddc ( 116 kB)
> [ 0.000000] .data : 0xc0346000 - 0xc035d700 ( 94 kB)
> [ 0.000000] .bss : 0xc035d700 - 0xc037cf24 ( 127 kB)
> [ 0.000000] SLUB: HWalign=32, Order=0-3, MinObjects=0, CPUs=1, Nodes=1
> [ 0.000000] Preemptible hierarchical RCU implementation.
> [ 0.000000] NR_IRQS:16 nr_irqs:16 16
> [ 0.000000] sched_clock: 32 bits at 100 Hz, resolution 10000000ns,
> wraps every 21474836480000000ns
> [ 0.010000] Calibrating delay loop... 147.86 BogoMIPS (lpj=739328)
> [ 0.090000] pid_max: default: 4096 minimum: 301
> [ 0.090000] Mount-cache hash table entries: 1024 (order: 0, 4096 bytes)
> [ 0.100000] Mountpoint-cache hash table entries: 1024 (order: 0, 4096 bytes)
> [ 0.110000] CPU: Testing write buffer coherency: ok
> [ 0.120000] Setting up static identity map for 0x296938 - 0x296980
> [ 0.140000] Unhandled fault: alignment exception (0x801) at 0x00000003

I've been seeing the same crash starting with next-20140818. I have slab
allocation debugging enabled and the above unhandled fault is apparently
caused by in-use poisoning:

	[    0.301383] Unable to handle kernel paging request at virtual address 5a5a5a5e

The below patch works "fixes" it for me, though I have no idea why it's
needed all of a sudden.

Thierry

diff --git a/fs/namespace.c b/fs/namespace.c
index 4246696dc678..a6815027f225 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -736,6 +736,7 @@ static struct mountpoint *new_mountpoint(struct dentry *dentry)
 		return ERR_PTR(ret);
 	}
 
+	INIT_HLIST_HEAD(&mp->m_list);
 	mp->m_dentry = dentry;
 	mp->m_count = 1;
 	hlist_add_head(&mp->m_hash, chain);

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