Re: [PATCHv3 02/10] ext4: Fix ext4_fc_stats trace point

[Nathan Chancellor]

On Thu, Mar 17, 2022 at 05:11:42PM +0100, Sven Schnelle wrote:
> Hi,
> 
> Ritesh Harjani <riteshh@xxxxxxxxxxxxx> writes:
> 
> > On 22/03/17 01:01PM, Sven Schnelle wrote:
> >> Ritesh Harjani <riteshh@xxxxxxxxxxxxx> writes:
> >>
> >> [    0.958403] Hardware name: IBM 3906 M04 704 (z/VM 7.1.0)
> >> [    0.958407] Workqueue: eval_map_wq eval_map_work_func
> >>
> >> [    0.958446] Krnl PSW : 0704e00180000000 000000000090a9d6 (number+0x25e/0x3c0)
> >> [    0.958456]            R:0 T:1 IO:1 EX:1 Key:0 M:1 W:0 P:0 AS:3 CC:2 PM:0 RI:0 EA:3
> >> [    0.958461] Krnl GPRS: 0000000000000058 00000000010de0ac 0000000000000001 00000000fffffffc
> >> [    0.958467]            0000038000047b80 0affffff010de0ab 0000000000000000 0000000000000000
> >> [    0.958481]            0000000000000020 0000038000000000 00000000010de0ad 00000000010de0ab
> >> [    0.958484]            0000000080312100 0000000000e68910 0000038000047b50 0000038000047ab8
> >> [    0.958494] Krnl Code: 000000000090a9c6: f0c84112b001        srp     274(13,%r4),1(%r11),8
> >> [    0.958494]            000000000090a9cc: 41202001            la      %r2,1(%r2)
> >> [    0.958494]           #000000000090a9d0: ecab0006c065        clgrj   %r10,%r11,12,000000000090a9dc
> >> [    0.958494]           >000000000090a9d6: d200b0004000        mvc     0(1,%r11),0(%r4)
> >> [    0.958494]            000000000090a9dc: 41b0b001            la      %r11,1(%r11)
> >> [    0.958494]            000000000090a9e0: a74bffff
> >>             aghi    %r4,-1
> >> [    0.958494]            000000000090a9e4: a727fff6            brctg   %r2,000000000090a9d0
> >> [    0.958494]            000000000090a9e8: a73affff            ahi     %r3,-1
> >> [    0.958575] Call Trace:
> >> [    0.958580]  [<000000000090a9d6>] number+0x25e/0x3c0
> >> [    0.958594] ([<0000000000289516>] update_event_printk+0xde/0x200)
> >> [    0.958602]  [<0000000000910020>] vsnprintf+0x4b0/0x7c8
> >> [    0.958606]  [<00000000009103e8>] snprintf+0x40/0x50
> >> [    0.958610]  [<00000000002893d2>] eval_replace+0x62/0xc8
> >> [    0.958614]  [<000000000028e2fe>] trace_event_eval_update+0x206/0x248
> >
> > This looks like you must have this patch from Steven as well [2].
> > Although I did test the patch and didn't see such a crash on my qemu box [3].

Indeed, commit b3bc8547d3be ("tracing: Have TRACE_DEFINE_ENUM affect
trace event types as well") from the ftrace tree is required to
reproduce this. The ftrace and ext4 changes alone are fine (my initial
bisect landed on a merge and I did two more bisects to confirm that).

> > [2]: https://lore.kernel.org/linux-ext4/20220310233234.4418186a@xxxxxxxxxxxxxxxxxx/
> > [3]: https://lore.kernel.org/linux-ext4/20220311051249.ltgqbjjothbrkbno@riteshh-domain/
> >
> > @Steven,
> > Sorry to bother. But does this crash strike anything obvious to you?
> 
> Looking at the oops output again made me realizes that the snprintf
> tries to write into pages that are mapped RO. Talking to Heiko he
> mentioned that s390 maps rodata/text RO when setting up the initial
> mapping while x86 has a RW mapping in the beginning and changes that
> later to RO. I haven't verified that, but that might be a reason why it
> works on x86.

For what it's worth, this is reproducible on all of my x86 boxes during
the initial boot on next-20220316 and newer. I am happy to test any
patches or provide further information as necessary.

Cheers,
Nathan