[PATCH] powerpc/mce: Fix a bug where mce loops on memory UE.

[Balbir Singh]

On Mon, Apr 23, 2018 at 2:59 PM, Mahesh J Salgaonkar
<mahesh at linux.vnet.ibm.com> wrote:
> From: Mahesh Salgaonkar <mahesh at linux.vnet.ibm.com>
>
> The current code extracts the physical address for UE errors and then
> hooks it up into memory failure infrastructure. On successful extraction
> of physical address it wrongly sets "handled = 1" which means this UE error
> has been recovered. Since MCE handler gets return value as handled = 1, it
> assumes that error has been recovered and goes back to same NIP. This causes
> MCE interrupt again and again in a loop leading to hard lockup.
>
> Also, initialize phys_addr to ULONG_MAX so that we don't end up queuing
> undesired page to hwpoison.
>
> Without this patch we see:
> [ 1476.541984] Severe Machine check interrupt [Recovered]
> [ 1476.541985]   NIP: [000000001002588c] PID: 7109 Comm: find
> [ 1476.541986]   Initiator: CPU
> [ 1476.541987]   Error type: UE [Load/Store]
> [ 1476.541988]     Effective address: 00007fffd2755940
> [ 1476.541989]     Physical address:  000020181a080000
> [...]
> [ 1476.542003] Severe Machine check interrupt [Recovered]
> [ 1476.542004]   NIP: [000000001002588c] PID: 7109 Comm: find
> [ 1476.542005]   Initiator: CPU
> [ 1476.542006]   Error type: UE [Load/Store]
> [ 1476.542006]     Effective address: 00007fffd2755940
> [ 1476.542007]     Physical address:  000020181a080000
> [ 1476.542010] Severe Machine check interrupt [Recovered]
> [ 1476.542012]   NIP: [000000001002588c] PID: 7109 Comm: find
> [ 1476.542013]   Initiator: CPU
> [ 1476.542014]   Error type: UE [Load/Store]
> [ 1476.542015]     Effective address: 00007fffd2755940
> [ 1476.542016]     Physical address:  000020181a080000
> [ 1476.542448] Memory failure: 0x20181a08: recovery action for dirty LRU page: Recovered
> [ 1476.542452] Memory failure: 0x20181a08: already hardware poisoned
> [ 1476.542453] Memory failure: 0x20181a08: already hardware poisoned
> [ 1476.542454] Memory failure: 0x20181a08: already hardware poisoned
> [ 1476.542455] Memory failure: 0x20181a08: already hardware poisoned
> [ 1476.542456] Memory failure: 0x20181a08: already hardware poisoned
> [ 1476.542457] Memory failure: 0x20181a08: already hardware poisoned
> [...]
> [ 1490.972174] Watchdog CPU:38 Hard LOCKUP
>
> After this patch we see:
>
> [  325.384336] Severe Machine check interrupt [Not recovered]

How did you test for this? If the error was recovered, shouldn't the
process have gotten
a SIGBUS and we should have prevented further access as a part of the handling
(memory_failure()). Do we just need a MF_MUST_KILL in the flags?

Why shouldn't we treat it as handled if we isolate the page?

Thanks,
Balbir Singh.