[PATCH v2] cxl/mce: Avoid alias page retirement for corrected errors
From: Shaikh Kamaluddin
Date: Wed Aug 12 2026 - 11:24:52 EST
cxl_handle_mce() offlines the aliased page of an Extended Linear Cache
(ELC) region for any MCE with a usable address in the region. This
includes corrected errors and non-memory errors, needlessly reducing
usable memory.
Restrict ELC MCE handling to uncorrected memory errors. Use
mce_is_correctable() so that MCE_AR_SEVERITY errors continue to retire
the alias when the reported page is handled elsewhere.
Fixes: 516e5bd0b6bf ("cxl: Add mce notifier to emit aliased address for extended linear cache")
Signed-off-by: Shaikh Kamaluddin <shaikhkamal2012@xxxxxxxxx>
---
Changelog:
v1 -> v2:
- Reworked commit message per review feedback
- Dropped redundant comment from the code
Reproduced under QEMU/vng. QEMU's cxl-type3 does not emulate the HMAT
extended-linear address_mode bit, so ELC was forced locally for testing
via a one-line debug hack in cxl_region_probe() (not part of this patch):
if (!p->cache_size && p->res)
p->cache_size = resource_size(p->res) / 2;
Steps:
1. Boot with an Intel CPU model under TCG (KVM host-passthrough will
otherwise leak the host's real vendor ID, and AMD/SMCA takes a
different mce_usable_address() path than the one under test):
vng -v -r ./arch/x86/boot/bzImage --disable-kvm --qemu-opts='-cpu Skylake-Server-v4,+mce,+mca -m 4G -machine q35,cxl=on -object memory-backend-ram,id=cxl-mem0,size=512M -device pxb-cxl,bus_nr=12,bus=pcie.0,id=cxl.0 -device cxl-rp,port=0,bus=cxl.0,id=root_port0,chassis=0,slot=0 -device cxl-type3,bus=root_port0,volatile-memdev=cxl-mem0,id=cxl-mem-device0 -M cxl-fmw.0.targets.0=cxl.0,cxl-fmw.0.size=512M'
2. modprobe mce-inject
$cxl list -M
$cxl list -D
$cxl create-region -m mem0 -d decoder0.0 -w 1 -g 256 -t ram
$dmesg | grep "DEBUG: forced cache_size"
$modprobe device_dax
$modprobe kmem
$ls /sys/bus/dax/devices/
$daxctl reconfigure-device dax0.0 --mode=system-ram
$lsmem
it will show as below:
dax0.0
[ 42.857642] Fallback order for Node 0: 0
[ 42.857828] Built 1 zonelists, mobility grouping on. Total pages: 1010805
[ 42.858446] Policy zone: Normal
[
{
"chardev":"dax0.0",
"size":536870912,
"target_node":0,
"align":2097152,
"mode":"system-ram",
"online_memblocks":4,
"total_memblocks":4,
"movable":true
}
]
reconfigured 1 device
RANGE SIZE STATE REMOVABLE BLOCK
0x0000000000000000-0x000000007fffffff 2G online yes 0-15
0x0000000100000000-0x000000017fffffff 2G online yes 32-47
0x0000000190000000-0x00000001afffffff 512M online yes 50-53
Memory block size: 128M
Total online memory: 4.5G
Total offline memory: 0B
3. Load the injector if not loaded earlier and derive the two MCi_STATUS values.
modprobe mce-inject
MCi_STATUS bit layout used here (arch/x86/include/asm/mce.h):
bit 63 VAL - record valid
bit 61 UC - uncorrected (0 = corrected error under test)
bit 60 EN - error reporting enabled
bit 59 MISCV - MCi_MISC valid
bit 58 ADDRV - MCi_ADDR valid
bits[15:0] MCACOD - bit 7 set = memory-error signature, per
mce_is_memory_error()'s Intel branch
python3 -c "
VAL, UC, EN, MISCV, ADDRV = 1<<63, 1<<61, 1<<60, 1<<59, 1<<58
MCACOD_MEM = 1<<7 # memory error signature
ce = VAL | EN | MISCV | ADDRV | MCACOD_MEM
uc = ce | UC
print(f'CE status = {hex(ce)}')
print(f'UC status = {hex(uc)}')"
# CE status = 0x9c00000000000080
# UC status = 0xbc00000000000080
MCi_MISC: address-mode field, bits[8:6], must be 2 (physical):
python3 -c "print(hex(2 << 6))"
# misc = 0x80
4. $ cd /sys/kernel/debug/mce-inject
$echo sw > flags
$echo 0x9C00000000000080 > status
$echo 0x80 > misc
$echo 0x190010000 > addr
$echo 9 > bank
[ 221.813395] mce: [Hardware Error]: Machine check events logged
[ 221.814716] cxl_mce_debug: entered status=0x9c00000000000080 addr=0x190010000 cache_size=0x10000000 res=[mem 0x190000000-0x1afffffff flags 0x200] usable=1
[ 221.816200] cxl_mce_debug: spa=0x190010000 contains=1
[ 221.817089] cxl_mce_debug: spa_alias=0x1a0010000 pfn=0x1a0010 pfn_valid=1
[ 221.817655] cxl_region region0: Offlining aliased SPA address0: 0x1a0010000
[ 221.821504] Memory failure: 0x1a0010: recovery action for free buddy page: Recovered
[ 221.823903] mce: [Hardware Error]: CPU 0: Machine Check: 0 Bank 9: 9c00000000000080
[ 221.824617] mce: [Hardware Error]: TSC a605ccda40 ADDR 190010000 MISC 80
[ 221.824997] mce: [Hardware Error]: PROCESSOR 0:50654 TIME 1786375908 SOCKET 0 APIC 0 microcode 1
root@virtme-ng:/sys/kernel/debug/mce-inject#
root@virtme-ng:/sys/kernel/debug/mce-inject# grep HardwareCorrupted /proc/meminfo
HardwareCorrupted: 4 kB
CE without this patch:
cxl_region region0: Offlining aliased SPA address0: 0x1a0010000
Memory failure: 0x1a0010: recovery action for free buddy page: Recovered
HardwareCorrupted: 4 kB
-------------------------------------
CE with this patch:
(no "Offlining aliased SPA" message logged)
HardwareCorrupted: 0 kB
$cd /sys/kernel/debug/mce-inject
$echo sw > flags
$echo 0x9C00000000000080 > status
$echo 0x80 > misc
$echo 0x190010000 > addr
$echo 9 > bank
addr bank cpu flags ipid misc README status synd
[ 61.681659] mce: [Hardware Error]: Machine check events logged
[ 61.683442] cxl_mce_debug: entered status=0x9c00000000000080 addr=0x190010000 cache_size=0x10000000 res=[mem 0x190000000-0x1afffffff flags 0x200] usable=1
[ 61.684528] mce: [Hardware Error]: CPU 0: Machine Check: 0 Bank 9: 9c00000000000080
[ 61.684974] mce: [Hardware Error]: TSC 2eeae32fe0 ADDR 190010000 MISC 80
[ 61.688880] mce: [Hardware Error]: PROCESSOR 0:50654 TIME 1786377156 SOCKET 0 APIC 0 microcode 1
[ 86.504456] clocksource: Watchdog remote CPU 11 read timed out
$grep HardwareCorrupted /proc/meminfo
HardwareCorrupted: 0 kB
-------------------------
For UC, same steps only status bit information will change :
$cd /sys/kernel/debug/mce-inject
$echo sw > flags
$echo 0xbc00000000000080 > status
$echo 0x80 > misc
$echo 0x190010000 > addr
$echo 9 > bank
$dmesg | grep cxl_mce_debug
$grep HardwareCorrupted /proc/meminfo
[ 313.398747] mce: [Hardware Error]: Machine check events logged
[ 313.400518] cxl_mce_debug: entered status=0xbc00000000000080 addr=0x190010000 cache_size=0x10000000 res=[mem 0x190000000-0x1afffffff flags 0x200] usable=1
[ 313.401727] cxl_mce_debug: spa=0x190010000 contains=1
[ 313.402157] cxl_mce_debug: spa_alias=0x1a0010000 pfn=0x1a0010 pfn_valid=1
[ 313.402663] cxl_region region0: Offlining aliased SPA address0: 0x1a0010000
[ 313.405999] Memory failure: 0x1a0010: recovery action for free buddy page: Recovered
[ 313.408375] mce: [Hardware Error]: CPU 0: Machine Check: 0 Bank 9: bc00000000000080
[ 313.408999] mce: [Hardware Error]: TSC e9dd2869e0 ADDR 190010000 MISC 80
[ 313.409367] mce: [Hardware Error]: PROCESSOR 0:50654 TIME 1786377750 SOCKET 0 APIC 0 microcode 1
Patched, UC: alias still offlined, confirming uncorrected handling is unchanged by this patch.
cxl_region region0: Offlining aliased SPA address0: 0x1a0010000
Memory failure: 0x1a0010: recovery action for free buddy page: Recovered
HardwareCorrupted: 4 kB
drivers/cxl/core/mce.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)
diff --git a/drivers/cxl/core/mce.c b/drivers/cxl/core/mce.c
index 65fed913b221..e3c561489a00 100644
--- a/drivers/cxl/core/mce.c
+++ b/drivers/cxl/core/mce.c
@@ -18,7 +18,13 @@ static int cxl_handle_mce(struct notifier_block *nb, unsigned long val,
u64 spa, spa_alias;
unsigned long pfn;
- if (!mce || !mce_usable_address(mce))
+ if (!mce)
+ return NOTIFY_DONE;
+
+ if (!mce_is_memory_error(mce) || mce_is_correctable(mce))
+ return NOTIFY_DONE;
+
+ if (!mce_usable_address(mce))
return NOTIFY_DONE;
spa = mce->addr & MCI_ADDR_PHYSADDR;
base-commit: 7098e9cd98a05c0c5de2fae0c2465f9d966fdd07
--
2.43.0