* [PATCH v2] cxl/mce: Avoid alias page retirement for corrected errors
@ 2026-08-12 15:17 Shaikh Kamaluddin
2026-08-12 23:02 ` Alison Schofield
0 siblings, 1 reply; 2+ messages in thread
From: Shaikh Kamaluddin @ 2026-08-12 15:17 UTC (permalink / raw)
To: Davidlohr Bueso, Jonathan Cameron, Dave Jiang, Alison Schofield,
Vishal Verma, Dan Williams, Ira Weiny, Li Ming, Ben Cheatham,
tony.luck, bp
Cc: linux-cxl, linux-edac, linux-kernel, stable, x86
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@gmail.com>
---
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
^ permalink raw reply related [flat|nested] 2+ messages in thread
* Re: [PATCH v2] cxl/mce: Avoid alias page retirement for corrected errors
2026-08-12 15:17 [PATCH v2] cxl/mce: Avoid alias page retirement for corrected errors Shaikh Kamaluddin
@ 2026-08-12 23:02 ` Alison Schofield
0 siblings, 0 replies; 2+ messages in thread
From: Alison Schofield @ 2026-08-12 23:02 UTC (permalink / raw)
To: Shaikh Kamaluddin
Cc: Davidlohr Bueso, Jonathan Cameron, Dave Jiang, Vishal Verma,
Dan Williams, Ira Weiny, Li Ming, Ben Cheatham, tony.luck, bp,
linux-cxl, linux-edac, linux-kernel, stable, x86
On Wed, Aug 12, 2026 at 08:47:59PM +0530, Shaikh Kamaluddin wrote:
> 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.
Hi Shaikh,
Thanks for revising this.
Sashiko had a complaint that needs to be considered about the AMD
specific MCE classification and makes me wonder about using
mce_is_memory_error() in the test at all.
Could the fix simply reject mce_is_correctable() and retain
the existing mce_usable_address() behavior? (And yeah, I'm thinking
I may have missed something in that complex description I asked
you to trim.)
Sashiko only replied to the linux-cxl mailing list so not all recipients
of your patch will see it:
https://sashiko.dev/#/patchset/20260812151759.14390-1-shaikhkamal2012%40gmail.com
BTW - it looks like Sashiko never looked at v1.
-- Alison
>
> Fixes: 516e5bd0b6bf ("cxl: Add mce notifier to emit aliased address for extended linear cache")
> Signed-off-by: Shaikh Kamaluddin <shaikhkamal2012@gmail.com>
> ---
> 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
>
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2026-08-12 23:02 ` Alison Schofield
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