Re: [PATCH v2] cxl/mce: Avoid alias page retirement for corrected errors

[Alison Schofield <alison.schofield@intel.com>]

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
>