[PATCH] cxl/mce: Only act on uncorrected memory errors

[PATCH] cxl/mce: Only act on uncorrected memory errors

[Shaikh Kamaluddin]

cxl_handle_mce() offlines the aliased page of an ELC region on any
record with a usable address; it does not check MCI_STATUS_UC or
filter non-memory errors. uc_decode_notifier(), the equivalent
handler for plain memory on the same chain at the same priority,
filters on mce->severity and leaves corrected errors untouched.
cxl_handle_mce() has no such gate, so a corrected error - which
the generic handler ignores - still causes the alias to be
permanently retired via memory_failure().

Corrected errors do reach the chain: machine_check_poll() logs
them via the same mce_gen_pool_process() path that feeds
x86_mce_decoder_chain, and cxl_extended_linear_cache_resize()
extends p->res to cover the DRAM half of the ELC pair, so a
routine DRAM CE carries an address inside the region resource.

Filter the record as nfit_handle_mce() does. Commit fc08a4703a41
("acpi, nfit: Fix the memory error check in nfit_handle_mce()") and
commit 5d96c9342c23 ("acpi/nfit, x86/mce: Handle only uncorrectable
machine checks") established this filter for an equivalent handler
on the same notifier chain; the consequence here is more severe, as
the CXL handler calls memory_failure() rather than recording a bad
block.

mce_is_correctable() is used instead of copying
uc_decode_notifier()'s AO/DEFERRED test because the alias must
still be offlined on MCE_AR_SEVERITY, where kill_me_maybe() owns
the reported page but nothing owns the alias.

Fixes: 516e5bd0b6bf ("cxl: Add mce notifier to emit aliased address for extended linear cache")

Signed-off-by: Shaikh Kamaluddin <shaikhkamal2012@gmail.com>
---
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 | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/cxl/core/mce.c b/drivers/cxl/core/mce.c
index 65fed913b221..ee70c1c9f9b9 100644
--- a/drivers/cxl/core/mce.c
+++ b/drivers/cxl/core/mce.c
@@ -18,7 +18,14 @@ 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;
+
+	/* Only uncorrected memory errors warrant taking down the alias page */
+	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

Re: [PATCH] cxl/mce: Only act on uncorrected memory errors

[Cheatham, Benjamin]

On 8/10/2026 1:30 PM, Shaikh Kamaluddin wrote:
> [You don't often get email from shaikhkamal2012@gmail.com. Learn why this is important at https://aka.ms/LearnAboutSenderIdentification ]
> 
> cxl_handle_mce() offlines the aliased page of an ELC region on any
> record with a usable address; it does not check MCI_STATUS_UC or
> filter non-memory errors. uc_decode_notifier(), the equivalent
> handler for plain memory on the same chain at the same priority,
> filters on mce->severity and leaves corrected errors untouched.
> cxl_handle_mce() has no such gate, so a corrected error - which
> the generic handler ignores - still causes the alias to be
> permanently retired via memory_failure().
> 
> Corrected errors do reach the chain: machine_check_poll() logs
> them via the same mce_gen_pool_process() path that feeds
> x86_mce_decoder_chain, and cxl_extended_linear_cache_resize()
> extends p->res to cover the DRAM half of the ELC pair, so a
> routine DRAM CE carries an address inside the region resource.
> 
> Filter the record as nfit_handle_mce() does. Commit fc08a4703a41
> ("acpi, nfit: Fix the memory error check in nfit_handle_mce()") and
> commit 5d96c9342c23 ("acpi/nfit, x86/mce: Handle only uncorrectable
> machine checks") established this filter for an equivalent handler
> on the same notifier chain; the consequence here is more severe, as
> the CXL handler calls memory_failure() rather than recording a bad
> block.
> 
> mce_is_correctable() is used instead of copying
> uc_decode_notifier()'s AO/DEFERRED test because the alias must
> still be offlined on MCE_AR_SEVERITY, where kill_me_maybe() owns
> the reported page but nothing owns the alias.
> 
> Fixes: 516e5bd0b6bf ("cxl: Add mce notifier to emit aliased address for extended linear cache")
> 
> Signed-off-by: Shaikh Kamaluddin <shaikhkamal2012@gmail.com>
> ---

This looks good to me, so:
Reviewed-by: Ben Cheatham <benjamin.cheatham@amd.com>

If you have the time, could you also share a script that does the below testing on the list? It may
be possible to integrate into the CXL testing suite (see https://github.com/pmem/ndctl.git), though
the QEMU usage may throw a wrench in that. Even if it's not possible, having the tests out there
for people to run would help with any future breakage.

Thanks,
Ben
> 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 | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/cxl/core/mce.c b/drivers/cxl/core/mce.c
> index 65fed913b221..ee70c1c9f9b9 100644
> --- a/drivers/cxl/core/mce.c
> +++ b/drivers/cxl/core/mce.c
> @@ -18,7 +18,14 @@ 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;
> +
> +       /* Only uncorrected memory errors warrant taking down the alias page */
> +       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
> 

Re: [PATCH] cxl/mce: Only act on uncorrected memory errors

[Alison Schofield]

On Tue, Aug 11, 2026 at 12:00:13AM +0530, Shaikh Kamaluddin wrote:
> cxl_handle_mce() offlines the aliased page of an ELC region on any
> record with a usable address; it does not check MCI_STATUS_UC or
> filter non-memory errors. uc_decode_notifier(), the equivalent
> handler for plain memory on the same chain at the same priority,
> filters on mce->severity and leaves corrected errors untouched.
> cxl_handle_mce() has no such gate, so a corrected error - which
> the generic handler ignores - still causes the alias to be
> permanently retired via memory_failure().
> 
> Corrected errors do reach the chain: machine_check_poll() logs
> them via the same mce_gen_pool_process() path that feeds
> x86_mce_decoder_chain, and cxl_extended_linear_cache_resize()
> extends p->res to cover the DRAM half of the ELC pair, so a
> routine DRAM CE carries an address inside the region resource.
> 
> Filter the record as nfit_handle_mce() does. Commit fc08a4703a41
> ("acpi, nfit: Fix the memory error check in nfit_handle_mce()") and
> commit 5d96c9342c23 ("acpi/nfit, x86/mce: Handle only uncorrectable
> machine checks") established this filter for an equivalent handler
> on the same notifier chain; the consequence here is more severe, as
> the CXL handler calls memory_failure() rather than recording a bad
> block.
> 
> mce_is_correctable() is used instead of copying
> uc_decode_notifier()'s AO/DEFERRED test because the alias must
> still be offlined on MCE_AR_SEVERITY, where kill_me_maybe() owns
> the reported page but nothing owns the alias.
> 
> Fixes: 516e5bd0b6bf ("cxl: Add mce notifier to emit aliased address for extended linear cache")
> 

Thanks for the patch Shaikh,

The patch looks good to me, and the reproduction details show that this has
been thoroughly investigated.

The commit log is harder to follow than the code change itself. It seems
that too much of the investigation has leaked into the commit log which
makes it very dense and not very clarifying. For what is ultimately a
small and straightforward behavioral fix, the log spends a lot of time
narrating the code paths that establish the problem, while the important
impact, a reduction in usable memory, gets lost.

The commit log should introduce the code change and make it easier to
understand. I'd prefer the usual behavioral progression: describe what
happens today, why that is wrong and its impact, then state how the patch
fixes it. Also, please spell out Extended Linear Cache (ELC) on first use.

The subject can describe the behavior being fixed more directly as well.

For example: (if you use this example, you need to proofread it, to
be sure I actually captured this correctly ;))

cxl/mce: Avoid alias page retirement for corrected errors

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")


more below...


> Signed-off-by: Shaikh Kamaluddin <shaikhkamal2012@gmail.com>
> ---

The detailed investigation and reproduction below the scissors is useful and
should stay there.

snip

> 
> diff --git a/drivers/cxl/core/mce.c b/drivers/cxl/core/mce.c
> index 65fed913b221..ee70c1c9f9b9 100644
> --- a/drivers/cxl/core/mce.c
> +++ b/drivers/cxl/core/mce.c
> @@ -18,7 +18,14 @@ 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;
> +
> +	/* Only uncorrected memory errors warrant taking down the alias page */
> +	if (!mce_is_memory_error(mce) || mce_is_correctable(mce))
> +		return NOTIFY_DONE;
> +

The comment restates what the if condition says. I think the comment can
simply be dropped.



> +	if (!mce_usable_address(mce))
>  		return NOTIFY_DONE;
>  
>  	spa = mce->addr & MCI_ADDR_PHYSADDR;
> 
> base-commit: 7098e9cd98a05c0c5de2fae0c2465f9d966fdd07
> -- 
> 2.43.0
> 

Re: [PATCH] cxl/mce: Only act on uncorrected memory errors

[shaikh kamaluddin]

On Mon, Aug 10, 2026 at 02:00:18PM -0500, Cheatham, Benjamin wrote:
> On 8/10/2026 1:30 PM, Shaikh Kamaluddin wrote:
> > [You don't often get email from shaikhkamal2012@gmail.com. Learn why this is important at https://aka.ms/LearnAboutSenderIdentification ]
> > 
> > cxl_handle_mce() offlines the aliased page of an ELC region on any
> > record with a usable address; it does not check MCI_STATUS_UC or
> > filter non-memory errors. uc_decode_notifier(), the equivalent
> > handler for plain memory on the same chain at the same priority,
> > filters on mce->severity and leaves corrected errors untouched.
> > cxl_handle_mce() has no such gate, so a corrected error - which
> > the generic handler ignores - still causes the alias to be
> > permanently retired via memory_failure().
> > 
> > Corrected errors do reach the chain: machine_check_poll() logs
> > them via the same mce_gen_pool_process() path that feeds
> > x86_mce_decoder_chain, and cxl_extended_linear_cache_resize()
> > extends p->res to cover the DRAM half of the ELC pair, so a
> > routine DRAM CE carries an address inside the region resource.
> > 
> > Filter the record as nfit_handle_mce() does. Commit fc08a4703a41
> > ("acpi, nfit: Fix the memory error check in nfit_handle_mce()") and
> > commit 5d96c9342c23 ("acpi/nfit, x86/mce: Handle only uncorrectable
> > machine checks") established this filter for an equivalent handler
> > on the same notifier chain; the consequence here is more severe, as
> > the CXL handler calls memory_failure() rather than recording a bad
> > block.
> > 
> > mce_is_correctable() is used instead of copying
> > uc_decode_notifier()'s AO/DEFERRED test because the alias must
> > still be offlined on MCE_AR_SEVERITY, where kill_me_maybe() owns
> > the reported page but nothing owns the alias.
> > 
> > Fixes: 516e5bd0b6bf ("cxl: Add mce notifier to emit aliased address for extended linear cache")
> > 
> > Signed-off-by: Shaikh Kamaluddin <shaikhkamal2012@gmail.com>
> > ---
> 
> This looks good to me, so:
> Reviewed-by: Ben Cheatham <benjamin.cheatham@amd.com>
> 
Thanks for review!
> If you have the time, could you also share a script that does the below testing on the list? It may
> be possible to integrate into the CXL testing suite (see https://github.com/pmem/ndctl.git), though
> the QEMU usage may throw a wrench in that. Even if it's not possible, having the tests out there
> for people to run would help with any future breakage.
Happy to share it. One question on where it'd fit best: ndctl
(github.com/pmem/ndctl.git) as you mentioned, or drivers/cxl's own
tools/testing/cxl/ in-tree? I'm open to either, or proposing it in
both if that's useful - happy to follow your lead on which is the
better home for it.

Thanks,
Shaikh 
> 
> Thanks,
> Ben
> > 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 | 9 ++++++++-
> >  1 file changed, 8 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/cxl/core/mce.c b/drivers/cxl/core/mce.c
> > index 65fed913b221..ee70c1c9f9b9 100644
> > --- a/drivers/cxl/core/mce.c
> > +++ b/drivers/cxl/core/mce.c
> > @@ -18,7 +18,14 @@ 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;
> > +
> > +       /* Only uncorrected memory errors warrant taking down the alias page */
> > +       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
> > 
> 

Re: [PATCH] cxl/mce: Only act on uncorrected memory errors

[shaikh kamaluddin]

On Tue, Aug 11, 2026 at 12:22:15PM -0700, Alison Schofield wrote:
> On Tue, Aug 11, 2026 at 12:00:13AM +0530, Shaikh Kamaluddin wrote:
> > cxl_handle_mce() offlines the aliased page of an ELC region on any
> > record with a usable address; it does not check MCI_STATUS_UC or
> > filter non-memory errors. uc_decode_notifier(), the equivalent
> > handler for plain memory on the same chain at the same priority,
> > filters on mce->severity and leaves corrected errors untouched.
> > cxl_handle_mce() has no such gate, so a corrected error - which
> > the generic handler ignores - still causes the alias to be
> > permanently retired via memory_failure().
> > 
> > Corrected errors do reach the chain: machine_check_poll() logs
> > them via the same mce_gen_pool_process() path that feeds
> > x86_mce_decoder_chain, and cxl_extended_linear_cache_resize()
> > extends p->res to cover the DRAM half of the ELC pair, so a
> > routine DRAM CE carries an address inside the region resource.
> > 
> > Filter the record as nfit_handle_mce() does. Commit fc08a4703a41
> > ("acpi, nfit: Fix the memory error check in nfit_handle_mce()") and
> > commit 5d96c9342c23 ("acpi/nfit, x86/mce: Handle only uncorrectable
> > machine checks") established this filter for an equivalent handler
> > on the same notifier chain; the consequence here is more severe, as
> > the CXL handler calls memory_failure() rather than recording a bad
> > block.
> > 
> > mce_is_correctable() is used instead of copying
> > uc_decode_notifier()'s AO/DEFERRED test because the alias must
> > still be offlined on MCE_AR_SEVERITY, where kill_me_maybe() owns
> > the reported page but nothing owns the alias.
> > 
> > Fixes: 516e5bd0b6bf ("cxl: Add mce notifier to emit aliased address for extended linear cache")
> > 
> 
> Thanks for the patch Shaikh,
> 
> The patch looks good to me, and the reproduction details show that this has
> been thoroughly investigated.
> 
> The commit log is harder to follow than the code change itself. It seems
> that too much of the investigation has leaked into the commit log which
> makes it very dense and not very clarifying. For what is ultimately a
> small and straightforward behavioral fix, the log spends a lot of time
> narrating the code paths that establish the problem, while the important
> impact, a reduction in usable memory, gets lost.
> 
> The commit log should introduce the code change and make it easier to
> understand. I'd prefer the usual behavioral progression: describe what
> happens today, why that is wrong and its impact, then state how the patch
> fixes it. Also, please spell out Extended Linear Cache (ELC) on first use.
> 
> The subject can describe the behavior being fixed more directly as well.
> 
> For example: (if you use this example, you need to proofread it, to
> be sure I actually captured this correctly ;))
> 
> cxl/mce: Avoid alias page retirement for corrected errors
> 
> 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")
>
Thanks for the detailed feedback, Alison - that's a much better
structure for the commit log, and matches what you captured.
> 
> more below...
> 
> 
> > Signed-off-by: Shaikh Kamaluddin <shaikhkamal2012@gmail.com>
> > ---
> 
> The detailed investigation and reproduction below the scissors is useful and
> should stay there.
> 
> snip
> 
> > 
> > diff --git a/drivers/cxl/core/mce.c b/drivers/cxl/core/mce.c
> > index 65fed913b221..ee70c1c9f9b9 100644
> > --- a/drivers/cxl/core/mce.c
> > +++ b/drivers/cxl/core/mce.c
> > @@ -18,7 +18,14 @@ 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;
> > +
> > +	/* Only uncorrected memory errors warrant taking down the alias page */
> > +	if (!mce_is_memory_error(mce) || mce_is_correctable(mce))
> > +		return NOTIFY_DONE;
> > +
> 
> The comment restates what the if condition says. I think the comment can
> simply be dropped.
>
Dropped the comment from the code  and sent the v2 
https://lore.kernel.org/all/20260812151759.14390-1-shaikhkamal2012@gmail.com/
>
> 
> > +	if (!mce_usable_address(mce))
> >  		return NOTIFY_DONE;
> >  
> >  	spa = mce->addr & MCI_ADDR_PHYSADDR;
> > 
> > base-commit: 7098e9cd98a05c0c5de2fae0c2465f9d966fdd07
> > -- 
> > 2.43.0
> > 

Re: [PATCH] cxl/mce: Only act on uncorrected memory errors

[Cheatham, Benjamin]

On 8/12/2026 10:59 AM, shaikh kamaluddin wrote:
> [You don't often get email from shaikhkamal2012@gmail.com. Learn why this is important at https://aka.ms/LearnAboutSenderIdentification ]
> 
> On Mon, Aug 10, 2026 at 02:00:18PM -0500, Cheatham, Benjamin wrote:
>> On 8/10/2026 1:30 PM, Shaikh Kamaluddin wrote:
>>> [You don't often get email from shaikhkamal2012@gmail.com. Learn why this is important at https://aka.ms/LearnAboutSenderIdentification ]
>>>
>>> cxl_handle_mce() offlines the aliased page of an ELC region on any
>>> record with a usable address; it does not check MCI_STATUS_UC or
>>> filter non-memory errors. uc_decode_notifier(), the equivalent
>>> handler for plain memory on the same chain at the same priority,
>>> filters on mce->severity and leaves corrected errors untouched.
>>> cxl_handle_mce() has no such gate, so a corrected error - which
>>> the generic handler ignores - still causes the alias to be
>>> permanently retired via memory_failure().
>>>
>>> Corrected errors do reach the chain: machine_check_poll() logs
>>> them via the same mce_gen_pool_process() path that feeds
>>> x86_mce_decoder_chain, and cxl_extended_linear_cache_resize()
>>> extends p->res to cover the DRAM half of the ELC pair, so a
>>> routine DRAM CE carries an address inside the region resource.
>>>
>>> Filter the record as nfit_handle_mce() does. Commit fc08a4703a41
>>> ("acpi, nfit: Fix the memory error check in nfit_handle_mce()") and
>>> commit 5d96c9342c23 ("acpi/nfit, x86/mce: Handle only uncorrectable
>>> machine checks") established this filter for an equivalent handler
>>> on the same notifier chain; the consequence here is more severe, as
>>> the CXL handler calls memory_failure() rather than recording a bad
>>> block.
>>>
>>> mce_is_correctable() is used instead of copying
>>> uc_decode_notifier()'s AO/DEFERRED test because the alias must
>>> still be offlined on MCE_AR_SEVERITY, where kill_me_maybe() owns
>>> the reported page but nothing owns the alias.
>>>
>>> Fixes: 516e5bd0b6bf ("cxl: Add mce notifier to emit aliased address for extended linear cache")
>>>
>>> Signed-off-by: Shaikh Kamaluddin <shaikhkamal2012@gmail.com>
>>> ---
>>
>> This looks good to me, so:
>> Reviewed-by: Ben Cheatham <benjamin.cheatham@amd.com>
>>
> Thanks for review!
>> If you have the time, could you also share a script that does the below testing on the list? It may
>> be possible to integrate into the CXL testing suite (see https://github.com/pmem/ndctl.git), though
>> the QEMU usage may throw a wrench in that. Even if it's not possible, having the tests out there
>> for people to run would help with any future breakage.
> Happy to share it. One question on where it'd fit best: ndctl
> (github.com/pmem/ndctl.git) as you mentioned, or drivers/cxl's own
> tools/testing/cxl/ in-tree? I'm open to either, or proposing it in
> both if that's useful - happy to follow your lead on which is the
> better home for it.
> 

If looks like there's already some mock functions for extended linear cache in tools/testing/cxl, so
I'd recommending trying to put it there to begin with. It may require updates to ndctl after the fact
to run the test(s) as well. If that looks too involved then sending the script out to the list standalone
should be fine. I don't know if anyone will pick it up, but it'll be searchable on lore if anyone wants
to test this.

Thanks,
Ben

Re: [PATCH] cxl/mce: Only act on uncorrected memory errors

[Alison Schofield]

On Wed, Aug 12, 2026 at 11:19:40AM -0500, Cheatham, Benjamin wrote:
> On 8/12/2026 10:59 AM, shaikh kamaluddin wrote:
> > [You don't often get email from shaikhkamal2012@gmail.com. Learn why this is important at https://aka.ms/LearnAboutSenderIdentification ]
> > 
> > On Mon, Aug 10, 2026 at 02:00:18PM -0500, Cheatham, Benjamin wrote:
> >> On 8/10/2026 1:30 PM, Shaikh Kamaluddin wrote:
> >>> [You don't often get email from shaikhkamal2012@gmail.com. Learn why this is important at https://aka.ms/LearnAboutSenderIdentification ]
> >>>
> >>> cxl_handle_mce() offlines the aliased page of an ELC region on any
> >>> record with a usable address; it does not check MCI_STATUS_UC or
> >>> filter non-memory errors. uc_decode_notifier(), the equivalent
> >>> handler for plain memory on the same chain at the same priority,
> >>> filters on mce->severity and leaves corrected errors untouched.
> >>> cxl_handle_mce() has no such gate, so a corrected error - which
> >>> the generic handler ignores - still causes the alias to be
> >>> permanently retired via memory_failure().
> >>>
> >>> Corrected errors do reach the chain: machine_check_poll() logs
> >>> them via the same mce_gen_pool_process() path that feeds
> >>> x86_mce_decoder_chain, and cxl_extended_linear_cache_resize()
> >>> extends p->res to cover the DRAM half of the ELC pair, so a
> >>> routine DRAM CE carries an address inside the region resource.
> >>>
> >>> Filter the record as nfit_handle_mce() does. Commit fc08a4703a41
> >>> ("acpi, nfit: Fix the memory error check in nfit_handle_mce()") and
> >>> commit 5d96c9342c23 ("acpi/nfit, x86/mce: Handle only uncorrectable
> >>> machine checks") established this filter for an equivalent handler
> >>> on the same notifier chain; the consequence here is more severe, as
> >>> the CXL handler calls memory_failure() rather than recording a bad
> >>> block.
> >>>
> >>> mce_is_correctable() is used instead of copying
> >>> uc_decode_notifier()'s AO/DEFERRED test because the alias must
> >>> still be offlined on MCE_AR_SEVERITY, where kill_me_maybe() owns
> >>> the reported page but nothing owns the alias.
> >>>
> >>> Fixes: 516e5bd0b6bf ("cxl: Add mce notifier to emit aliased address for extended linear cache")
> >>>
> >>> Signed-off-by: Shaikh Kamaluddin <shaikhkamal2012@gmail.com>
> >>> ---
> >>
> >> This looks good to me, so:
> >> Reviewed-by: Ben Cheatham <benjamin.cheatham@amd.com>
> >>
> > Thanks for review!
> >> If you have the time, could you also share a script that does the below testing on the list? It may
> >> be possible to integrate into the CXL testing suite (see https://github.com/pmem/ndctl.git), though
> >> the QEMU usage may throw a wrench in that. Even if it's not possible, having the tests out there
> >> for people to run would help with any future breakage.
> > Happy to share it. One question on where it'd fit best: ndctl
> > (github.com/pmem/ndctl.git) as you mentioned, or drivers/cxl's own
> > tools/testing/cxl/ in-tree? I'm open to either, or proposing it in
> > both if that's useful - happy to follow your lead on which is the
> > better home for it.
> > 
> 
> If looks like there's already some mock functions for extended linear cache in tools/testing/cxl, so
> I'd recommending trying to put it there to begin with. It may require updates to ndctl after the fact
> to run the test(s) as well. If that looks too involved then sending the script out to the list standalone
> should be fine. I don't know if anyone will pick it up, but it'll be searchable on lore if anyone wants
> to test this.


This sounded interesting! I gave it a try with cxl_test and mce-inject,
and it looks like this can be tested without the QEMU CXL topology or the
forced cache_size hack.

I built with CONFIG_X86_MCE_INJECT=m and loaded cxl_test with its
existing ELC support:

# modprobe cxl_test extended_linear_cache=1
# cxl list -R
[
  {
    "region":"region0",
    "resource":70300293136384,
    "size":1073741824,
    "extended_linear_cache_size":536870912,
    "type":"ram",
    "interleave_ways":2,
    "interleave_granularity":4096,
    "decode_state":"commit",
    "locked":false
  }
]

Using 0x3ff010010000 as the injected SPA, I first injected the
corrected error from your example. On the patched kernel there was no
CXL offlining message, as expected.

I then changed only the status to the uncorrectable case:

# cd /sys/kernel/debug/mce-inject
# echo sw > flags
# echo 0xbc00000000000080 > status
# echo 0x80 > misc
# echo 0x3ff010010000 > addr
# echo 9 > bank

and got:

cxl_region region0: Offlining aliased SPA address0: 0x3ff030010000
Memory failure: 0x3ff030010: memory outside kernel control
mce: [Hardware Error]: CPU 0: Machine Check: 0 Bank 9: bc00000000000080
mce: [Hardware Error]: TSC 23c5a5a9188 ADDR 3ff010010000 MISC 80
mce: [Hardware Error]: PROCESSOR 0:50657 TIME 1786573477 SOCKET 0 APIC 0 microcode 5003302

So the cxl_test ELC plus mce-inject looks sufficient to exercise the
path: the CE is ignored with the patch, while the UC reaches the alias
offlining path and computes the expected alias.

The "memory outside kernel control" is because I did not put the
aliased memory into system RAM for this quick test.

This would be a test case addition for the the cxl-elc.sh unit test.
It seems like a tiny, close-the-barn-door-after-the-horse-got-out,
kind of test case, but maybe not?  Maybe it opens the door to more
things can do we mce-inject elsewhere?

I'll leave that to Shaikh if they want to add the new test case.

-- Alison


> 
> Thanks,
> Ben