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

[shaikh kamaluddin]

On Mon, Aug 17, 2026 at 04:12:49PM -0700, Alison Schofield wrote:
> On Mon, Aug 17, 2026 at 05:23:37PM +0530, shaikh kamaluddin wrote:
> > On Wed, Aug 12, 2026 at 03:42:18PM -0700, Alison Schofield wrote:
> > > 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@xxxxxxxxx. 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@xxxxxxxxx. 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@xxxxxxxxx>
> > > > >>> ---
> > > > >>
> > > > >> This looks good to me, so:
> > > > >> Reviewed-by: Ben Cheatham <benjamin.cheatham@xxxxxxx>
> > > > >>
> > > > > 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
> > 
> > 
> > Hi Alison,
> > 
> > Thanks for trying this. This is very helpful.
> > 
> > I had initially started with the same cxl_test + mce-inject approach before moving to the vng/QEMU CXL Type-3 setup.
> > 
> > On the current cxl/next tree, I was first blocked while building tools/testing/cxl with LLVM/ld.lld. modpost was failing on wrapped CXL symbols, for example:
> > 
> > .export_symbol section references '__wrap_devm_cxl_add_rch_dport',
> > but it does not seem to be an export symbol
> > 
> > .export_symbol section references '__wrap_devm_cxl_add_dport_by_dev',
> > but it does not seem to be an export symbol
> > 
> > .export_symbol section references '__wrap_cxl_await_media_ready',
> > but it does not seem to be an export symbol
> > 
> > There were similar failures for some of the decoder/CDAT wrappers.
> > 
> > This appears to be the same issue addressed by the patch currently under review:
> > 
> > [PATCH] tools/testing/cxl: Don't wrap cxl_core's own exported symbols
> > 
> > https://lore.kernel.org/linux-cxl/20260721084009.38100-1-icheng@xxxxxxxxxx/
> > 
> > The patch avoids globally wrapping the CXL core symbols that are also defined/exported by cxl_core, and instead applies those wrappers only to the modules that need them.
> 
> Hi Shaikh,
> 
> I applied the patch to the base commit included in the patch:
> >> base-commit: 7098e9cd98a05c0c5de2fae0c2465f9d966fdd07
> 
> I don't use LLVM toolchain, but I do see Richards patch you refer to.
> We'll need to get that queued for 7.4.
> 
> > 
> > With that patch applied, tools/testing/cxl builds successfully for me. However, I still hit a runtime crash when loading the ELC setup:
> > 
> > # modprobe cxl_test extended_linear_cache=1
> > 
> > BUG: unable to handle page fault for address: 0000000000003358
> > #PF: supervisor read access in kernel mode
> > RIP: __alloc_frozen_pages_noprof+0x12e/0x320
> > CR2: 0000000000003358
> > Workqueue: async async_run_entry_fn
> 
> 
> So there must be something different about your cxl-test address 
> space. I have 
> region resource: 0x3ff010000000
> region size:     0x40000000
> ELC size:        0x20000000
> 
> You cannot even load w elc = 1 !
> 
> Does "modprobe cxl_test" work for you?
> If yes, how far can you get? Does cxl-topology.sh work for you?
> I wouldn't have you try the whole suite since you are telling me
> cxl-elc.sh  already fails.
> 
> Maybe we need to look at where cxl test is loaded for you and why
> the ELC address shennanigans fail in your environment.
> 
> Let me know what you see. 
>

Hi Alison,

I found the cause of the `cxl_test` runtime crash.

My virtme-ng guest has only NUMA node 0 available and online:

# cat /sys/devices/system/node/possible
0

# cat /sys/devices/system/node/online
0


The crash I was seeing matches the issue fixed by Davidlohr's patch:
https://lore.kernel.org/linux-cxl/aoOmo-DZudbSOV3N@xxxxxxxxxxxxxxxxxx/T/#m60009c527f2017c031dad319366418578fcab1fd

`cxl/test: Map mock device nodes to an online node`

The test code has changed since that patch was posted. The original patch updates the three `set_dev_node()` 
sites in `cxl_mem_init()`, while in current cxl/next the corresponding paths are under `cxl_type3_mem_init()`.

I applied the equivalent change there:

set_dev_node(&pdev->dev, numa_map_to_online_node(i % 2)); to all three Type-3 memdev paths.
With that applied, the repeated crash is gone and this now succeeds:

# modprobe cxl_test extended_linear_cache=1

I can also get a committed test region:

# cxl list -R
[
  {
    "region":"region1",
    "resource":1031060586496,
    "size":1073741824,
    "type":"ram",
    "interleave_ways":2,
    "interleave_granularity":4096,
    "decode_state":"commit"
  }
]


So the earlier failure was not related to the ELC address-space layout itself; it was the mock memdev 
being assigned to NUMA node 1 on my single-node guest.

I also noticed current cxl/next has a new `set_dev_node(&pdev->dev, i % 2)` in `cxl_type2_mem_init()`. 
`NR_CXL_TYPE2_ACCEL` is currently 1, so it does not hit this problem today, but I mentioned it on 
Davidlohr's patch thread as a possible consistency/future-proofing update.

I have not yet checked why my `cxl list -R` output does not show `extended_linear_cache_size` like yours. 
I will look at that separately now that `cxl_test` loads cleanly.

Thanks,
Shaikh

> Thanks,
> Alison
> 
> 
> 
> > 
> > So the build issue and this runtime crash appear to be separate problems. The runtime failure is what led me to use the vng/QEMU CXL Type-3 setup for validating the MCE change, where I was able to exercise both the CE and UC cases.
> > 
> > Since you were able to run:
> > 
> > modprobe cxl_test extended_linear_cache=1
> >         +
> > mce-inject
> > 
> > successfully, could you please share the kernel configuration and any patches you have on top of cxl/next? That would help me compare the working setup with mine and identify what I am still missing on the cxl_test side.
> > 
> > Your result also confirms that, once I get this setup stable, extending the existing cxl-elc.sh test looks like the right direction for regression coverage. My plan would be to derive the SPA and expected alias dynamically from the ELC region, inject a CE and verify that the alias-offlining path is not reached, then inject a UC and verify that the expected aliased SPA is still offlined.
> > 
> > This is a small regression test for the current issue, but exercising mce-inject through the CXL test infrastructure may also provide useful coverage for other CXL RAS/MCE paths in the future.
> > 
> > For the current patch validation, I still think the vng/QEMU Type-3 setup is useful as an end-to-end test since it can exercise the actual CXL region and system-RAM memory_failure() path, while cxl_test + mce-inject looks better suited for the lightweight automated regression test.
> > 
> > Thanks,
> > Shaikh
> > 
> > 
> > > 
> > > > 
> > > > Thanks,
> > > > Ben