Re: [PATCH v3 0/3] mm: Implement ECC handling for pfn with no struct page

Liam R. Howlett
2025-10-21T16:31:56Z

* ankita@nvidia.com <ankita@nvidia.com> [251021 06:23]:
> From: Ankit Agrawal <ankita@nvidia.com>
>
> The kernel MM currently handles ECC errors / poison only on memory page
> backed by struct page. The handling is currently missing for the PFNMAP
> memory that does not have struct pages. The series adds such support.
>
> Implement a new ECC handling for memory without struct pages. Kernel MM
> expose registration APIs to allow modules that are managing the device
> to register its device memory region. MM then tracks such regions using
> interval tree.
>
> The mechanism is largely similar to that of ECC on pfn with struct pages.
> If there is an ECC error on a pfn, all the mapping to it are identified
> and a SIGBUS is sent to the user space processes owning those mappings.
> Note that there is one primary difference versus the handling of the
> poison on struct pages, which is to skip unmapping to the poisoned PFN.
> This is done to handle the huge PFNMAP support added recently [1] that
> enables VM_PFNMAP vmas to map at PMD level. Otherwise, a poison to a PFN
> would need breaking the PMD mapping into PTEs to unmap only the poisoned
> PFN. This can have a major performance impact.

Is the performance impact really a concern in the event of failed
memory? Does this happen enough to warrant this special case?

Surely it's not failing hardware that may cause performance impacts, so
is this triggered in some other way that I'm missing or a conversation
pointer?

>
> nvgrace-gpu-vfio-pci module maps the device memory to user VA (Qemu) using
> remap_pfn_range without being added to the kernel [2]. These device memory
> PFNs are not backed by struct page. So make nvgrace-gpu-vfio-pci module
> make use of the mechanism to get poison handling support on the device
> memory.
>
> Patch rebased to v6.17-rc7.
>
> Signed-off-by: Ankit Agrawal <ankita@nvidia.com>
> ---
>
> Link: https://lore.kernel.org/all/20231123003513.24292-1-ankita@nvidia.com/ [v2]
>
> v2 -> v3
> - Rebased to v6.17-rc7.
> - Skipped the unmapping of PFNMAP during reception of poison. Suggested by
> Jason Gunthorpe, Jiaqi Yan, Vikram Sethi (Thanks!)
> - Updated the check to prevent multiple registration to the same PFN
> range using interval_tree_iter_first. Thanks Shameer Kolothum for the
> suggestion.
> - Removed the callback function in the nvgrace-gpu requiring tracking of
> poisoned PFN as it isn't required anymore.
> - Introduced seperate collect_procs_pfn function to collect the list of
> processes mapping to the poisoned PFN.
>
> v1 -> v2
> - Change poisoned page tracking from bitmap to hashtable.
> - Addressed miscellaneous comments in v1.
>
> Link: https://lore.kernel.org/all/20240826204353.2228736-1-peterx@redhat.com/ [1]
> Link: https://lore.kernel.org/all/20240220115055.23546-1-ankita@nvidia.com/ [2]
>
> Ankit Agrawal (3):
> mm: handle poisoning of pfn without struct pages
> mm: Change ghes code to allow poison of non-struct pfn
> vfio/nvgrace-gpu: register device memory for poison handling
>
> MAINTAINERS | 1 +
> drivers/acpi/apei/ghes.c | 6 --
> drivers/vfio/pci/nvgrace-gpu/main.c | 45 +++++++++-
> include/linux/memory-failure.h | 17 ++++
> include/linux/mm.h | 1 +
> include/ras/ras_event.h | 1 +
> mm/Kconfig | 1 +
> mm/memory-failure.c | 128 +++++++++++++++++++++++++++-
> 8 files changed, 192 insertions(+), 8 deletions(-)
> create mode 100644 include/linux/memory-failure.h
>
> --
> 2.34.1
>