Re: [PATCH v3 1/3] mm: handle poisoning of pfn without struct pages

[Miaohe Lin <linmiaohe@huawei.com>]

On 2025/10/21 18:23, ankita@nvidia.com wrote:
> From: Ankit Agrawal <ankita@nvidia.com>
> 
> The kernel MM currently does not handle ECC errors / poison on a memory
> region that is not backed by struct pages. If a memory region mapped
> using remap_pfn_range() for example, but not added to the kernel, MM
> will not have associated struct pages. Add a new mechanism to handle
> memory failure on such memory.
> 
> Make kernel MM expose a function to allow modules managing the device
> memory to register the device memory SPA and the address space associated
> it. MM maintains this information as an interval tree. On poison, MM can
> search for the range that the poisoned PFN belong and use the address_space
> to determine the mapping VMA.
> 
> In this implementation, kernel MM follows the following sequence that is
> largely similar to the memory_failure() handler for struct page backed
> memory:
> 1. memory_failure() is triggered on reception of a poison error. An
> absence of struct page is detected and consequently memory_failure_pfn()
> is executed.
> 2. memory_failure_pfn() collects the processes mapped to the PFN.
> 3. memory_failure_pfn() sends SIGBUS to all the processes mapping the
> poisoned PFN using kill_procs().
> 
> Note that there is one primary difference versus the handling of the
> poison on struct pages, which is to skip unmapping to the faulty PFN.
> This is done to handle the huge PFNMAP support added recently [1] that
> enables VM_PFNMAP vmas to map in either PMD level. Otherwise, a poison
> to a PFN would need breaking the PMD mapping into PTEs to unmap only
> the poisoned PFN. This will have a major performance impact.
> 
> Link: https://lore.kernel.org/all/20240826204353.2228736-1-peterx@redhat.com/ [1]
> 
> Signed-off-by: Ankit Agrawal <ankita@nvidia.com>

Thanks for your patch. Some comments below.

> ---
>  MAINTAINERS                    |   1 +
>  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 ++++++++++++++++++++++++++++++++-
>  6 files changed, 148 insertions(+), 1 deletion(-)
>  create mode 100644 include/linux/memory-failure.h
> 
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 520fb4e379a3..463d062d0386 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -11359,6 +11359,7 @@ M:	Miaohe Lin <linmiaohe@huawei.com>
>  R:	Naoya Horiguchi <nao.horiguchi@gmail.com>
>  L:	linux-mm@kvack.org
>  S:	Maintained
> +F:	include/linux/memory-failure.h
>  F:	mm/hwpoison-inject.c
>  F:	mm/memory-failure.c
>  
> diff --git a/include/linux/memory-failure.h b/include/linux/memory-failure.h
> new file mode 100644
> index 000000000000..bc326503d2d2
> --- /dev/null
> +++ b/include/linux/memory-failure.h
> @@ -0,0 +1,17 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _LINUX_MEMORY_FAILURE_H
> +#define _LINUX_MEMORY_FAILURE_H
> +
> +#include <linux/interval_tree.h>
> +
> +struct pfn_address_space;

Do we need this declaration?

> +
> +struct pfn_address_space {
> +	struct interval_tree_node node;
> +	struct address_space *mapping;
> +};
> +

<snip>

> +static int memory_failure_pfn(unsigned long pfn, int flags)
> +{
> +	struct interval_tree_node *node;
> +	LIST_HEAD(tokill);
> +
> +	mutex_lock(&pfn_space_lock);
> +	/*
> +	 * Modules registers with MM the address space mapping to the device memory they
> +	 * manage. Iterate to identify exactly which address space has mapped to this
> +	 * failing PFN.
> +	 */
> +	for (node = interval_tree_iter_first(&pfn_space_itree, pfn, pfn); node;
> +	     node = interval_tree_iter_next(node, pfn, pfn)) {
> +		struct pfn_address_space *pfn_space =
> +			container_of(node, struct pfn_address_space, node);
> +
> +		collect_procs_pfn(pfn_space->mapping, pfn, &tokill);
> +	}
> +	mutex_unlock(&pfn_space_lock);
> +
> +	/*
> +	 * Unlike System-RAM there is no possibility to swap in a different
> +	 * physical page at a given virtual address, so all userspace
> +	 * consumption of direct PFN memory necessitates SIGBUS (i.e.
> +	 * MF_MUST_KILL)
> +	 */
> +	flags |= MF_ACTION_REQUIRED | MF_MUST_KILL;
> +
> +	kill_procs(&tokill, true, pfn, flags);
> +

If pfn doesn't belong to any address space mapping, it's still counted as MF_RECOVERED?

> +	return action_result(pfn, MF_MSG_PFN_MAP, MF_RECOVERED);
> +}
> +
>  /**
>   * memory_failure - Handle memory failure of a page.
>   * @pfn: Page Number of the corrupted page
> @@ -2259,6 +2380,11 @@ int memory_failure(unsigned long pfn, int flags)
>  	if (!(flags & MF_SW_SIMULATED))
>  		hw_memory_failure = true;
>  
> +	if (!pfn_valid(pfn) && !arch_is_platform_page(PFN_PHYS(pfn))) {

It's better to have some comments for this case.

> +		res = memory_failure_pfn(pfn, flags);
> +		goto unlock_mutex;
> +	}
> +
>  	p = pfn_to_online_page(pfn);
>  	if (!p) {
>  		res = arch_memory_failure(pfn, flags);

Can we move above memory_failure_pfn block here? I'm worried that too many scenario branches might
lead to confusion.

Thanks.
.