Message-ID: <68f7bd68b1056_1668f310046@iweiny-mobl.notmuch>
Date: Tue, 21 Oct 2025 12:05:44 -0500
From: Ira Weiny <ira.weiny@...el.com>
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Subject: Re: [PATCH v3 1/3] mm: handle poisoning of pfn without struct pages

ankita@ wrote:
> From: Ankit Agrawal <ankita@...dia.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]
> 

[snip]

> @@ -2216,6 +2222,121 @@ static void kill_procs_now(struct page *p, unsigned long pfn, int flags,
>  	kill_procs(&tokill, true, pfn, flags);
>  }
>  
> +int register_pfn_address_space(struct pfn_address_space *pfn_space)
> +{
> +	if (!pfn_space)
> +		return -EINVAL;

Is there a reason callers may be passing NULL?

> +
> +	mutex_lock(&pfn_space_lock);
> +
> +	if (interval_tree_iter_first(&pfn_space_itree,
> +				     pfn_space->node.start,
> +				     pfn_space->node.last)) {
> +		mutex_unlock(&pfn_space_lock);

Register and unregister are good places to use guard().

> +		return -EBUSY;
> +	}
> +
> +	interval_tree_insert(&pfn_space->node, &pfn_space_itree);
> +	mutex_unlock(&pfn_space_lock);
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(register_pfn_address_space);
> +
> +void unregister_pfn_address_space(struct pfn_address_space *pfn_space)
> +{
> +	if (!pfn_space)
> +		return;
> +
> +	mutex_lock(&pfn_space_lock);
> +	interval_tree_remove(&pfn_space->node, &pfn_space_itree);
> +	mutex_unlock(&pfn_space_lock);
> +}
> +EXPORT_SYMBOL_GPL(unregister_pfn_address_space);
> +
> +static void add_to_kill_pfn(struct task_struct *tsk,
> +			    struct vm_area_struct *vma,
> +			    struct list_head *to_kill,
> +			    unsigned long pfn)
> +{
> +	struct to_kill *tk;
> +
> +	tk = kmalloc(sizeof(*tk), GFP_ATOMIC);
> +	if (!tk)
> +		return;
> +
> +	/* Check for pgoff not backed by struct page */
> +	tk->addr = vma_address(vma, pfn, 1);
> +	tk->size_shift = PAGE_SHIFT;
> +
> +	if (tk->addr == -EFAULT)
> +		pr_info("Unable to find address %lx in %s\n",
> +			pfn, tsk->comm);

If the pfn is not in the process why is it added to the kill list?

> +
> +	get_task_struct(tsk);
> +	tk->tsk = tsk;
> +	list_add_tail(&tk->nd, to_kill);
> +}
> +
> +/*
> + * Collect processes when the error hit a PFN not backed by struct page.
> + */
> +static void collect_procs_pfn(struct address_space *mapping,
> +			      unsigned long pfn, struct list_head *to_kill)
> +{
> +	struct vm_area_struct *vma;
> +	struct task_struct *tsk;
> +
> +	i_mmap_lock_read(mapping);
> +	rcu_read_lock();
> +	for_each_process(tsk) {
> +		struct task_struct *t = tsk;
> +
> +		t = task_early_kill(tsk, true);
> +		if (!t)
> +			continue;
> +		vma_interval_tree_foreach(vma, &mapping->i_mmap, pfn, pfn) {
> +			if (vma->vm_mm == t->mm)
> +				add_to_kill_pfn(t, vma, to_kill, pfn);
> +		}
> +	}
> +	rcu_read_unlock();
> +	i_mmap_unlock_read(mapping);
> +}
> +
> +static int memory_failure_pfn(unsigned long pfn, int flags)
> +{
> +	struct interval_tree_node *node;
> +	LIST_HEAD(tokill);
> +
> +	mutex_lock(&pfn_space_lock);

scoped_guard()  Or probably wrap this part in a guarded function.

Ira

> +	/*
> +	 * 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);
> +
> +	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))) {
> +		res = memory_failure_pfn(pfn, flags);
> +		goto unlock_mutex;
> +	}
> +
>  	p = pfn_to_online_page(pfn);
>  	if (!p) {
>  		res = arch_memory_failure(pfn, flags);
> -- 
> 2.34.1
> 
> 

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