Re: [PATCH v2 1/3] arm64: mm: Don't sleep in split_kernel_leaf_mapping() when in atomic context

["David Hildenbrand (Red Hat)" <davidhildenbrandkernel@gmail.com>]

On 06.11.25 17:09, Ryan Roberts wrote:
> It has been reported that split_kernel_leaf_mapping() is trying to sleep
> in non-sleepable context. It does this when acquiring the
> pgtable_split_lock mutex, when either CONFIG_DEBUG_PAGEALLOC or
> CONFIG_KFENCE are enabled, which change linear map permissions within
> softirq context during memory allocation and/or freeing. All other paths
> into this function are called from sleepable context and so are safe.
> 
> But it turns out that the memory for which these 2 features may attempt
> to modify the permissions is always mapped by pte, so there is no need
> to attempt to split the mapping. So let's exit early in these cases and
> avoid attempting to take the mutex.
> 
> There is one wrinkle to this approach; late-initialized kfence allocates
> it's pool from the buddy which may be block mapped. So we must hook that
> allocation and convert it to pte-mappings up front. Previously this was
> done as a side-effect of kfence protecting all the individual pages in
> its pool at init-time, but this no longer works due to the added early
> exit path in split_kernel_leaf_mapping().
> 
> So instead, do this via the existing arch_kfence_init_pool() arch hook,
> and reuse the existing linear_map_split_to_ptes() infrastructure.
> 
> Closes: https://lore.kernel.org/all/f24b9032-0ec9-47b1-8b95-c0eeac7a31c5@roeck-us.net/
> Fixes: a166563e7ec3 ("arm64: mm: support large block mapping when rodata=full")
> Tested-by: Guenter Roeck <groeck@google.com>
> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
> ---

Reviewed-by: David Hildenbrand (Red Hat) <david@kernel.org>