[PATCH] arm64: mte: Do not flag the zero page as PG_mte_tagged

[Lance Yang]

On 2025/9/24 20:49, David Hildenbrand wrote:
> On 24.09.25 14:48, Lance Yang wrote:
>>
>>
>> On 2025/9/24 20:31, Catalin Marinas wrote:
>>> Commit 68d54ceeec0e ("arm64: mte: Allow PTRACE_PEEKMTETAGS access to the
>>> zero page") attempted to fix ptrace() reading of tags from the zero page
>>> by marking it as PG_mte_tagged during cpu_enable_mte(). The same commit
>>> also changed the ptrace() tag access permission check to the VM_MTE vma
>>> flag while turning the page flag test into a WARN_ON_ONCE().
>>>
>>> Attempting to set the PG_mte_tagged flag early with
>>> CONFIG_DEFERRED_STRUCT_PAGE_INIT enabled may either hang (after commit
>>> d77e59a8fccd "arm64: mte: Lock a page for MTE tag initialisation") or
>>> have the flags cleared later during page_alloc_init_late(). In addition,
>>> pages_identical() -> memcmp_pages() will reject any comparison with the
>>> zero page as it is marked as tagged.
>>>
>>> Partially revert the above commit to avoid setting PG_mte_tagged on the
>>> zero page. Update the __access_remote_tags() warning on untagged pages
>>> to ignore the zero page since it is known to have the tags initialised.
>>>
>>> Note that all user mapping of the zero page are marked as pte_special().
>>> The arm64 set_pte_at() will not call mte_sync_tags() on such pages, so
>>> PG_mte_tagged will remain cleared.
>>>
>>> Signed-off-by: Catalin Marinas <catalin.marinas at arm.com>
>>> Fixes: 68d54ceeec0e ("arm64: mte: Allow PTRACE_PEEKMTETAGS access to 
>>> the zero page")
>>> Reported-by: Gergely Kovacs <Gergely.Kovacs2 at arm.com>
>>> Cc: <stable at vger.kernel.org> # 5.10.x
>>> Cc: Will Deacon <will at kernel.org>
>>> Cc: David Hildenbrand <david at redhat.com>
>>> Cc: Lance Yang <lance.yang at linux.dev>
>>> ---
>>>
>>> For reference, discussion on page merging here:
>>>
>>> https://lore.kernel.org/r/aNKJ5glToE4hMhWA@arm.com
>>
>> Cool. LGTM.
>> Acked-by: Lance Yang <lance.yang at linux.dev>
> 
> 
> Likely you can provide even a Tested-by? Just saying :)

Right, so:
Tested-by: Lance Yang <lance.yang at linux.dev>