Re: [PATCH] hugetlbfs: add MTE support

[Yang Shi <yang@os.amperecomputing.com>]

On 7/2/24 6:09 AM, David Hildenbrand wrote:
> On 02.07.24 14:34, Catalin Marinas wrote:
>> On Tue, Jun 25, 2024 at 04:37:17PM -0700, Yang Shi wrote:
>>> MTE can be supported on ram based filesystem. It is supported on tmpfs.
>>> There is use case to use MTE on hugetlbfs as well, adding MTE support.
>>>
>>> Signed-off-by: Yang Shi <yang@os.amperecomputing.com>
>>> ---
>>>   fs/hugetlbfs/inode.c | 2 +-
>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
>>> index ecad73a4f713..c34faef62daf 100644
>>> --- a/fs/hugetlbfs/inode.c
>>> +++ b/fs/hugetlbfs/inode.c
>>> @@ -110,7 +110,7 @@ static int hugetlbfs_file_mmap(struct file 
>>> *file, struct vm_area_struct *vma)
>>>        * way when do_mmap unwinds (may be important on powerpc
>>>        * and ia64).
>>>        */
>>> -    vm_flags_set(vma, VM_HUGETLB | VM_DONTEXPAND);
>>> +    vm_flags_set(vma, VM_HUGETLB | VM_DONTEXPAND | VM_MTE_ALLOWED);
>>>       vma->vm_ops = &hugetlb_vm_ops;
>>
>> Last time I checked, about a year ago, this was not sufficient. One
>> issue is that there's no arch_clear_hugetlb_flags() implemented by your
>> patch, leaving PG_arch_{2,3} set on a page. The other issue was that I
>> initially tried to do this only on the head page but this did not go
>> well with the folio_copy() -> copy_highpage() which expects the
>> PG_arch_* flags on each individual page. The alternative was for
>> arch_clear_hugetlb_flags() to iterate over all the pages in a folio.
>
> This would likely also add a blocker for 
> ARCH_WANT_OPTIMIZE_HUGETLB_VMEMMAP on arm64 (no idea if there are now 
> ways to move forward with that now, or if we are still not sure if we 
> can actually add support), correct?

IIUC, it is not. We just need to guarantee each subpage has 
PG_mte_tagged flag and allocated tags. The HVO just maps the 7 vmemmap 
pages for sub pages to the first page, they still see the flag and the 
space for tag is not impacted, right? Did I miss something?

>
>