Re: [PATCH] hugetlbfs: add MTE support

[Catalin Marinas <catalin.marinas@arm.com>]

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.

I'd also like to see some tests added to
tools/testing/selftest/arm64/mte to exercise MAP_HUGETLB with PROT_MTE:
write/read tags, a series of mman+munmap (mostly to check if old page
flags are still around), force some copy on write. I don't think we
should merge the patch without proper tests.

An untested hunk on top of your changes:

diff --git a/arch/arm64/include/asm/hugetlb.h b/arch/arm64/include/asm/hugetlb.h
index 3954cbd2ff56..5357b00b9087 100644
--- a/arch/arm64/include/asm/hugetlb.h
+++ b/arch/arm64/include/asm/hugetlb.h
@@ -20,7 +20,19 @@ extern bool arch_hugetlb_migration_supported(struct hstate *h);
 
 static inline void arch_clear_hugetlb_flags(struct folio *folio)
 {
-	clear_bit(PG_dcache_clean, &folio->flags);
+	unsigned long i, nr_pages = folio_nr_pages(folio);
+	const unsigned long clear_flags = BIT(PG_dcache_clean) |
+		BIT(PG_arch_2) | BIT(PG_arch_3);
+
+	if (!system_supports_mte()) {
+		clear_bit(PG_dcache_clean, &folio->flags);
+		return;
+	}
+
+	for (i = 0; i < nr_pages; i++) {
+		struct page *page = folio_page(folio, i);
+		page->flags &= ~clear_flags;
+	}
 }
 #define arch_clear_hugetlb_flags arch_clear_hugetlb_flags
 
diff --git a/arch/arm64/include/asm/mman.h b/arch/arm64/include/asm/mman.h
index 5966ee4a6154..304dfc499e68 100644
--- a/arch/arm64/include/asm/mman.h
+++ b/arch/arm64/include/asm/mman.h
@@ -28,7 +28,8 @@ static inline unsigned long arch_calc_vm_flag_bits(unsigned long flags)
 	 * backed by tags-capable memory. The vm_flags may be overridden by a
 	 * filesystem supporting MTE (RAM-based).
 	 */
-	if (system_supports_mte() && (flags & MAP_ANONYMOUS))
+	if (system_supports_mte() &&
+	    (flags & (MAP_ANONYMOUS | MAP_HUGETLB)))
 		return VM_MTE_ALLOWED;
 
 	return 0;

-- 
Catalin