On 7/2/24 5:34 AM, 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@xxxxxxxxxxxxxxxxxxxxxx>
---
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.
Thanks for pointing this out. I did miss this point. I took a quick look
at when the PG_ flags are set. IIUC, it is set by post_alloc_hook() for
order-0 anonymous folio (clearing page and tags) and set_ptes() for
others (just clear tags), for example, THP and hugetlb.
I can see THP does set the PG_mte_tagged flag for each sub pages. But it
seems it does not do it for hugetlb if I read the code correctly. The
call path is:
hugetlb_fault() ->
hugetlb_no_page->
set_huge_pte_at ->
__set_ptes() ->
__sync_cache_and_tags() ->
The __set_ptes() is called in a loop:
if (!pte_present(pte)) {
for (i = 0; i < ncontig; i++, ptep++, addr += pgsize)
__set_ptes(mm, addr, ptep, pte, 1);
return;
}
The ncontig and pgsize are returned by num_contig_ptes(). For example,
2M hugetlb, ncontig is 1 and pgsize is 2M IIUC. So it means actually
just the head page has PG_mte_tagged set. If so the copy_highpage() will
just copy the old head page's flag to the new head page, and the tag.
All the sub pages don't have PG_mte_tagged set.
Is it expected behavior? I'm supposed we need tags for every sub pages
too, right?
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;
Do we really need this change? IIRC, the mmap_region() will call
hugetlbfs's mmap and set VM_MTE_ALLOWED in vma->vm_flags, then update
vma->vm_page_prot with the new vma->vm_flags.
If this is needed, MTE for tmpfs won't work, right?
return 0;
