Re: + mm-revert-thp-make-madv_hugepage-check-for-mm-def_flags.patch added to -mm tree

[Oleg Nesterov <oleg@redhat.com>]

On 02/26, Alex Thorlton wrote:
>
> + * MADV_HUGEPAGE after PRCTL_THP_DISABLE is broken on s390 because
> + * qemu blindly does madvise(MADV_HUGEPAGE) after s390_enable_sie().
> + */
> +#ifdef CONFIG_S390
> +               if (mm_has_pgste(vma->vm_mm))
>                         return -EINVAL;
> +#endif

The comment is not really right...

And personally I think that

	@@ -504,6 +504,9 @@ static int gmap_connect_pgtable(unsigned long address, unsigned long segment,
		if (!pmd_present(*pmd) &&
		    __pte_alloc(mm, vma, pmd, vmaddr))
			return -ENOMEM;
	+       /* large pmds cannot yet be handled */
	+       if (pmd_large(*pmd))
	+               return -EFAULT;

change still makes sense, so that we can simply revert this s390-
specific hack in hugepage_madvise().

I'd suggest the patch below on top of your changes, but I won't argue.

It would be nice to also change thp_split_mm() to not not play with
mm->def_flags, but I am not sure if we can do this.

Oleg.
---

Subject: [PATCH] s390: make sure MADV_HUGEPAGE fails after s390_enable_sie()

As Christian pointed out, the recent 'Revert "thp: make MADV_HUGEPAGE
check for mm->def_flags"' breaks qemu, it does QEMU_MADV_HUGEPAGE for
all kvm pages but this doesn't work after s390_enable_sie/thp_split_mm.

Reported-by: Christian Borntraeger <borntraeger@de.ibm.com>
Suggested-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index a4310a5..0e08d92 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1970,11 +1970,22 @@ int hugepage_madvise(struct vm_area_struct *vma,
 {
 	switch (advice) {
 	case MADV_HUGEPAGE:
+#ifdef CONFIG_S390
+		/*
+		 * MADV_HUGEPAGE is broken after s390_enable_sie(), qemu
+		 * blindly does madvise(MADV_HUGEPAGE) for for all kvm pages
+		 * and expects it must fail on s390. Avoid a possible SIGSEGV
+		 * until qemu is changed.
+		 */
+		if (mm_has_pgste(vma->vm_mm))
+			return -EINVAL;
+#endif
 		/*
 		 * Be somewhat over-protective like KSM for now!
 		 */
 		if (*vm_flags & (VM_HUGEPAGE | VM_NO_THP))
 			return -EINVAL;
+
 		*vm_flags &= ~VM_NOHUGEPAGE;
 		*vm_flags |= VM_HUGEPAGE;
 		/*