Re: [PATCH 2/4] mm, s390: Ignore MADV_HUGEPAGE on s390 to prevent SIGSEGV in qemu

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.

Paolo suggested that instead of failing on the call to madvise, we
simply ignore the call (return 0).

Reported-by: Christian Borntraeger <redacted>
Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Suggested-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Alex Thorlton <redacted>
Cc: Gerald Schaefer <redacted>
Cc: Martin Schwidefsky <redacted>
Cc: Heiko Carstens <redacted>
Cc: Christian Borntraeger <redacted>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Kirill A. Shutemov" <redacted>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Rik van Riel <redacted>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Andrea Arcangeli <redacted>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: "Eric W. Biederman" <redacted>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: linux390@de.ibm.com
Cc: linux-s390@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-mm@kvack.org
Cc: linux-api@vger.kernel.org

---
 mm/huge_memory.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index a4310a5..61d234d 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c

@@ -1970,6 +1970,15 @@ int hugepage_madvise(struct vm_area_struct *vma,
 {
 	switch (advice) {
 	case MADV_HUGEPAGE:
+#ifdef CONFIG_S390
+		/*
+		 * qemu blindly sets MADV_HUGEPAGE on all allocations, but s390
+		 * can't handle this properly after s390_enable_sie, so we simply
+		 * ignore the madvise to prevent qemu from causing a SIGSEGV.
+		 */
+		if (mm_has_pgste(vma->vm_mm))
+			return 0;
+#endif
 		/*
 		 * Be somewhat over-protective like KSM for now!
 		 */