[PATCH 1/3] Revert "thp: make MADV_HUGEPAGE check for mm->def_flags"

[Alex Thorlton]

This reverts commit 8e72033f2a489b6c98c4e3c7cc281b1afd6cb85c, and adds
in code to fix up any issues caused by the revert.

The revert is necessary because hugepage_madvise would return -EINVAL
when VM_NOHUGEPAGE is set, which will break subsequent chunks of this
patch set.

Here's a snip of an e-mail from Gerald detailing the original purpose
of this code, and providing justification for the revert:

<snip>
The intent of 8e72033f2a48 was to guard against any future programming
errors that may result in an madvice(MADV_HUGEPAGE) on guest mappings,
which would crash the kernel.

Martin suggested adding the bit to arch/s390/mm/pgtable.c, if 8e72033f2a48
was to be reverted, because that check will also prevent a kernel crash
in the case described above, it will now send a SIGSEGV instead.

This would now also allow to do the madvise on other parts, if needed,
so it is a more flexible approach. One could also say that it would have
been better to do it this way right from the beginning... 
</snip>

Signed-off-by: Alex Thorlton <athorlton@xxxxxxx>
Suggested-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Gerald Schaefer <gerald.schaefer@xxxxxxxxxx>
Cc: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
Cc: Heiko Carstens <heiko.carstens@xxxxxxxxxx>
Cc: Christian Borntraeger <borntraeger@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: linux390@xxxxxxxxxx
Cc: linux-s390@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: linux-mm@xxxxxxxxx
Cc: linux-api@xxxxxxxxxxxxxxx

---
 arch/s390/mm/pgtable.c | 3 +++
 mm/huge_memory.c       | 4 ----
 2 files changed, 3 insertions(+), 4 deletions(-)

diff --git a/arch/s390/mm/pgtable.c b/arch/s390/mm/pgtable.c
index 3584ed9..a87cdb4 100644
--- a/arch/s390/mm/pgtable.c
+++ b/arch/s390/mm/pgtable.c
@@ -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;
 	/* pmd now points to a valid segment table entry. */
 	rmap = kmalloc(sizeof(*rmap), GFP_KERNEL|__GFP_REPEAT);
 	if (!rmap)
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 82166bf..a4310a5 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1968,8 +1968,6 @@ out:
 int hugepage_madvise(struct vm_area_struct *vma,
 		     unsigned long *vm_flags, int advice)
 {
-	struct mm_struct *mm = vma->vm_mm;
-
 	switch (advice) {
 	case MADV_HUGEPAGE:
 		/*
@@ -1977,8 +1975,6 @@ int hugepage_madvise(struct vm_area_struct *vma,
 		 */
 		if (*vm_flags & (VM_HUGEPAGE | VM_NO_THP))
 			return -EINVAL;
-		if (mm->def_flags & VM_NOHUGEPAGE)
-			return -EINVAL;
 		*vm_flags &= ~VM_NOHUGEPAGE;
 		*vm_flags |= VM_HUGEPAGE;
 		/*
-- 
1.7.12.4

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