avoid breaking huge pmd invariants in case of vma_adjust failures

An huge pmd can only be mapped if the corresponding 2M virtual range
is fully contained in the vma. At times the VM calls split_vma twice,
if the first split_vma succeeds and the second fail, the first
split_vma remains in effect and it's not rolled back. For split_vma or
vma_adjust to fail an allocation failure is needed so it's a very
unlikely event (the out of memory killer would normally fire before
any allocation failure is visible to kernel and userland and if an out
of memory condition happens it's unlikely to happen exactly
here). Nevertheless it's safer to ensure that no huge pmd can be left
around if the vma is adjusted in a way that can't fit hugepages
anymore at the new vm_start/vm_end address.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>

3 files changed, 99 insertions, 2 deletions

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index b4fca2d0d63e..8358986dc881 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -106,6 +106,19 @@ extern void __split_huge_page_pmd(struct mm_struct *mm, pmd_t *pmd);
 
 extern unsigned long vma_address(struct page *page, struct vm_area_struct *vma);
 extern int hugepage_madvise(unsigned long *vm_flags);
+extern void __vma_adjust_trans_huge(struct vm_area_struct *vma,
+				    unsigned long start,
+				    unsigned long end,
+				    long adjust_next);
+static inline void vma_adjust_trans_huge(struct vm_area_struct *vma,
+					 unsigned long start,
+					 unsigned long end,
+					 long adjust_next)
+{
+	if (!vma->anon_vma || vma->vm_ops || vma->vm_file)
+		return;
+	__vma_adjust_trans_huge(vma, start, end, adjust_next);
+}
 static inline int PageTransHuge(struct page *page)
 {
 	VM_BUG_ON(PageTail(page));
@@ -138,6 +151,12 @@ static inline int hugepage_madvise(unsigned long *vm_flags)
 	BUG_ON(0);
 	return 0;
 }
+static inline void vma_adjust_trans_huge(struct vm_area_struct *vma,
+					 unsigned long start,
+					 unsigned long end,
+					 long adjust_next)
+{
+}
 #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
 
 #endif /* _LINUX_HUGE_MM_H */
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 428cc69bd81d..253fa2cdd24f 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1071,8 +1071,16 @@ pmd_t *page_check_address_pmd(struct page *page,
 		goto out;
 	if (pmd_page(*pmd) != page)
 		goto out;
-	VM_BUG_ON(flag == PAGE_CHECK_ADDRESS_PMD_NOTSPLITTING_FLAG &&
-		  pmd_trans_splitting(*pmd));
+	/*
+	 * split_vma() may create temporary aliased mappings. There is
+	 * no risk as long as all huge pmd are found and have their
+	 * splitting bit set before __split_huge_page_refcount
+	 * runs. Finding the same huge pmd more than once during the
+	 * same rmap walk is not a problem.
+	 */
+	if (flag == PAGE_CHECK_ADDRESS_PMD_NOTSPLITTING_FLAG &&
+	    pmd_trans_splitting(*pmd))
+		goto out;
 	if (pmd_trans_huge(*pmd)) {
 		VM_BUG_ON(flag == PAGE_CHECK_ADDRESS_PMD_SPLITTING_FLAG &&
 			  !pmd_trans_splitting(*pmd));
@@ -2174,3 +2182,71 @@ void __split_huge_page_pmd(struct mm_struct *mm, pmd_t *pmd)
 	put_page(page);
 	BUG_ON(pmd_trans_huge(*pmd));
 }
+
+static void split_huge_page_address(struct mm_struct *mm,
+				    unsigned long address)
+{
+	pgd_t *pgd;
+	pud_t *pud;
+	pmd_t *pmd;
+
+	VM_BUG_ON(!(address & ~HPAGE_PMD_MASK));
+
+	pgd = pgd_offset(mm, address);
+	if (!pgd_present(*pgd))
+		return;
+
+	pud = pud_offset(pgd, address);
+	if (!pud_present(*pud))
+		return;
+
+	pmd = pmd_offset(pud, address);
+	if (!pmd_present(*pmd))
+		return;
+	/*
+	 * Caller holds the mmap_sem write mode, so a huge pmd cannot
+	 * materialize from under us.
+	 */
+	split_huge_page_pmd(mm, pmd);
+}
+
+void __vma_adjust_trans_huge(struct vm_area_struct *vma,
+			     unsigned long start,
+			     unsigned long end,
+			     long adjust_next)
+{
+	/*
+	 * If the new start address isn't hpage aligned and it could
+	 * previously contain an hugepage: check if we need to split
+	 * an huge pmd.
+	 */
+	if (start & ~HPAGE_PMD_MASK &&
+	    (start & HPAGE_PMD_MASK) >= vma->vm_start &&
+	    (start & HPAGE_PMD_MASK) + HPAGE_PMD_SIZE <= vma->vm_end)
+		split_huge_page_address(vma->vm_mm, start);
+
+	/*
+	 * If the new end address isn't hpage aligned and it could
+	 * previously contain an hugepage: check if we need to split
+	 * an huge pmd.
+	 */
+	if (end & ~HPAGE_PMD_MASK &&
+	    (end & HPAGE_PMD_MASK) >= vma->vm_start &&
+	    (end & HPAGE_PMD_MASK) + HPAGE_PMD_SIZE <= vma->vm_end)
+		split_huge_page_address(vma->vm_mm, end);
+
+	/*
+	 * If we're also updating the vma->vm_next->vm_start, if the new
+	 * vm_next->vm_start isn't page aligned and it could previously
+	 * contain an hugepage: check if we need to split an huge pmd.
+	 */
+	if (adjust_next > 0) {
+		struct vm_area_struct *next = vma->vm_next;
+		unsigned long nstart = next->vm_start;
+		nstart += adjust_next << PAGE_SHIFT;
+		if (nstart & ~HPAGE_PMD_MASK &&
+		    (nstart & HPAGE_PMD_MASK) >= next->vm_start &&
+		    (nstart & HPAGE_PMD_MASK) + HPAGE_PMD_SIZE <= next->vm_end)
+			split_huge_page_address(next->vm_mm, nstart);
+	}
+}
diff --git a/mm/mmap.c b/mm/mmap.c
index 4115e7441df6..7d1535283a1a 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -589,6 +589,8 @@ again:			remove_next = 1 + (end > next->vm_end);
 		}
 	}
 
+	vma_adjust_trans_huge(vma, start, end, adjust_next);
+
 	/*
 	 * When changing only vma->vm_end, we don't really need anon_vma
 	 * lock. This is a fairly rare case by itself, but the anon_vma