[PATCH] mm/mremap.c: refactor finding vma and checking vma is alllowed to expand

[Li Xinhai <lixinhai.lxh@xxxxxxxxx>]

Function vma_to_resize)() is called to find the vma to be remapped and
also check if expand size is allowed or not. This function assume that all
call sites should make sure new_len >= old_len, and currently this
assumption is fullfilled at those two call sites, so no real problem at
present.

After this patch, we explicitly check new_len < old_len case, and separate
a new function for checking if expand size is allowed or not. Also rename
vma_to_resize to vma_to_remap, since the vma to be remapped would not
always require resize.

Cc: John Hubbard <jhubbard@xxxxxxxxxx>
Cc: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
Signed-off-by: Li Xinhai <lixinhai.lxh@xxxxxxxxx>
---
 mm/mremap.c | 79 ++++++++++++++++++++++++++++++-----------------------
 1 file changed, 45 insertions(+), 34 deletions(-)

diff --git a/mm/mremap.c b/mm/mremap.c
index c5590afe7165..22eb4e9f35d6 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -621,13 +621,52 @@ static unsigned long move_vma(struct vm_area_struct *vma,
 	return new_addr;
 }
 
-static struct vm_area_struct *vma_to_resize(unsigned long addr,
+static struct vm_area_struct *vma_allow_expand(struct vm_area_struct *vma,
+	unsigned long addr, unsigned long old_len, unsigned long new_len,
+	unsigned long *p)
+{
+	struct mm_struct *mm = current->mm;
+	unsigned long pgoff;
+
+	pgoff = (addr - vma->vm_start) >> PAGE_SHIFT;
+	pgoff += vma->vm_pgoff;
+	if (pgoff + (new_len >> PAGE_SHIFT) < pgoff)
+		return ERR_PTR(-EINVAL);
+
+	if (vma->vm_flags & (VM_DONTEXPAND | VM_PFNMAP))
+		return ERR_PTR(-EFAULT);
+
+	if (vma->vm_flags & VM_LOCKED) {
+		unsigned long locked, lock_limit;
+
+		locked = mm->locked_vm << PAGE_SHIFT;
+		lock_limit = rlimit(RLIMIT_MEMLOCK);
+		locked += new_len - old_len;
+		if (locked > lock_limit && !capable(CAP_IPC_LOCK))
+			return ERR_PTR(-EAGAIN);
+	}
+
+	if (!may_expand_vm(mm, vma->vm_flags,
+				(new_len - old_len) >> PAGE_SHIFT))
+		return ERR_PTR(-ENOMEM);
+
+	if (vma->vm_flags & VM_ACCOUNT) {
+		unsigned long charged = (new_len - old_len) >> PAGE_SHIFT;
+
+		if (security_vm_enough_memory_mm(mm, charged))
+			return ERR_PTR(-ENOMEM);
+		*p = charged;
+	}
+
+	return vma;
+}
+
+static struct vm_area_struct *vma_to_remap(unsigned long addr,
 	unsigned long old_len, unsigned long new_len, unsigned long flags,
 	unsigned long *p)
 {
 	struct mm_struct *mm = current->mm;
 	struct vm_area_struct *vma = find_vma(mm, addr);
-	unsigned long pgoff;
 
 	if (!vma || vma->vm_start > addr)
 		return ERR_PTR(-EFAULT);
@@ -656,39 +695,11 @@ static struct vm_area_struct *vma_to_resize(unsigned long addr,
 	if (old_len > vma->vm_end - addr)
 		return ERR_PTR(-EFAULT);
 
-	if (new_len == old_len)
+	if (new_len <= old_len)
 		return vma;
 
 	/* Need to be careful about a growing mapping */
-	pgoff = (addr - vma->vm_start) >> PAGE_SHIFT;
-	pgoff += vma->vm_pgoff;
-	if (pgoff + (new_len >> PAGE_SHIFT) < pgoff)
-		return ERR_PTR(-EINVAL);
-
-	if (vma->vm_flags & (VM_DONTEXPAND | VM_PFNMAP))
-		return ERR_PTR(-EFAULT);
-
-	if (vma->vm_flags & VM_LOCKED) {
-		unsigned long locked, lock_limit;
-		locked = mm->locked_vm << PAGE_SHIFT;
-		lock_limit = rlimit(RLIMIT_MEMLOCK);
-		locked += new_len - old_len;
-		if (locked > lock_limit && !capable(CAP_IPC_LOCK))
-			return ERR_PTR(-EAGAIN);
-	}
-
-	if (!may_expand_vm(mm, vma->vm_flags,
-				(new_len - old_len) >> PAGE_SHIFT))
-		return ERR_PTR(-ENOMEM);
-
-	if (vma->vm_flags & VM_ACCOUNT) {
-		unsigned long charged = (new_len - old_len) >> PAGE_SHIFT;
-		if (security_vm_enough_memory_mm(mm, charged))
-			return ERR_PTR(-ENOMEM);
-		*p = charged;
-	}
-
-	return vma;
+	return vma_allow_expand(vma, addr, old_len, new_len, p);
 }
 
 static unsigned long mremap_to(unsigned long addr, unsigned long old_len,
@@ -743,7 +754,7 @@ static unsigned long mremap_to(unsigned long addr, unsigned long old_len,
 		old_len = new_len;
 	}
 
-	vma = vma_to_resize(addr, old_len, new_len, flags, &charged);
+	vma = vma_to_remap(addr, old_len, new_len, flags, &charged);
 	if (IS_ERR(vma)) {
 		ret = PTR_ERR(vma);
 		goto out;
@@ -894,7 +905,7 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
 	/*
 	 * Ok, we need to grow..
 	 */
-	vma = vma_to_resize(addr, old_len, new_len, flags, &charged);
+	vma = vma_to_remap(addr, old_len, new_len, flags, &charged);
 	if (IS_ERR(vma)) {
 		ret = PTR_ERR(vma);
 		goto out;
-- 
2.18.4