[RFC v7 PATCH 4/4] mm: unmap special vmas with regular do_munmap()

[Yang Shi <yang.shi@linux.alibaba.com>]

Unmapping vmas, which have VM_HUGETLB | VM_PFNMAP flag set or
have uprobes set, need get done with write mmap_sem held since
they may update vm_flags.

So, it might be not safe enough to deal with these kind of special
mappings with read mmap_sem. Deal with such mappings with regular
do_munmap() call.

Michal suggested to make this as a separate patch for safer and more
bisectable sake.

Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
---
 mm/mmap.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/mm/mmap.c b/mm/mmap.c
index 2234d5a..06cb83c 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2766,6 +2766,16 @@ static inline void munlock_vmas(struct vm_area_struct *vma,
 	}
 }
 
+static inline bool can_zap_with_rlock(struct vm_area_struct *vma)
+{
+	if ((vma->vm_file &&
+	     vma_has_uprobes(vma, vma->vm_start, vma->vm_end)) ||
+	     (vma->vm_flags | (VM_HUGETLB | VM_PFNMAP)))
+		return false;
+
+	return true;
+}
+
 /*
  * Zap pages with read mmap_sem held
  *
@@ -2808,6 +2818,17 @@ static int do_munmap_zap_rlock(struct mm_struct *mm, unsigned long start,
 			goto out;
 	}
 
+	/*
+	 * Unmapping vmas, which have VM_HUGETLB | VM_PFNMAP flag set or
+	 * have uprobes set, need get done with write mmap_sem held since
+	 * they may update vm_flags. Deal with such mappings with regular
+	 * do_munmap() call.
+	 */
+	for (vma = start_vma; vma && vma->vm_start < end; vma = vma->vm_next) {
+		if (!can_zap_with_rlock(vma))
+			goto regular_path;
+	}
+
 	/* Handle mlocked vmas */
 	if (mm->locked_vm) {
 		vma = start_vma;
@@ -2828,6 +2849,9 @@ static int do_munmap_zap_rlock(struct mm_struct *mm, unsigned long start,
 
 	return 0;
 
+regular_path:
+	ret = do_munmap(mm, start, len, uf);
+
 out:
 	up_write(&mm->mmap_sem);
 	return ret;
-- 
1.8.3.1