Re: [PATCH v2 2/7] mm: Extend copy_vma()

Hi, Kirill,

On 21.05.2019 11:18, Kirill A. Shutemov wrote:

On Mon, May 20, 2019 at 05:00:12PM +0300, Kirill Tkhai wrote:

quoted

This prepares the function to copy a vma between
two processes. Two new arguments are introduced.

This kind of changes requires a lot more explanation in commit message,
describing all possible corner cases> For instance, I would really like to see a story on why logic around
need_rmap_locks is safe after the change.

Let me fast answer on the below question firstly, and later I'll write
wide explanations, since this requires much more time.
 

quoted

Signed-off-by: Kirill Tkhai <redacted>
---
 include/linux/mm.h |    4 ++--
 mm/mmap.c          |   33 ++++++++++++++++++++++++---------
 mm/mremap.c        |    4 ++--
 3 files changed, 28 insertions(+), 13 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 0e8834ac32b7..afe07e4a76f8 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h

@@ -2329,8 +2329,8 @@ extern void __vma_link_rb(struct mm_struct *, struct vm_area_struct *,
 	struct rb_node **, struct rb_node *);
 extern void unlink_file_vma(struct vm_area_struct *);
 extern struct vm_area_struct *copy_vma(struct vm_area_struct **,
-	unsigned long addr, unsigned long len, pgoff_t pgoff,
-	bool *need_rmap_locks);
+	struct mm_struct *, unsigned long addr, unsigned long len,
+	pgoff_t pgoff, bool *need_rmap_locks, bool clear_flags_ctx);
 extern void exit_mmap(struct mm_struct *);
 
 static inline int check_data_rlimit(unsigned long rlim,

diff --git a/mm/mmap.c b/mm/mmap.c
index 57803a0a3a5c..99778e724ad1 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c

@@ -3195,19 +3195,21 @@ int insert_vm_struct(struct mm_struct *mm, struct vm_area_struct *vma)
 }
 
 /*
- * Copy the vma structure to a new location in the same mm,
- * prior to moving page table entries, to effect an mremap move.
+ * Copy the vma structure to new location in the same vma
+ * prior to moving page table entries, to effect an mremap move;
  */
 struct vm_area_struct *copy_vma(struct vm_area_struct **vmap,
-	unsigned long addr, unsigned long len, pgoff_t pgoff,
-	bool *need_rmap_locks)
+				struct mm_struct *mm, unsigned long addr,
+				unsigned long len, pgoff_t pgoff,
+				bool *need_rmap_locks, bool clear_flags_ctx)
 {
 	struct vm_area_struct *vma = *vmap;
 	unsigned long vma_start = vma->vm_start;
-	struct mm_struct *mm = vma->vm_mm;
+	struct vm_userfaultfd_ctx uctx;
 	struct vm_area_struct *new_vma, *prev;
 	struct rb_node **rb_link, *rb_parent;
 	bool faulted_in_anon_vma = true;
+	unsigned long flags;
 
 	/*
 	 * If anonymous vma has not yet been faulted, update new pgoff

@@ -3220,15 +3222,25 @@ struct vm_area_struct *copy_vma(struct vm_area_struct **vmap,
 
 	if (find_vma_links(mm, addr, addr + len, &prev, &rb_link, &rb_parent))
 		return NULL;	/* should never get here */
-	new_vma = vma_merge(mm, prev, addr, addr + len, vma->vm_flags,
-			    vma->anon_vma, vma->vm_file, pgoff, vma_policy(vma),
-			    vma->vm_userfaultfd_ctx);
+
+	uctx = vma->vm_userfaultfd_ctx;
+	flags = vma->vm_flags;
+	if (clear_flags_ctx) {
+		uctx = NULL_VM_UFFD_CTX;
+		flags &= ~(VM_UFFD_MISSING | VM_UFFD_WP | VM_MERGEABLE |
+			   VM_LOCKED | VM_LOCKONFAULT | VM_WIPEONFORK |
+			   VM_DONTCOPY);
+	}

Why is the new logic required? No justification given.

Ditto.

quoted

+
+	new_vma = vma_merge(mm, prev, addr, addr + len, flags, vma->anon_vma,
+			    vma->vm_file, pgoff, vma_policy(vma), uctx);
 	if (new_vma) {
 		/*
 		 * Source vma may have been merged into new_vma
 		 */
 		if (unlikely(vma_start >= new_vma->vm_start &&
-			     vma_start < new_vma->vm_end)) {
+			     vma_start < new_vma->vm_end) &&
+			     vma->vm_mm == mm) {

How can vma_merge() succeed if vma->vm_mm != mm?

We don't use vma as an argument of vma_merge(). We use vma as a source of
vma->anon_vma, vma->vm_file and vma_policy().

We search some new_vma in mm with the same characteristics as vma has in vma->vm_mm.
In case of success vma_merge() returns it for us. For example, it may success, when
vma->vm_mm is mm_struct of forked process, while mm is mm_struct of its parent.

[...]

Kirill