+ mm-mmap-vma_merge-set-mid-to-null-if-not-applicable.patch added to mm-unstable branch

+ mm-mmap-vma_merge-set-mid-to-null-if-not-applicable.patch added to mm-unstable branch

[Andrew Morton]

The patch titled
     Subject: mm/mmap/vma_merge: set mid to NULL if not applicable
has been added to the -mm mm-unstable branch.  Its filename is
     mm-mmap-vma_merge-set-mid-to-null-if-not-applicable.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-mmap-vma_merge-set-mid-to-null-if-not-applicable.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Vlastimil Babka <vbabka@suse.cz>
Subject: mm/mmap/vma_merge: set mid to NULL if not applicable
Date: Thu, 9 Mar 2023 12:12:54 +0100

There are several places where we test if 'mid' is really the area NNNN in
the diagram and the tests have two variants and are non-obvious to follow.
Instead, set 'mid' to NULL up-front if it's not the NNNN area, and
simplify the tests.

Also update the description in comment accordingly.

Link: https://lkml.kernel.org/r/20230309111258.24079-7-vbabka@suse.cz
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Lorenzo Stoakes <lstoakes@gmail.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/mmap.c |   21 +++++++++++++--------
 1 file changed, 13 insertions(+), 8 deletions(-)

--- a/mm/mmap.c~mm-mmap-vma_merge-set-mid-to-null-if-not-applicable
+++ a/mm/mmap.c
@@ -861,10 +861,11 @@ can_vma_merge_after(struct vm_area_struc
  *
  * The following mprotect cases have to be considered, where AAAA is
  * the area passed down from mprotect_fixup, never extending beyond one
- * vma, PPPPPP is the prev vma specified, and NNNNNN the next vma after:
+ * vma, PPPPPP is the prev vma specified, NNNN is a vma that overlaps
+ * the area AAAA and XXXXXX the next vma after AAAA:
  *
  *     AAAA             AAAA                   AAAA
- *    PPPPPPNNNNNN    PPPPPPXXXXXX       PPPPPPNNNNNN
+ *    PPPPPPXXXXXX    PPPPPPXXXXXX       PPPPPPNNNNNN
  *    cannot merge    might become       might become
  *                    PPXXXXXXXXXX       PPPPPPPPPPNN
  *    mmap, brk or    case 4 below       case 5 below
@@ -892,9 +893,10 @@ can_vma_merge_after(struct vm_area_struc
  *
  * In the code below:
  * PPPP is represented by *prev
- * NNNN is represented by *mid (and possibly equal to *next)
- * XXXX is represented by *next or not represented at all.
- * AAAA is not represented - it will be merged or the function will return NULL
+ * NNNN is represented by *mid or not represented at all (NULL)
+ * XXXX is represented by *next or not represented at all (NULL)
+ * AAAA is not represented - it will be merged and the vma containing the
+ *      area is returned, or the function will return NULL
  */
 struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
 			struct vm_area_struct *prev, unsigned long addr,
@@ -931,6 +933,9 @@ struct vm_area_struct *vma_merge(struct
 	else
 		next = mid;
 
+	if (mid && end <= mid->vm_start)
+		mid = NULL;
+
 	/* verify some invariant that must be enforced by the caller */
 	VM_WARN_ON(prev && addr <= prev->vm_start);
 	VM_WARN_ON(mid && end > mid->vm_end);
@@ -965,7 +970,7 @@ struct vm_area_struct *vma_merge(struct
 		remove = next;				/* case 1 */
 		vma_end = next->vm_end;
 		err = dup_anon_vma(prev, next);
-		if (mid != next) {			/* case 6 */
+		if (mid) {				/* case 6 */
 			remove = mid;
 			remove2 = next;
 			if (!next->anon_vma)
@@ -973,7 +978,7 @@ struct vm_area_struct *vma_merge(struct
 		}
 	} else if (merge_prev) {
 		err = 0;				/* case 2 */
-		if (mid && end > mid->vm_start) {
+		if (mid) {
 			err = dup_anon_vma(prev, mid);
 			if (end == mid->vm_end) {	/* case 7 */
 				remove = mid;
@@ -995,7 +1000,7 @@ struct vm_area_struct *vma_merge(struct
 			vma_end = next->vm_end;
 			vma_pgoff = next->vm_pgoff;
 			err = 0;
-			if (mid != next) {		/* case 8 */
+			if (mid) {			/* case 8 */
 				vma_pgoff = mid->vm_pgoff;
 				remove = mid;
 				err = dup_anon_vma(next, mid);
_

Patches currently in -mm which might be from vbabka@suse.cz are

mm-page_alloc-reduce-page-alloc-free-sanity-checks.patch
mm-mmap-vma_merge-use-only-primary-pointers-for-preparing-merge.patch
mm-mmap-vma_merge-use-the-proper-vma-pointer-in-case-3.patch
mm-mmap-vma_merge-use-the-proper-vma-pointers-in-cases-1-and-6.patch
mm-mmap-vma_merge-use-the-proper-vma-pointer-in-case-4.patch
mm-mmap-vma_merge-initialize-mid-and-next-in-natural-order.patch
mm-mmap-vma_merge-set-mid-to-null-if-not-applicable.patch
mm-mmap-vma_merge-rename-adj_next-to-adj_start.patch
mm-mmap-vma_merge-convert-mergeability-checks-to-return-bool.patch
mm-mmap-start-distinguishing-if-vma-can-be-removed-in-mergeability-test.patch
mm-mremap-simplify-vma-expansion-again.patch