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Subject: [RFC PATCH v2 3/4] [PATCH 3/4] mm: enable merging of VMAs with different anon_vmas
Date:   Fri, 11 Mar 2022 18:46:01 +0100
Message-Id: <20220311174602.288010-4-matenajakub@gmail.com>
In-Reply-To: <20220311174602.288010-1-matenajakub@gmail.com>
References: <20220311174602.288010-1-matenajakub@gmail.com>
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Enable merging of a VMA even when it is linked to different
anon_vma than the one it is being merged to, but only if the VMA
in question does not share any page with a parent or child process.
Every anonymous page stores a pointer to its anon_vma in the parameter
mapping, which is now updated as part of the merge process.

Signed-off-by: Jakub Matěna <matenajakub@gmail.com>
---
 include/linux/rmap.h | 17 ++++++++++++++++-
 mm/mmap.c            | 15 ++++++++++++++-
 mm/rmap.c            | 40 ++++++++++++++++++++++++++++++++++++++++
 3 files changed, 70 insertions(+), 2 deletions(-)

diff --git a/include/linux/rmap.h b/include/linux/rmap.h
index e704b1a4c06c..c8508a4ebc46 100644
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -137,10 +137,13 @@ static inline void anon_vma_unlock_read(struct anon_vma *anon_vma)
  */
 void anon_vma_init(void);	/* create anon_vma_cachep */
 int  __anon_vma_prepare(struct vm_area_struct *);
+void reconnect_pages(struct vm_area_struct *vma, struct vm_area_struct *next);
 void unlink_anon_vmas(struct vm_area_struct *);
 int anon_vma_clone(struct vm_area_struct *, struct vm_area_struct *);
 int anon_vma_fork(struct vm_area_struct *, struct vm_area_struct *);
 
+bool rbt_no_children(struct anon_vma *av);
+
 static inline int anon_vma_prepare(struct vm_area_struct *vma)
 {
 	if (likely(vma->anon_vma))
@@ -149,10 +152,22 @@ static inline int anon_vma_prepare(struct vm_area_struct *vma)
 	return __anon_vma_prepare(vma);
 }
 
+/**
+ * anon_vma_merge() - Merge anon_vmas of the given VMAs
+ * @vma: VMA being merged to
+ * @next: VMA being merged
+ */
 static inline void anon_vma_merge(struct vm_area_struct *vma,
 				  struct vm_area_struct *next)
 {
-	VM_BUG_ON_VMA(vma->anon_vma != next->anon_vma, vma);
+	struct anon_vma *anon_vma1 = vma->anon_vma;
+	struct anon_vma *anon_vma2 = next->anon_vma;
+
+	VM_BUG_ON_VMA(anon_vma1 && anon_vma2 && anon_vma1 != anon_vma2 &&
+			((anon_vma2 != anon_vma2->root)
+			|| !rbt_no_children(anon_vma2)), vma);
+
+	reconnect_pages(vma, next);
 	unlink_anon_vmas(next);
 }
 
diff --git a/mm/mmap.c b/mm/mmap.c
index 4f9c6ca7ff4e..ccb24862e670 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1065,7 +1065,20 @@ static inline int is_mergeable_anon_vma(struct anon_vma *anon_vma1,
 	if ((!anon_vma1 || !anon_vma2) && (!vma ||
 		list_is_singular(&vma->anon_vma_chain)))
 		return 1;
-	return anon_vma1 == anon_vma2;
+	if (anon_vma1 == anon_vma2)
+		return 1;
+	/*
+	 * Different anon_vma but not shared by several processes
+	 */
+	else if ((anon_vma1 && anon_vma2) &&
+			(anon_vma1 == anon_vma1->root)
+			&& (rbt_no_children(anon_vma1)))
+		return 1;
+	/*
+	 * Different anon_vma and shared -> unmergeable
+	 */
+	else
+		return 0;
 }
 
 /*
diff --git a/mm/rmap.c b/mm/rmap.c
index 96273d6a9796..b296e1e1aec3 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -387,6 +387,46 @@ int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma)
 	return -ENOMEM;
 }
 
+/**
+ * reconnect_pages() - Reconnect physical pages from old to vma
+ * @vma: VMA to newly contain all physical pages of old
+ * @old: old VMA being merged to vma
+ */
+void reconnect_pages(struct vm_area_struct *vma, struct vm_area_struct *old)
+{
+	struct anon_vma *anon_vma1 = vma->anon_vma;
+	struct anon_vma *anon_vma2 = old->anon_vma;
+	unsigned long pg_iter;
+	int pg_iters;
+
+	if (anon_vma1 == anon_vma2 || anon_vma1 == NULL || anon_vma2 == NULL)
+		return; /* Nothing to do */
+
+	/* Modify page->mapping for all pages in old */
+	pg_iter = 0;
+	pg_iters = (old->vm_end - old->vm_start) >> PAGE_SHIFT;
+
+	for (; pg_iter < pg_iters; ++pg_iter) {
+		/* Get the physical page */
+		unsigned long shift = pg_iter << PAGE_SHIFT;
+		struct page *phys_page = follow_page(old, old->vm_start + shift, FOLL_GET);
+		struct anon_vma *page_anon_vma;
+
+		/* Do some checks and lock the page */
+		if (phys_page == NULL)
+			continue; /* Virtual memory page is not mapped */
+		lock_page(phys_page);
+		page_anon_vma = page_get_anon_vma(phys_page);
+		if (page_anon_vma != NULL) { /* NULL in case of ZERO_PAGE */
+			VM_BUG_ON_VMA(page_anon_vma != old->anon_vma, old);
+			/* Update physical page's mapping */
+			page_move_anon_rmap(phys_page, vma);
+		}
+		unlock_page(phys_page);
+		put_page(phys_page);
+	}
+}
+
 /*
  * Used by rbt_no_children to check node subtree.
  * Check if none of the VMAs connected to the node subtree via
-- 
2.34.1