Merge branch 'anonvma'
* anonvma:
anonvma: when setting up page->mapping, we need to pick the _oldest_ anonvma
anon_vma: clone the anon_vma chain in the right order
vma_adjust: fix the copying of anon_vma chains
Simplify and comment on anon_vma re-use for anon_vma_prepare()
diff --git a/mm/mmap.c b/mm/mmap.c
index 75557c6..f90ea92 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -507,11 +507,12 @@
struct address_space *mapping = NULL;
struct prio_tree_root *root = NULL;
struct file *file = vma->vm_file;
- struct anon_vma *anon_vma = NULL;
long adjust_next = 0;
int remove_next = 0;
if (next && !insert) {
+ struct vm_area_struct *exporter = NULL;
+
if (end >= next->vm_end) {
/*
* vma expands, overlapping all the next, and
@@ -519,7 +520,7 @@
*/
again: remove_next = 1 + (end > next->vm_end);
end = next->vm_end;
- anon_vma = next->anon_vma;
+ exporter = next;
importer = vma;
} else if (end > next->vm_start) {
/*
@@ -527,7 +528,7 @@
* mprotect case 5 shifting the boundary up.
*/
adjust_next = (end - next->vm_start) >> PAGE_SHIFT;
- anon_vma = next->anon_vma;
+ exporter = next;
importer = vma;
} else if (end < vma->vm_end) {
/*
@@ -536,28 +537,19 @@
* mprotect case 4 shifting the boundary down.
*/
adjust_next = - ((vma->vm_end - end) >> PAGE_SHIFT);
- anon_vma = next->anon_vma;
+ exporter = vma;
importer = next;
}
- }
- /*
- * When changing only vma->vm_end, we don't really need anon_vma lock.
- */
- if (vma->anon_vma && (insert || importer || start != vma->vm_start))
- anon_vma = vma->anon_vma;
- if (anon_vma) {
/*
* Easily overlooked: when mprotect shifts the boundary,
* make sure the expanding vma has anon_vma set if the
* shrinking vma had, to cover any anon pages imported.
*/
- if (importer && !importer->anon_vma) {
- /* Block reverse map lookups until things are set up. */
- if (anon_vma_clone(importer, vma)) {
+ if (exporter && exporter->anon_vma && !importer->anon_vma) {
+ if (anon_vma_clone(importer, exporter))
return -ENOMEM;
- }
- importer->anon_vma = anon_vma;
+ importer->anon_vma = exporter->anon_vma;
}
}
@@ -825,6 +817,61 @@
}
/*
+ * Rough compatbility check to quickly see if it's even worth looking
+ * at sharing an anon_vma.
+ *
+ * They need to have the same vm_file, and the flags can only differ
+ * in things that mprotect may change.
+ *
+ * NOTE! The fact that we share an anon_vma doesn't _have_ to mean that
+ * we can merge the two vma's. For example, we refuse to merge a vma if
+ * there is a vm_ops->close() function, because that indicates that the
+ * driver is doing some kind of reference counting. But that doesn't
+ * really matter for the anon_vma sharing case.
+ */
+static int anon_vma_compatible(struct vm_area_struct *a, struct vm_area_struct *b)
+{
+ return a->vm_end == b->vm_start &&
+ mpol_equal(vma_policy(a), vma_policy(b)) &&
+ a->vm_file == b->vm_file &&
+ !((a->vm_flags ^ b->vm_flags) & ~(VM_READ|VM_WRITE|VM_EXEC)) &&
+ b->vm_pgoff == a->vm_pgoff + ((b->vm_start - a->vm_start) >> PAGE_SHIFT);
+}
+
+/*
+ * Do some basic sanity checking to see if we can re-use the anon_vma
+ * from 'old'. The 'a'/'b' vma's are in VM order - one of them will be
+ * the same as 'old', the other will be the new one that is trying
+ * to share the anon_vma.
+ *
+ * NOTE! This runs with mm_sem held for reading, so it is possible that
+ * the anon_vma of 'old' is concurrently in the process of being set up
+ * by another page fault trying to merge _that_. But that's ok: if it
+ * is being set up, that automatically means that it will be a singleton
+ * acceptable for merging, so we can do all of this optimistically. But
+ * we do that ACCESS_ONCE() to make sure that we never re-load the pointer.
+ *
+ * IOW: that the "list_is_singular()" test on the anon_vma_chain only
+ * matters for the 'stable anon_vma' case (ie the thing we want to avoid
+ * is to return an anon_vma that is "complex" due to having gone through
+ * a fork).
+ *
+ * We also make sure that the two vma's are compatible (adjacent,
+ * and with the same memory policies). That's all stable, even with just
+ * a read lock on the mm_sem.
+ */
+static struct anon_vma *reusable_anon_vma(struct vm_area_struct *old, struct vm_area_struct *a, struct vm_area_struct *b)
+{
+ if (anon_vma_compatible(a, b)) {
+ struct anon_vma *anon_vma = ACCESS_ONCE(old->anon_vma);
+
+ if (anon_vma && list_is_singular(&old->anon_vma_chain))
+ return anon_vma;
+ }
+ return NULL;
+}
+
+/*
* find_mergeable_anon_vma is used by anon_vma_prepare, to check
* neighbouring vmas for a suitable anon_vma, before it goes off
* to allocate a new anon_vma. It checks because a repetitive
@@ -834,28 +881,16 @@
*/
struct anon_vma *find_mergeable_anon_vma(struct vm_area_struct *vma)
{
+ struct anon_vma *anon_vma;
struct vm_area_struct *near;
- unsigned long vm_flags;
near = vma->vm_next;
if (!near)
goto try_prev;
- /*
- * Since only mprotect tries to remerge vmas, match flags
- * which might be mprotected into each other later on.
- * Neither mlock nor madvise tries to remerge at present,
- * so leave their flags as obstructing a merge.
- */
- vm_flags = vma->vm_flags & ~(VM_READ|VM_WRITE|VM_EXEC);
- vm_flags |= near->vm_flags & (VM_READ|VM_WRITE|VM_EXEC);
-
- if (near->anon_vma && vma->vm_end == near->vm_start &&
- mpol_equal(vma_policy(vma), vma_policy(near)) &&
- can_vma_merge_before(near, vm_flags,
- NULL, vma->vm_file, vma->vm_pgoff +
- ((vma->vm_end - vma->vm_start) >> PAGE_SHIFT)))
- return near->anon_vma;
+ anon_vma = reusable_anon_vma(near, vma, near);
+ if (anon_vma)
+ return anon_vma;
try_prev:
/*
* It is potentially slow to have to call find_vma_prev here.
@@ -868,14 +903,9 @@
if (!near)
goto none;
- vm_flags = vma->vm_flags & ~(VM_READ|VM_WRITE|VM_EXEC);
- vm_flags |= near->vm_flags & (VM_READ|VM_WRITE|VM_EXEC);
-
- if (near->anon_vma && near->vm_end == vma->vm_start &&
- mpol_equal(vma_policy(near), vma_policy(vma)) &&
- can_vma_merge_after(near, vm_flags,
- NULL, vma->vm_file, vma->vm_pgoff))
- return near->anon_vma;
+ anon_vma = reusable_anon_vma(near, near, vma);
+ if (anon_vma)
+ return anon_vma;
none:
/*
* There's no absolute need to look only at touching neighbours:
diff --git a/mm/rmap.c b/mm/rmap.c
index eaa7a09..4bad326 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -182,7 +182,7 @@
{
struct anon_vma_chain *avc, *pavc;
- list_for_each_entry(pavc, &src->anon_vma_chain, same_vma) {
+ list_for_each_entry_reverse(pavc, &src->anon_vma_chain, same_vma) {
avc = anon_vma_chain_alloc();
if (!avc)
goto enomem_failure;
@@ -734,9 +734,20 @@
static void __page_set_anon_rmap(struct page *page,
struct vm_area_struct *vma, unsigned long address)
{
- struct anon_vma *anon_vma = vma->anon_vma;
+ struct anon_vma_chain *avc;
+ struct anon_vma *anon_vma;
- BUG_ON(!anon_vma);
+ BUG_ON(!vma->anon_vma);
+
+ /*
+ * We must use the _oldest_ possible anon_vma for the page mapping!
+ *
+ * So take the last AVC chain entry in the vma, which is the deepest
+ * ancestor, and use the anon_vma from that.
+ */
+ avc = list_entry(vma->anon_vma_chain.prev, struct anon_vma_chain, same_vma);
+ anon_vma = avc->anon_vma;
+
anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON;
page->mapping = (struct address_space *) anon_vma;
page->index = linear_page_index(vma, address);
