@@ -1902,6 +1902,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
{
struct anon_vma *anon_vma;
int ret = 1;
+ struct address_space *mapping;
BUG_ON(is_huge_zero_page(page));
BUG_ON(!PageAnon(page));
@@ -1913,10 +1914,24 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
* page_lock_anon_vma_read except the write lock is taken to serialise
* against parallel split or collapse operations.
*/
- anon_vma = page_get_anon_vma(page);
- if (!anon_vma)
- goto out;
- anon_vma_lock_write(anon_vma);
+ for (;;) {
+ mapping = ACCESS_ONCE(page->mapping);
+ anon_vma = page_get_anon_vma(page);
+ if (!anon_vma)
+ goto out;
+ anon_vma_lock_write(anon_vma);
+ /*
+ * We don't hold the page lock here so
+ * remap_pages_huge_pmd can change the anon_vma from
+ * under us until we obtain the anon_vma lock. Verify
+ * that we obtained the anon_vma lock before
+ * remap_pages did.
+ */
+ if (likely(mapping == ACCESS_ONCE(page->mapping)))
+ break;
+ anon_vma_unlock_write(anon_vma);
+ put_anon_vma(anon_vma);
+ }
ret = 0;
if (!PageCompound(page))
@@ -492,6 +492,7 @@ struct anon_vma *page_lock_anon_vma_read(struct page *page)
struct anon_vma *root_anon_vma;
unsigned long anon_mapping;
+repeat:
rcu_read_lock();
anon_mapping = (unsigned long) ACCESS_ONCE(page->mapping);
if ((anon_mapping & PAGE_MAPPING_FLAGS) != PAGE_MAPPING_ANON)
@@ -530,6 +531,14 @@ struct anon_vma *page_lock_anon_vma_read(struct page *page)
rcu_read_unlock();
anon_vma_lock_read(anon_vma);
+ /* check if remap_anon_pages changed the anon_vma */
+ if (unlikely((unsigned long) ACCESS_ONCE(page->mapping) != anon_mapping)) {
+ anon_vma_unlock_read(anon_vma);
+ put_anon_vma(anon_vma);
+ anon_vma = NULL;
+ goto repeat;
+ }
+
if (atomic_dec_and_test(&anon_vma->refcount)) {
/*
* Oops, we held the last refcount, release the lock
As far as the rmap code is concerned, rmap_pages only alters the page->mapping and page->index. It does it while holding the page lock. However there are a few places that in presence of anon pages are allowed to do rmap walks without the page lock (split_huge_page and page_referenced_anon). Those places that are doing rmap walks without taking the page lock first, must be updated to re-check that the page->mapping didn't change after they obtained the anon_vma lock. remap_pages takes the anon_vma lock for writing before altering the page->mapping, so if the page->mapping is still the same after obtaining the anon_vma lock (without the page lock), the rmap walks can go ahead safely (and remap_pages will wait them to complete before proceeding). remap_pages serializes against itself with the page lock. All other places taking the anon_vma lock while holding the mmap_sem for writing, don't need to check if the page->mapping has changed after taking the anon_vma lock, regardless of the page lock, because remap_pages holds the mmap_sem for reading. There's one constraint enforced to allow this simplification: the source pages passed to remap_pages must be mapped only in one vma, but this is not a limitation when used to handle userland page faults. The source addresses passed to remap_pages should be set as VM_DONTCOPY with MADV_DONTFORK to avoid any risk of the mapcount of the pages increasing, if fork runs in parallel in another thread, before or while remap_pages runs. Signed-off-by: Andrea Arcangeli <aarcange@redhat.com> --- mm/huge_memory.c | 23 +++++++++++++++++++---- mm/rmap.c | 9 +++++++++ 2 files changed, 28 insertions(+), 4 deletions(-)