Commit f1819427 authored by Hugh Dickins's avatar Hugh Dickins Committed by Linus Torvalds

mm: fix hang on anon_vma->root->lock

After several hours, kbuild tests hang with anon_vma_prepare() spinning on
a newly allocated anon_vma's lock - on a box with CONFIG_TREE_PREEMPT_RCU=y
(which makes this very much more likely, but it could happen without).

The ever-subtle page_lock_anon_vma() now needs a further twist: since
anon_vma_prepare() and anon_vma_fork() are liable to change the ->root
of a reused anon_vma structure at any moment, page_lock_anon_vma()
needs to check page_mapped() again before succeeding, otherwise
page_unlock_anon_vma() might address a different root->lock.
Signed-off-by: default avatarHugh Dickins <>
Reviewed-by: default avatarRik van Riel <>
Cc: Christoph Lameter <>
Cc: Peter Zijlstra <>
Cc: Andrea Arcangeli <>
Signed-off-by: default avatarLinus Torvalds <>
parent d4348c67
......@@ -316,7 +316,7 @@ void __init anon_vma_init(void)
struct anon_vma *page_lock_anon_vma(struct page *page)
struct anon_vma *anon_vma;
struct anon_vma *anon_vma, *root_anon_vma;
unsigned long anon_mapping;
......@@ -327,8 +327,21 @@ struct anon_vma *page_lock_anon_vma(struct page *page)
goto out;
anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON);
return anon_vma;
root_anon_vma = ACCESS_ONCE(anon_vma->root);
* If this page is still mapped, then its anon_vma cannot have been
* freed. But if it has been unmapped, we have no security against
* the anon_vma structure being freed and reused (for another anon_vma:
* SLAB_DESTROY_BY_RCU guarantees that - so the spin_lock above cannot
* corrupt): with anon_vma_prepare() or anon_vma_fork() redirecting
* anon_vma->root before page_unlock_anon_vma() is called to unlock.
if (page_mapped(page))
return anon_vma;
return NULL;
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