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anon_vma_prepare: properly lock even newly allocated entries

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The anon_vma code is very subtle, and we end up doing optimistic lookups
of anon_vmas under RCU in page_lock_anon_vma() with no locking.  Other
CPU's can also see the newly allocated entry immediately after we've
exposed it by setting "vma->anon_vma" to the new value.

We protect against the anon_vma being destroyed by having the SLAB
marked as SLAB_DESTROY_BY_RCU, so the RCU lookup can depend on the
allocation not being destroyed - but it might still be free'd and
re-allocated here to a new vma.

As a result, we should not do the anon_vma list ops on a newly allocated
vma without proper locking.

Acked-by: Nick Piggin <npiggin@suse.de>
Acked-by: Hugh Dickins <hugh@veritas.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Linus Torvalds 2008-10-19 10:32:20 -07:00
parent 0cfd81031a
commit d9d332e087
1 changed files with 32 additions and 10 deletions
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42
mm/rmap.c
View file

@ -55,7 +55,33 @@
struct kmem_cache *anon_vma_cachep; struct kmem_cache *anon_vma_cachep;
/* This must be called under the mmap_sem. */ /**
* anon_vma_prepare - attach an anon_vma to a memory region
* @vma: the memory region in question
*
* This makes sure the memory mapping described by 'vma' has
* an 'anon_vma' attached to it, so that we can associate the
* anonymous pages mapped into it with that anon_vma.
*
* The common case will be that we already have one, but if
* if not we either need to find an adjacent mapping that we
* can re-use the anon_vma from (very common when the only
* reason for splitting a vma has been mprotect()), or we
* allocate a new one.
*
* Anon-vma allocations are very subtle, because we may have
* optimistically looked up an anon_vma in page_lock_anon_vma()
* and that may actually touch the spinlock even in the newly
* allocated vma (it depends on RCU to make sure that the
* anon_vma isn't actually destroyed).
*
* As a result, we need to do proper anon_vma locking even
* for the new allocation. At the same time, we do not want
* to do any locking for the common case of already having
* an anon_vma.
*
* This must be called with the mmap_sem held for reading.
*/
int anon_vma_prepare(struct vm_area_struct *vma) int anon_vma_prepare(struct vm_area_struct *vma)
{ {
struct anon_vma *anon_vma = vma->anon_vma; struct anon_vma *anon_vma = vma->anon_vma;
@ -63,20 +89,17 @@ int anon_vma_prepare(struct vm_area_struct *vma)
might_sleep(); might_sleep();
if (unlikely(!anon_vma)) { if (unlikely(!anon_vma)) {
struct mm_struct *mm = vma->vm_mm; struct mm_struct *mm = vma->vm_mm;
struct anon_vma *allocated, *locked; struct anon_vma *allocated;
anon_vma = find_mergeable_anon_vma(vma); anon_vma = find_mergeable_anon_vma(vma);
if (anon_vma) { allocated = NULL;
allocated = NULL; if (!anon_vma) {
locked = anon_vma;
spin_lock(&locked->lock);
} else {
anon_vma = anon_vma_alloc(); anon_vma = anon_vma_alloc();
if (unlikely(!anon_vma)) if (unlikely(!anon_vma))
return -ENOMEM; return -ENOMEM;
allocated = anon_vma; allocated = anon_vma;
locked = NULL;
} }
spin_lock(&anon_vma->lock);
/* page_table_lock to protect against threads */ /* page_table_lock to protect against threads */
spin_lock(&mm->page_table_lock); spin_lock(&mm->page_table_lock);
@ -87,8 +110,7 @@ int anon_vma_prepare(struct vm_area_struct *vma)
} }
spin_unlock(&mm->page_table_lock); spin_unlock(&mm->page_table_lock);
if (locked) spin_unlock(&anon_vma->lock);
spin_unlock(&locked->lock);
if (unlikely(allocated)) if (unlikely(allocated))
anon_vma_free(allocated); anon_vma_free(allocated);
} }