[PATCH 3/5] track the root (oldest) anon_vma
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From: Rik van Riel on 12 May 2010 13:50 Subject: track the root (oldest) anon_vma Track the root (oldest) anon_vma in each anon_vma tree. Because we only take the lock on the root anon_vma, we cannot use the lock on higher-up anon_vmas to lock anything. This makes it impossible to do an indirect lookup of the root anon_vma, since the data structures could go away from under us. However, a direct pointer is safe because the root anon_vma is always the last one that gets freed on munmap or exit, by virtue of the same_vma list order and unlink_anon_vmas walking the list forward. Signed-off-by: Rik van Riel <riel(a)redhat.com> --- include/linux/rmap.h | 1 + mm/rmap.c | 20 +++++++++++++++++--- 2 files changed, 18 insertions(+), 3 deletions(-) diff --git a/include/linux/rmap.h b/include/linux/rmap.h index 72ecd87..457ae1e 100644 --- a/include/linux/rmap.h +++ b/include/linux/rmap.h @@ -26,6 +26,7 @@ */ struct anon_vma { spinlock_t lock; /* Serialize access to vma list */ + struct anon_vma *root; /* Root of this anon_vma tree */ #ifdef CONFIG_KSM atomic_t ksm_refcount; #endif diff --git a/mm/rmap.c b/mm/rmap.c index 6102f77..e34cb56 100644 --- a/mm/rmap.c +++ b/mm/rmap.c @@ -132,6 +132,11 @@ int anon_vma_prepare(struct vm_area_struct *vma) if (unlikely(!anon_vma)) goto out_enomem_free_avc; allocated = anon_vma; + /* + * This VMA had no anon_vma yet. This anon_vma is + * the root of any anon_vma tree that might form. + */ + anon_vma->root = anon_vma; } anon_vma_lock(anon_vma); @@ -203,7 +208,7 @@ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src) */ int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma) { - struct anon_vma_chain *avc; + struct anon_vma_chain *avc, *root_avc; struct anon_vma *anon_vma; /* Don't bother if the parent process has no anon_vma here. */ @@ -224,9 +229,18 @@ int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma) avc = anon_vma_chain_alloc(); if (!avc) goto out_error_free_anon_vma; - anon_vma_chain_link(vma, avc, anon_vma); + + /* + * Get the root anon_vma on the list by depending on the ordering + * of the same_vma list setup by previous invocations of anon_vma_fork. + * The root anon_vma will always be referenced by the last item + * in the anon_vma_chain list. + */ + root_avc = list_entry(vma->anon_vma_chain.prev, struct anon_vma_chain, same_vma); + anon_vma->root = root_avc->anon_vma; /* Mark this anon_vma as the one where our new (COWed) pages go. */ vma->anon_vma = anon_vma; + anon_vma_chain_link(vma, avc, anon_vma); return 0; @@ -261,7 +275,7 @@ void unlink_anon_vmas(struct vm_area_struct *vma) { struct anon_vma_chain *avc, *next; - /* Unlink each anon_vma chained to the VMA. */ + /* Unlink each anon_vma chained to the VMA, from newest to oldest. */ list_for_each_entry_safe(avc, next, &vma->anon_vma_chain, same_vma) { anon_vma_unlink(avc); list_del(&avc->same_vma); -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ |