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@@ -320,8 +320,22 @@ void __init anon_vma_init(void)
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}
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/*
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- * Getting a lock on a stable anon_vma from a page off the LRU is
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- * tricky: page_lock_anon_vma rely on RCU to guard against the races.
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+ * Getting a lock on a stable anon_vma from a page off the LRU is tricky!
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+ *
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+ * Since there is no serialization what so ever against page_remove_rmap()
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+ * the best this function can do is return a locked anon_vma that might
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+ * have been relevant to this page.
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+ *
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+ * The page might have been remapped to a different anon_vma or the anon_vma
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+ * returned may already be freed (and even reused).
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+ *
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+ * All users of this function must be very careful when walking the anon_vma
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+ * chain and verify that the page in question is indeed mapped in it
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+ * [ something equivalent to page_mapped_in_vma() ].
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+ *
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+ * Since anon_vma's slab is DESTROY_BY_RCU and we know from page_remove_rmap()
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+ * that the anon_vma pointer from page->mapping is valid if there is a
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+ * mapcount, we can dereference the anon_vma after observing those.
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*/
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struct anon_vma *page_lock_anon_vma(struct page *page)
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{
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