[PATCH 20 of 32] add pmd_huge_pte to mm_struct

[Andrea Arcangeli <aarcange@redhat.com>]

From: Andrea Arcangeli <aarcange@redhat.com>

This increase the size of the mm struct a bit but it is needed to preallocate
one pte for each hugepage so that split_huge_page will not require a fail path.
Guarantee of success is a fundamental property of split_huge_page to avoid
decrasing swapping reliability and to avoid adding -ENOMEM fail paths that
would otherwise force the hugepage-unaware VM code to learn rolling back in the
middle of its pte mangling operations (if something we need it to learn
handling pmd_trans_huge natively rather being capable of rollback). When
split_huge_page runs a pte is needed to succeed the split, to map the newly
splitted regular pages with a regular pte.  This way all existing VM code
remains backwards compatible by just adding a split_huge_page* one liner. The
memory waste of those preallocated ptes is negligible and so it is worth it.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Acked-by: Rik van Riel <riel@redhat.com>
---

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -291,6 +291,9 @@ struct mm_struct {
 #ifdef CONFIG_MMU_NOTIFIER
 	struct mmu_notifier_mm *mmu_notifier_mm;
 #endif
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+	pgtable_t pmd_huge_pte; /* protected by page_table_lock */
+#endif
 };
 
 /* Future-safe accessor for struct mm_struct's cpu_vm_mask. */
diff --git a/kernel/fork.c b/kernel/fork.c
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -498,6 +498,9 @@ void __mmdrop(struct mm_struct *mm)
 	mm_free_pgd(mm);
 	destroy_context(mm);
 	mmu_notifier_mm_destroy(mm);
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+	VM_BUG_ON(mm->pmd_huge_pte);
+#endif
 	free_mm(mm);
 }
 EXPORT_SYMBOL_GPL(__mmdrop);
@@ -638,6 +641,10 @@ struct mm_struct *dup_mm(struct task_str
 	mm->token_priority = 0;
 	mm->last_interval = 0;
 
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+	mm->pmd_huge_pte = NULL;
+#endif
+
 	if (!mm_init(mm, tsk))
 		goto fail_nomem;
 

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