linux-kernel - [PATCH next] hugetlb: pte_alloc_huge() to replace huge pte_alloc

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Date:   Tue, 18 Apr 2023 22:22:02 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <muchun.song@...ux.dev>,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-mm@...ck.org
Subject: [PATCH next] hugetlb: pte_alloc_huge() to replace huge
 pte_alloc_map()

Some architectures can have their hugetlb pages down at the lowest PTE
level: their huge_pte_alloc() using pte_alloc_map(), but without any
following pte_unmap().  Since none of these arches uses CONFIG_HIGHPTE,
this is not seen as a problem at present; but would become a problem if
forthcoming changes were to add an rcu_read_lock() into pte_offset_map(),
with the rcu_read_unlock() expected in pte_unmap().

Similarly in their huge_pte_offset(): pte_offset_kernel() is good enough
for that, but it's probably less confusing if we define pte_offset_huge()
along with pte_alloc_huge().  Only define them without CONFIG_HIGHPTE:
so there would be a build error to signal if ever more work is needed.

For ease of development, define these now for 6.4-rc1, ahead of any use:
then architectures can integrate patches using them, independent from mm.

Signed-off-by: Hugh Dickins <hughd@...gle.com>
---

 include/linux/hugetlb.h | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -183,6 +183,23 @@ extern struct list_head huge_boot_pages;

 /* arch callbacks */

+#ifndef CONFIG_HIGHPTE
+/*
+ * pte_offset_huge() and pte_alloc_huge() are helpers for those architectures
+ * which may go down to the lowest PTE level in their huge_pte_offset() and
+ * huge_pte_alloc(): to avoid reliance on pte_offset_map() without pte_unmap().
+ */
+static inline pte_t *pte_offset_huge(pmd_t *pmd, unsigned long address)
+{
+	return pte_offset_kernel(pmd, address);
+}
+static inline pte_t *pte_alloc_huge(struct mm_struct *mm, pmd_t *pmd,
+				    unsigned long address)
+{
+	return pte_alloc(mm, pmd) ? NULL : pte_offset_huge(pmd, address);
+}
+#endif
+
 pte_t *huge_pte_alloc(struct mm_struct *mm, struct vm_area_struct *vma,
 			unsigned long addr, unsigned long sz);
 /*