mm/gup: move gup_must_unshare() to mm/internal.h

This function is only used in gup.c and closely related.  It touches
FOLL_PIN so it must be moved before the next patch.

Link: https://lkml.kernel.org/r/12-v2-987e91b59705+36b-gup_tidy_jgg@nvidia.com
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Claudio Imbrenda <imbrenda@linux.ibm.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Mike Rapoport (IBM) <rppt@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/mm.h

@@ -3180,71 +3180,6 @@ static inline int vm_fault_to_errno(vm_fault_t vm_fault, int foll_flags)
 	return 0;
 }
 
-/*
- * Indicates for which pages that are write-protected in the page table,
- * whether GUP has to trigger unsharing via FAULT_FLAG_UNSHARE such that the
- * GUP pin will remain consistent with the pages mapped into the page tables
- * of the MM.
- *
- * Temporary unmapping of PageAnonExclusive() pages or clearing of
- * PageAnonExclusive() has to protect against concurrent GUP:
- * * Ordinary GUP: Using the PT lock
- * * GUP-fast and fork(): mm->write_protect_seq
- * * GUP-fast and KSM or temporary unmapping (swap, migration): see
- *    page_try_share_anon_rmap()
- *
- * Must be called with the (sub)page that's actually referenced via the
- * page table entry, which might not necessarily be the head page for a
- * PTE-mapped THP.
- *
- * If the vma is NULL, we're coming from the GUP-fast path and might have
- * to fallback to the slow path just to lookup the vma.
- */
-static inline bool gup_must_unshare(struct vm_area_struct *vma,
-				    unsigned int flags, struct page *page)
-{
-	/*
-	 * FOLL_WRITE is implicitly handled correctly as the page table entry
-	 * has to be writable -- and if it references (part of) an anonymous
-	 * folio, that part is required to be marked exclusive.
-	 */
-	if ((flags & (FOLL_WRITE | FOLL_PIN)) != FOLL_PIN)
-		return false;
-	/*
-	 * Note: PageAnon(page) is stable until the page is actually getting
-	 * freed.
-	 */
-	if (!PageAnon(page)) {
-		/*
-		 * We only care about R/O long-term pining: R/O short-term
-		 * pinning does not have the semantics to observe successive
-		 * changes through the process page tables.
-		 */
-		if (!(flags & FOLL_LONGTERM))
-			return false;
-
-		/* We really need the vma ... */
-		if (!vma)
-			return true;
-
-		/*
-		 * ... because we only care about writable private ("COW")
-		 * mappings where we have to break COW early.
-		 */
-		return is_cow_mapping(vma->vm_flags);
-	}
-
-	/* Paired with a memory barrier in page_try_share_anon_rmap(). */
-	if (IS_ENABLED(CONFIG_HAVE_FAST_GUP))
-		smp_rmb();
-
-	/*
-	 * Note that PageKsm() pages cannot be exclusive, and consequently,
-	 * cannot get pinned.
-	 */
-	return !PageAnonExclusive(page);
-}
-
 /*
  * Indicates whether GUP can follow a PROT_NONE mapped page, or whether
  * a (NUMA hinting) fault is required.

mm/internal.h

@@ -858,6 +858,71 @@ int migrate_device_coherent_page(struct page *page);
 struct folio *try_grab_folio(struct page *page, int refs, unsigned int flags);
 int __must_check try_grab_page(struct page *page, unsigned int flags);
 
+/*
+ * Indicates for which pages that are write-protected in the page table,
+ * whether GUP has to trigger unsharing via FAULT_FLAG_UNSHARE such that the
+ * GUP pin will remain consistent with the pages mapped into the page tables
+ * of the MM.
+ *
+ * Temporary unmapping of PageAnonExclusive() pages or clearing of
+ * PageAnonExclusive() has to protect against concurrent GUP:
+ * * Ordinary GUP: Using the PT lock
+ * * GUP-fast and fork(): mm->write_protect_seq
+ * * GUP-fast and KSM or temporary unmapping (swap, migration): see
+ *    page_try_share_anon_rmap()
+ *
+ * Must be called with the (sub)page that's actually referenced via the
+ * page table entry, which might not necessarily be the head page for a
+ * PTE-mapped THP.
+ *
+ * If the vma is NULL, we're coming from the GUP-fast path and might have
+ * to fallback to the slow path just to lookup the vma.
+ */
+static inline bool gup_must_unshare(struct vm_area_struct *vma,
+				    unsigned int flags, struct page *page)
+{
+	/*
+	 * FOLL_WRITE is implicitly handled correctly as the page table entry
+	 * has to be writable -- and if it references (part of) an anonymous
+	 * folio, that part is required to be marked exclusive.
+	 */
+	if ((flags & (FOLL_WRITE | FOLL_PIN)) != FOLL_PIN)
+		return false;
+	/*
+	 * Note: PageAnon(page) is stable until the page is actually getting
+	 * freed.
+	 */
+	if (!PageAnon(page)) {
+		/*
+		 * We only care about R/O long-term pining: R/O short-term
+		 * pinning does not have the semantics to observe successive
+		 * changes through the process page tables.
+		 */
+		if (!(flags & FOLL_LONGTERM))
+			return false;
+
+		/* We really need the vma ... */
+		if (!vma)
+			return true;
+
+		/*
+		 * ... because we only care about writable private ("COW")
+		 * mappings where we have to break COW early.
+		 */
+		return is_cow_mapping(vma->vm_flags);
+	}
+
+	/* Paired with a memory barrier in page_try_share_anon_rmap(). */
+	if (IS_ENABLED(CONFIG_HAVE_FAST_GUP))
+		smp_rmb();
+
+	/*
+	 * Note that PageKsm() pages cannot be exclusive, and consequently,
+	 * cannot get pinned.
+	 */
+	return !PageAnonExclusive(page);
+}
+
 extern bool mirrored_kernelcore;
 
 static inline bool vma_soft_dirty_enabled(struct vm_area_struct *vma)