[PATCH v3 11/11] mm: rename vm_ops->find_special_page() to vm_ops->find_normal_page()

[David Hildenbrand <david@redhat.com>]

... and hide it behind a kconfig option. There is really no need for
any !xen code to perform this check.

The naming is a bit off: we want to find the "normal" page when a PTE
was marked "special". So it's really not "finding a special" page.

Improve the documentation, and add a comment in the code where XEN ends
up performing the pte_mkspecial() through a hypercall. More details can
be found in commit 923b2919e2c3 ("xen/gntdev: mark userspace PTEs as
special on x86 PV guests").

Cc: David Vrabel <david.vrabel@citrix.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
 drivers/xen/Kconfig              |  1 +
 drivers/xen/gntdev.c             |  5 +++--
 include/linux/mm.h               | 18 +++++++++++++-----
 mm/Kconfig                       |  2 ++
 mm/memory.c                      | 12 ++++++++++--
 tools/testing/vma/vma_internal.h | 18 +++++++++++++-----
 6 files changed, 42 insertions(+), 14 deletions(-)

diff --git a/drivers/xen/Kconfig b/drivers/xen/Kconfig
index 24f485827e039..f9a35ed266ecf 100644
--- a/drivers/xen/Kconfig
+++ b/drivers/xen/Kconfig
@@ -138,6 +138,7 @@ config XEN_GNTDEV
 	depends on XEN
 	default m
 	select MMU_NOTIFIER
+	select FIND_NORMAL_PAGE
 	help
 	  Allows userspace processes to use grants.
 
diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
index 1f21607656182..26f13b37c78e6 100644
--- a/drivers/xen/gntdev.c
+++ b/drivers/xen/gntdev.c
@@ -321,6 +321,7 @@ static int find_grant_ptes(pte_t *pte, unsigned long addr, void *data)
 	BUG_ON(pgnr >= map->count);
 	pte_maddr = arbitrary_virt_to_machine(pte).maddr;
 
+	/* Note: this will perform a pte_mkspecial() through the hypercall. */
 	gnttab_set_map_op(&map->map_ops[pgnr], pte_maddr, flags,
 			  map->grants[pgnr].ref,
 			  map->grants[pgnr].domid);
@@ -528,7 +529,7 @@ static void gntdev_vma_close(struct vm_area_struct *vma)
 	gntdev_put_map(priv, map);
 }
 
-static struct page *gntdev_vma_find_special_page(struct vm_area_struct *vma,
+static struct page *gntdev_vma_find_normal_page(struct vm_area_struct *vma,
 						 unsigned long addr)
 {
 	struct gntdev_grant_map *map = vma->vm_private_data;
@@ -539,7 +540,7 @@ static struct page *gntdev_vma_find_special_page(struct vm_area_struct *vma,
 static const struct vm_operations_struct gntdev_vmops = {
 	.open = gntdev_vma_open,
 	.close = gntdev_vma_close,
-	.find_special_page = gntdev_vma_find_special_page,
+	.find_normal_page = gntdev_vma_find_normal_page,
 };
 
 /* ------------------------------------------------------------------ */
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 8ca7d2fa71343..3868ca1a25f9c 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -657,13 +657,21 @@ struct vm_operations_struct {
 	struct mempolicy *(*get_policy)(struct vm_area_struct *vma,
 					unsigned long addr, pgoff_t *ilx);
 #endif
+#ifdef CONFIG_FIND_NORMAL_PAGE
 	/*
-	 * Called by vm_normal_page() for special PTEs to find the
-	 * page for @addr.  This is useful if the default behavior
-	 * (using pte_page()) would not find the correct page.
+	 * Called by vm_normal_page() for special PTEs in @vma at @addr. This
+	 * allows for returning a "normal" page from vm_normal_page() even
+	 * though the PTE indicates that the "struct page" either does not exist
+	 * or should not be touched: "special".
+	 *
+	 * Do not add new users: this really only works when a "normal" page
+	 * was mapped, but then the PTE got changed to something weird (+
+	 * marked special) that would not make pte_pfn() identify the originally
+	 * inserted page.
 	 */
-	struct page *(*find_special_page)(struct vm_area_struct *vma,
-					  unsigned long addr);
+	struct page *(*find_normal_page)(struct vm_area_struct *vma,
+					 unsigned long addr);
+#endif /* CONFIG_FIND_NORMAL_PAGE */
 };
 
 #ifdef CONFIG_NUMA_BALANCING
diff --git a/mm/Kconfig b/mm/Kconfig
index e443fe8cd6cf2..59a04d0b2e272 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -1381,6 +1381,8 @@ config PT_RECLAIM
 
 	  Note: now only empty user PTE page table pages will be reclaimed.
 
+config FIND_NORMAL_PAGE
+	def_bool n
 
 source "mm/damon/Kconfig"
 
diff --git a/mm/memory.c b/mm/memory.c
index 6f806bf3cc994..002c28795d8b7 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -639,6 +639,12 @@ static void print_bad_page_map(struct vm_area_struct *vma,
  * trivial. Secondly, an architecture may not have a spare page table
  * entry bit, which requires a more complicated scheme, described below.
  *
+ * With CONFIG_FIND_NORMAL_PAGE, we might have the "special" bit set on
+ * page table entries that actually map "normal" pages: however, that page
+ * cannot be looked up through the PFN stored in the page table entry, but
+ * instead will be looked up through vm_ops->find_normal_page(). So far, this
+ * only applies to PTEs.
+ *
  * A raw VM_PFNMAP mapping (ie. one that is not COWed) is always considered a
  * special mapping (even if there are underlying and valid "struct pages").
  * COWed pages of a VM_PFNMAP are always normal.
@@ -679,8 +685,10 @@ static inline struct page *__vm_normal_page(struct vm_area_struct *vma,
 {
 	if (IS_ENABLED(CONFIG_ARCH_HAS_PTE_SPECIAL)) {
 		if (unlikely(special)) {
-			if (vma->vm_ops && vma->vm_ops->find_special_page)
-				return vma->vm_ops->find_special_page(vma, addr);
+#ifdef CONFIG_FIND_NORMAL_PAGE
+			if (vma->vm_ops && vma->vm_ops->find_normal_page)
+				return vma->vm_ops->find_normal_page(vma, addr);
+#endif /* CONFIG_FIND_NORMAL_PAGE */
 			if (vma->vm_flags & (VM_PFNMAP | VM_MIXEDMAP))
 				return NULL;
 			if (is_zero_pfn(pfn) || is_huge_zero_pfn(pfn))
diff --git a/tools/testing/vma/vma_internal.h b/tools/testing/vma/vma_internal.h
index 3639aa8dd2b06..cb1c2a8afe265 100644
--- a/tools/testing/vma/vma_internal.h
+++ b/tools/testing/vma/vma_internal.h
@@ -467,13 +467,21 @@ struct vm_operations_struct {
 	struct mempolicy *(*get_policy)(struct vm_area_struct *vma,
 					unsigned long addr, pgoff_t *ilx);
 #endif
+#ifdef CONFIG_FIND_NORMAL_PAGE
 	/*
-	 * Called by vm_normal_page() for special PTEs to find the
-	 * page for @addr.  This is useful if the default behavior
-	 * (using pte_page()) would not find the correct page.
+	 * Called by vm_normal_page() for special PTEs in @vma at @addr. This
+	 * allows for returning a "normal" page from vm_normal_page() even
+	 * though the PTE indicates that the "struct page" either does not exist
+	 * or should not be touched: "special".
+	 *
+	 * Do not add new users: this really only works when a "normal" page
+	 * was mapped, but then the PTE got changed to something weird (+
+	 * marked special) that would not make pte_pfn() identify the originally
+	 * inserted page.
 	 */
-	struct page *(*find_special_page)(struct vm_area_struct *vma,
-					  unsigned long addr);
+	struct page *(*find_normal_page)(struct vm_area_struct *vma,
+					 unsigned long addr);
+#endif /* CONFIG_FIND_NORMAL_PAGE */
 };
 
 struct vm_unmapped_area_info {
-- 
2.50.1