[PATCH v3 06/13] mm: add remap_pfn_range_prepare(), remap_pfn_range_complete()

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

We need the ability to split PFN remap between updating the VMA and
performing the actual remap, in order to do away with the legacy
f_op->mmap hook.

To do so, update the PFN remap code to provide shared logic, and also make
remap_pfn_range_notrack() static, as its one user, io_mapping_map_user()
was removed in commit 9a4f90e24661 ("mm: remove mm/io-mapping.c").

Then, introduce remap_pfn_range_prepare(), which accepts VMA descriptor
and PFN parameters, and remap_pfn_range_complete() which accepts the same
parameters as remap_pfn_rangte().

remap_pfn_range_prepare() will set the cow vma->vm_pgoff if necessary, so
it must be supplied with a correct PFN to do so.  If the caller must hold
locks to be able to do this, those locks should be held across the
operation, and mmap_abort() should be provided to revoke the lock should
an error arise.

While we're here, also clean up the duplicated #ifdef
__HAVE_PFNMAP_TRACKING check and put into a single #ifdef/#else block.

We would prefer to define these functions in mm/internal.h, however we
will do the same for io_remap*() and these have arch defines that require
access to the remap functions.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
---
 include/linux/mm.h |  25 +++++++--
 mm/memory.c        | 128 ++++++++++++++++++++++++++++-----------------
 2 files changed, 102 insertions(+), 51 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index dd1fec5f028a..3277e035006d 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -489,6 +489,21 @@ extern unsigned int kobjsize(const void *objp);
  */
 #define VM_SPECIAL (VM_IO | VM_DONTEXPAND | VM_PFNMAP | VM_MIXEDMAP)
 
+/*
+ * Physically remapped pages are special. Tell the
+ * rest of the world about it:
+ *   VM_IO tells people not to look at these pages
+ *	(accesses can have side effects).
+ *   VM_PFNMAP tells the core MM that the base pages are just
+ *	raw PFN mappings, and do not have a "struct page" associated
+ *	with them.
+ *   VM_DONTEXPAND
+ *      Disable vma merging and expanding with mremap().
+ *   VM_DONTDUMP
+ *      Omit vma from core dump, even when VM_IO turned off.
+ */
+#define VM_REMAP_FLAGS (VM_IO | VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP)
+
 /* This mask prevents VMA from being scanned with khugepaged */
 #define VM_NO_KHUGEPAGED (VM_SPECIAL | VM_HUGETLB)
 
@@ -3622,10 +3637,12 @@ unsigned long change_prot_numa(struct vm_area_struct *vma,
 
 struct vm_area_struct *find_extend_vma_locked(struct mm_struct *,
 		unsigned long addr);
-int remap_pfn_range(struct vm_area_struct *, unsigned long addr,
-			unsigned long pfn, unsigned long size, pgprot_t);
-int remap_pfn_range_notrack(struct vm_area_struct *vma, unsigned long addr,
-		unsigned long pfn, unsigned long size, pgprot_t prot);
+int remap_pfn_range(struct vm_area_struct *vma, unsigned long addr,
+		    unsigned long pfn, unsigned long size, pgprot_t pgprot);
+void remap_pfn_range_prepare(struct vm_area_desc *desc, unsigned long pfn);
+int remap_pfn_range_complete(struct vm_area_struct *vma, unsigned long addr,
+		unsigned long pfn, unsigned long size, pgprot_t pgprot);
+
 int vm_insert_page(struct vm_area_struct *, unsigned long addr, struct page *);
 int vm_insert_pages(struct vm_area_struct *vma, unsigned long addr,
 			struct page **pages, unsigned long *num);
diff --git a/mm/memory.c b/mm/memory.c
index 41e641823558..4be4a9dc0fd8 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2900,8 +2900,27 @@ static inline int remap_p4d_range(struct mm_struct *mm, pgd_t *pgd,
 	return 0;
 }
 
+static int get_remap_pgoff(vm_flags_t vm_flags, unsigned long addr,
+		unsigned long end, unsigned long vm_start, unsigned long vm_end,
+		unsigned long pfn, pgoff_t *vm_pgoff_p)
+{
+	/*
+	 * There's a horrible special case to handle copy-on-write
+	 * behaviour that some programs depend on. We mark the "original"
+	 * un-COW'ed pages by matching them up with "vma->vm_pgoff".
+	 * See vm_normal_page() for details.
+	 */
+	if (is_cow_mapping(vm_flags)) {
+		if (addr != vm_start || end != vm_end)
+			return -EINVAL;
+		*vm_pgoff_p = pfn;
+	}
+
+	return 0;
+}
+
 static int remap_pfn_range_internal(struct vm_area_struct *vma, unsigned long addr,
-		unsigned long pfn, unsigned long size, pgprot_t prot)
+		unsigned long pfn, unsigned long size, pgprot_t prot, bool set_vma)
 {
 	pgd_t *pgd;
 	unsigned long next;
@@ -2912,32 +2931,17 @@ static int remap_pfn_range_internal(struct vm_area_struct *vma, unsigned long ad
 	if (WARN_ON_ONCE(!PAGE_ALIGNED(addr)))
 		return -EINVAL;
 
-	/*
-	 * Physically remapped pages are special. Tell the
-	 * rest of the world about it:
-	 *   VM_IO tells people not to look at these pages
-	 *	(accesses can have side effects).
-	 *   VM_PFNMAP tells the core MM that the base pages are just
-	 *	raw PFN mappings, and do not have a "struct page" associated
-	 *	with them.
-	 *   VM_DONTEXPAND
-	 *      Disable vma merging and expanding with mremap().
-	 *   VM_DONTDUMP
-	 *      Omit vma from core dump, even when VM_IO turned off.
-	 *
-	 * There's a horrible special case to handle copy-on-write
-	 * behaviour that some programs depend on. We mark the "original"
-	 * un-COW'ed pages by matching them up with "vma->vm_pgoff".
-	 * See vm_normal_page() for details.
-	 */
-	if (is_cow_mapping(vma->vm_flags)) {
-		if (addr != vma->vm_start || end != vma->vm_end)
-			return -EINVAL;
-		vma->vm_pgoff = pfn;
+	if (set_vma) {
+		err = get_remap_pgoff(vma->vm_flags, addr, end,
+				      vma->vm_start, vma->vm_end,
+				      pfn, &vma->vm_pgoff);
+		if (err)
+			return err;
+		vm_flags_set(vma, VM_REMAP_FLAGS);
+	} else {
+		VM_WARN_ON_ONCE((vma->vm_flags & VM_REMAP_FLAGS) != VM_REMAP_FLAGS);
 	}
 
-	vm_flags_set(vma, VM_IO | VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP);
-
 	BUG_ON(addr >= end);
 	pfn -= addr >> PAGE_SHIFT;
 	pgd = pgd_offset(mm, addr);
@@ -2957,11 +2961,10 @@ static int remap_pfn_range_internal(struct vm_area_struct *vma, unsigned long ad
  * Variant of remap_pfn_range that does not call track_pfn_remap.  The caller
  * must have pre-validated the caching bits of the pgprot_t.
  */
-int remap_pfn_range_notrack(struct vm_area_struct *vma, unsigned long addr,
-		unsigned long pfn, unsigned long size, pgprot_t prot)
+static int remap_pfn_range_notrack(struct vm_area_struct *vma, unsigned long addr,
+		unsigned long pfn, unsigned long size, pgprot_t prot, bool set_vma)
 {
-	int error = remap_pfn_range_internal(vma, addr, pfn, size, prot);
-
+	int error = remap_pfn_range_internal(vma, addr, pfn, size, prot, set_vma);
 	if (!error)
 		return 0;
 
@@ -2974,6 +2977,18 @@ int remap_pfn_range_notrack(struct vm_area_struct *vma, unsigned long addr,
 	return error;
 }
 
+void remap_pfn_range_prepare(struct vm_area_desc *desc, unsigned long pfn)
+{
+	/*
+	 * We set addr=VMA start, end=VMA end here, so this won't fail, but we
+	 * check it again on complete and will fail there if specified addr is
+	 * invalid.
+	 */
+	get_remap_pgoff(desc->vm_flags, desc->start, desc->end,
+			desc->start, desc->end, pfn, &desc->pgoff);
+	desc->vm_flags |= VM_REMAP_FLAGS;
+}
+
 #ifdef __HAVE_PFNMAP_TRACKING
 static inline struct pfnmap_track_ctx *pfnmap_track_ctx_alloc(unsigned long pfn,
 		unsigned long size, pgprot_t *prot)
@@ -3002,23 +3017,9 @@ void pfnmap_track_ctx_release(struct kref *ref)
 	pfnmap_untrack(ctx->pfn, ctx->size);
 	kfree(ctx);
 }
-#endif /* __HAVE_PFNMAP_TRACKING */
 
-/**
- * remap_pfn_range - remap kernel memory to userspace
- * @vma: user vma to map to
- * @addr: target page aligned user address to start at
- * @pfn: page frame number of kernel physical memory address
- * @size: size of mapping area
- * @prot: page protection flags for this mapping
- *
- * Note: this is only safe if the mm semaphore is held when called.
- *
- * Return: %0 on success, negative error code otherwise.
- */
-#ifdef __HAVE_PFNMAP_TRACKING
-int remap_pfn_range(struct vm_area_struct *vma, unsigned long addr,
-		    unsigned long pfn, unsigned long size, pgprot_t prot)
+static int remap_pfn_range_track(struct vm_area_struct *vma, unsigned long addr,
+		unsigned long pfn, unsigned long size, pgprot_t prot, bool set_vma)
 {
 	struct pfnmap_track_ctx *ctx = NULL;
 	int err;
@@ -3044,7 +3045,7 @@ int remap_pfn_range(struct vm_area_struct *vma, unsigned long addr,
 		return -EINVAL;
 	}
 
-	err = remap_pfn_range_notrack(vma, addr, pfn, size, prot);
+	err = remap_pfn_range_notrack(vma, addr, pfn, size, prot, set_vma);
 	if (ctx) {
 		if (err)
 			kref_put(&ctx->kref, pfnmap_track_ctx_release);
@@ -3054,11 +3055,44 @@ int remap_pfn_range(struct vm_area_struct *vma, unsigned long addr,
 	return err;
 }
 
+/**
+ * remap_pfn_range - remap kernel memory to userspace
+ * @vma: user vma to map to
+ * @addr: target page aligned user address to start at
+ * @pfn: page frame number of kernel physical memory address
+ * @size: size of mapping area
+ * @prot: page protection flags for this mapping
+ *
+ * Note: this is only safe if the mm semaphore is held when called.
+ *
+ * Return: %0 on success, negative error code otherwise.
+ */
+int remap_pfn_range(struct vm_area_struct *vma, unsigned long addr,
+		    unsigned long pfn, unsigned long size, pgprot_t prot)
+{
+	return remap_pfn_range_track(vma, addr, pfn, size, prot,
+				     /* set_vma = */true);
+}
+
+int remap_pfn_range_complete(struct vm_area_struct *vma, unsigned long addr,
+		unsigned long pfn, unsigned long size, pgprot_t prot)
+{
+	/* With set_vma = false, the VMA will not be modified. */
+	return remap_pfn_range_track(vma, addr, pfn, size, prot,
+				     /* set_vma = */false);
+}
 #else
 int remap_pfn_range(struct vm_area_struct *vma, unsigned long addr,
 		    unsigned long pfn, unsigned long size, pgprot_t prot)
 {
-	return remap_pfn_range_notrack(vma, addr, pfn, size, prot);
+	return remap_pfn_range_notrack(vma, addr, pfn, size, prot, /* set_vma = */true);
+}
+
+int remap_pfn_range_complete(struct vm_area_struct *vma, unsigned long addr,
+			     unsigned long pfn, unsigned long size, pgprot_t prot)
+{
+	return remap_pfn_range_notrack(vma, addr, pfn, size, prot,
+				       /* set_vma = */false);
 }
 #endif
 EXPORT_SYMBOL(remap_pfn_range);
-- 
2.51.0