[PATCH 39/39] mm/vma: introduce and use vma[_flags]_can_gup()

[Lorenzo Stoakes (ARM)]

GUP cannot be used for VMAs which set VMA_IO_BIT - because memory-mapped
I/O must not be accessed on the user's behalf - or VMA_PFNMAP_BIT - because
PFN maps have no folios which the kernel is permitted to access.

Rather than keeping these checks open-coded, abstract them to
vma_flags_can_gup() and its VMA wrapper vma_can_gup().

This is useful as there are a number of additional places within the kernel
that need to check whether a mapping can be accessed via GUP.

Therefore, update all such occurrences.

While here, drop a reference to 'special' and replace a use of the
deprecated VMA flags API in vma_dump_size().

No functional change intended.

Signed-off-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>
---
 fs/coredump.c      |  4 ++--
 include/linux/mm.h | 29 +++++++++++++++++++++++++++++
 mm/gup.c           |  7 +++----
 mm/hmm.c           |  3 +--
 mm/memory.c        | 14 ++++++++------
 mm/mempolicy.c     |  3 ++-
 6 files changed, 45 insertions(+), 15 deletions(-)

diff --git a/fs/coredump.c b/fs/coredump.c
index fb21fb6703dd..9f729c594c47 100644
--- a/fs/coredump.c
+++ b/fs/coredump.c
@@ -1616,8 +1616,8 @@ static unsigned long vma_dump_size(struct vm_area_struct *vma,
 		return 0;
 	}
 
-	/* Do not dump I/O mapped devices or special mappings */
-	if (vma->vm_flags & VM_IO)
+	/* Do not dump memory-mapped I/O, which may have side effects on read. */
+	if (vma_test(vma, VMA_IO_BIT))
 		return 0;
 
 	/* By default, dump shared memory if mapped from an anonymous file. */
diff --git a/include/linux/mm.h b/include/linux/mm.h
index b5784685272a..1902d4c77481 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1777,6 +1777,35 @@ static inline bool vma_is_persistent(const struct vm_area_struct *vma)
 	return vma_flags_is_persistent(&vma->flags);
 }
 
+/**
+ * vma_flags_can_gup() - Do the specified VMA flags permit GUP to access the
+ * mapping's pages?
+ * @flags: The VMA flags to test.
+ *
+ * GUP cannot access pages belonging to mappings whose pages are not permitted
+ * to be accessed (VMA_PFNMAP_BIT) and must not manipulate or provide access to
+ * memory-mapped I/O ranges to users (VMA_IO_BIT).
+ *
+ * Returns: true if GUP may access pages from the mapping, otherwise false.
+ */
+static inline bool vma_flags_can_gup(const vma_flags_t *flags)
+{
+	return !vma_flags_test_any(flags, VMA_IO_BIT, VMA_PFNMAP_BIT);
+}
+
+/**
+ * vma_can_gup() - May GUP obtain pages from @vma?
+ * @vma: The VMA to test.
+ *
+ * See vma_flags_can_gup() for details.
+ *
+ * Returns: true if GUP may access pages from the mapping, otherwise false.
+ */
+static inline bool vma_can_gup(const struct vm_area_struct *vma)
+{
+	return vma_flags_can_gup(&vma->flags);
+}
+
 /**
  * vma_kernel_pagesize - Default page size granularity for this VMA.
  * @vma: The user mapping.
diff --git a/mm/gup.c b/mm/gup.c
index 66b306911703..f4d0cfcb602b 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1204,7 +1204,7 @@ static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
 	int foreign = (gup_flags & FOLL_REMOTE);
 	bool vma_anon = vma_is_anonymous(vma);
 
-	if (vm_flags & (VM_IO | VM_PFNMAP))
+	if (!vma_can_gup(vma))
 		return -EFAULT;
 
 	if ((gup_flags & FOLL_ANON) && !vma_anon)
@@ -1955,7 +1955,7 @@ int __mm_populate(unsigned long start, unsigned long len, int ignore_errors)
 		 * range with the first VMA. Also, skip undesirable VMA types.
 		 */
 		nend = min(end, vma->vm_end);
-		if (vma->vm_flags & (VM_IO | VM_PFNMAP))
+		if (!vma_can_gup(vma))
 			continue;
 		if (nstart < vma->vm_start)
 			nstart = vma->vm_start;
@@ -2017,8 +2017,7 @@ static long __get_user_pages_locked(struct mm_struct *mm, unsigned long start,
 			break;
 
 		/* protect what we can, including chardevs */
-		if ((vma->vm_flags & (VM_IO | VM_PFNMAP)) ||
-		    !(vm_flags & vma->vm_flags))
+		if (!vma_can_gup(vma) || !(vm_flags & vma->vm_flags))
 			break;
 
 		if (pages) {
diff --git a/mm/hmm.c b/mm/hmm.c
index 2f1e98c6b644..e9569b82a1f0 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -595,8 +595,7 @@ static int hmm_vma_walk_test(unsigned long start, unsigned long end,
 	struct hmm_range *range = hmm_vma_walk->range;
 	struct vm_area_struct *vma = walk->vma;
 
-	if (!(vma->vm_flags & (VM_IO | VM_PFNMAP)) &&
-	    vma->vm_flags & VM_READ)
+	if (vma_can_gup(vma) && vma_test(vma, VMA_READ_BIT))
 		return 0;
 
 	/*
diff --git a/mm/memory.c b/mm/memory.c
index 9a38c7d4cc40..cb56d67b17ca 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2417,11 +2417,11 @@ static bool vm_mixed_zeropage_allowed(struct vm_area_struct *vma)
 	 * be problematic as soon as the zeropage gets replaced by a different
 	 * page due to vma->vm_ops->pfn_mkwrite, because what's mapped would
 	 * now differ to what GUP looked up. FSDAX is incompatible to
-	 * FOLL_LONGTERM and VM_IO is incompatible to GUP completely (see
-	 * check_vma_flags).
+	 * FOLL_LONGTERM and memory-mapped I/O is incompatible to GUP completely
+	 * (see vma_can_gup()).
 	 */
 	return vma->vm_ops && vma->vm_ops->pfn_mkwrite &&
-	       (vma_is_fsdax(vma) || vma->vm_flags & VM_IO);
+	       (vma_is_fsdax(vma) || vma_test(vma, VMA_IO_BIT));
 }
 
 static int validate_page_before_insert(struct vm_area_struct *vma,
@@ -7116,7 +7116,8 @@ int follow_pfnmap_start(struct follow_pfnmap_args *args)
 	if (unlikely(address < vma->vm_start || address >= vma->vm_end))
 		goto out;
 
-	if (!(vma->vm_flags & (VM_IO | VM_PFNMAP)))
+	/* Only mappings GUP cannot handle are followed here. */
+	if (vma_can_gup(vma))
 		goto out;
 retry:
 	pgdp = pgd_offset(mm, address);
@@ -7310,8 +7311,9 @@ static int __access_remote_vm(struct mm_struct *mm, unsigned long addr,
 			}
 
 			/*
-			 * Check if this is a VM_IO | VM_PFNMAP VMA, which
-			 * we can access using slightly different code.
+			 * GUP failed, perhaps because this is a mapping it
+			 * cannot handle (see vma_can_gup()) - such mappings may
+			 * provide access via vm_ops->access() instead.
 			 */
 			bytes = 0;
 #ifdef CONFIG_HAVE_IOREMAP_PROT
diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index aeb99c5933cb..ed444061631c 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -2011,7 +2011,8 @@ SYSCALL_DEFINE5(get_mempolicy, int __user *, policy,
 
 bool vma_migratable(struct vm_area_struct *vma)
 {
-	if (vma->vm_flags & (VM_IO | VM_PFNMAP))
+	/* Pages which GUP cannot obtain cannot be migrated either. */
+	if (!vma_can_gup(vma))
 		return false;
 
 	/*

-- 
2.55.0