[PATCH RFC v4 04/12] mm/gup: let check_vma_flags() ignore selected VMA flags

[Rik van Riel <riel@surriel.com>]

check_vma_flags() rejects a VMA whose flags gup cannot handle. A caller
that wants to reach a COWed page in a VM_IO or VM_PFNMAP VMA, where that
page does have a struct page, cannot express the exception and has to
repeat the flag test itself.

Add an ignore_flags argument that check_vma_flags() clears from its local
copy of vma->vm_flags before the flag checks. A caller can drop a single
rejection this way while keeping every other check. All current callers
pass 0.

This prepares for get_user_page_vma(), which passes VM_IO | VM_PFNMAP to
reach those COWed pages.

No functional change intended.

Assisted-by: Claude:claude-opus-4.8
Signed-off-by: Rik van Riel <riel@surriel.com>
---
 mm/gup.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index 0692119b7904..dcece7b5253c 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1197,13 +1197,21 @@ static bool writable_file_mapping_allowed(struct vm_area_struct *vma,
 	return !vma_needs_dirty_tracking(vma);
 }
 
-static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
+static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags,
+			   vm_flags_t ignore_flags)
 {
 	vm_flags_t vm_flags = vma->vm_flags;
 	int write = (gup_flags & FOLL_WRITE);
 	int foreign = (gup_flags & FOLL_REMOTE);
 	bool vma_anon = vma_is_anonymous(vma);
 
+	/*
+	 * Opt out of the flag checks that read this local copy (the
+	 * VM_IO/VM_PFNMAP gate and the write/read/cow bits); checks that
+	 * re-read vma->vm_flags through helpers are unaffected.
+	 */
+	vm_flags &= ~ignore_flags;
+
 	if (vm_flags & (VM_IO | VM_PFNMAP))
 		return -EFAULT;
 
@@ -1387,7 +1395,7 @@ static long __get_user_pages(struct mm_struct *mm,
 					ret = -ENOMEM;
 					goto out;
 				}
-				if (check_vma_flags(vma, gup_flags)) {
+				if (check_vma_flags(vma, gup_flags, 0)) {
 					ret = -EINVAL;
 					goto out;
 				}
@@ -1408,7 +1416,7 @@ static long __get_user_pages(struct mm_struct *mm,
 				ret = -EFAULT;
 				goto out;
 			}
-			ret = check_vma_flags(vma, gup_flags);
+			ret = check_vma_flags(vma, gup_flags, 0);
 			if (ret)
 				goto out;
 		}
-- 
2.53.0-Meta