On 20.12.24 20:01, David Hildenbrand wrote:

On 17.12.24 06:12, Alistair Popple wrote:

quoted

In preparation for using insert_page() for DAX, enhance
insert_page_into_pte_locked() to handle establishing writable
mappings.  Recall that DAX returns VM_FAULT_NOPAGE after installing a
PTE which bypasses the typical set_pte_range() in finish_fault.

Signed-off-by: Alistair Popple <apopple@nvidia.com>
Suggested-by: Dan Williams <redacted>

---

Changes since v2:

   - New patch split out from "mm/memory: Add dax_insert_pfn"
---
   mm/memory.c | 45 +++++++++++++++++++++++++++++++++++++--------
   1 file changed, 37 insertions(+), 8 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 06bb29e..cd82952 100644
--- a/mm/memory.c
+++ b/mm/memory.c

@@ -2126,19 +2126,47 @@ static int validate_page_before_insert(struct vm_area_struct *vma,
   }
   
   static int insert_page_into_pte_locked(struct vm_area_struct *vma, pte_t *pte,
-			unsigned long addr, struct page *page, pgprot_t prot)
+				unsigned long addr, struct page *page,
+				pgprot_t prot, bool mkwrite)
   {
   	struct folio *folio = page_folio(page);
+	pte_t entry = ptep_get(pte);
   	pte_t pteval;
   
-	if (!pte_none(ptep_get(pte)))
-		return -EBUSY;
+	if (!pte_none(entry)) {
+		if (!mkwrite)
+			return -EBUSY;
+
+		/*
+		 * For read faults on private mappings the PFN passed in may not
+		 * match the PFN we have mapped if the mapped PFN is a writeable
+		 * COW page.  In the mkwrite case we are creating a writable PTE
+		 * for a shared mapping and we expect the PFNs to match. If they
+		 * don't match, we are likely racing with block allocation and
+		 * mapping invalidation so just skip the update.
+		 */

Would it make sense to instead have here

/* See insert_pfn(). */

But ...

quoted

+		if (pte_pfn(entry) != page_to_pfn(page)) {
+			WARN_ON_ONCE(!is_zero_pfn(pte_pfn(entry)));
+			return -EFAULT;
+		}
+		entry = maybe_mkwrite(entry, vma);
+		entry = pte_mkyoung(entry);
+		if (ptep_set_access_flags(vma, addr, pte, entry, 1))
+			update_mmu_cache(vma, addr, pte);

... I am not sure if we want the above at all. Someone inserted a page,
which is refcounted + mapcounted already.

Now you ignore that and do like the second insertion "worked" ?

No, that feels wrong, I suspect you will run into refcount+mapcount issues.

If there is already something, inserting must fail IMHO. If you want to
change something to upgrade write permissions, then a different
interface should be used.

Ah, now I realize that the early exit saves you because we won't adjust 
the refcount +mapcount.

I still wonder if that really belongs in here, I would prefer to not 
play such tricks to upgrade write permissions if possible.

-- 
Cheers,

David / dhildenb