On Fri, Dec 20, 2024 at 08:06:48PM +0100, David Hildenbrand wrote:

On 20.12.24 20:01, David Hildenbrand wrote:

quoted

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.

Right.
 

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

As you have pointed out this was all inspired (ie. mostly copied)
from the existing insert_pfn() implementation which is used from
vmf_insert_mixed{_mkwrite}().

I agree a different interface to upgrade permissions would be nice. However
it's tricky because in general callers of these functions (eg. FS DAX) aren't
aware if the page is already mapped by a PTE/PMD. They only know a fault has
occured and the faulting permissions.

This wouldn't be impossible to fix - the mm does provide vm_ops->page_mkwrite()
for permission upgrades. The difficulty is that most filesystems that support
FS DAX (ie. ext4, XFS) don't treat a vm_ops->page_mkwrite() call any differently
from a vm_ops->fault() call due to write fault. Therefore the FS DAX code is
unaware of whether or not this is a permission upgrade or initial writeable
mapping of the page in the VMA.

A further issue in there is currently no vm_ops->huge_mkwrite() callback.

Obviously this could all be plumbed through the MM/FS layers, but that would
require a separate patch series. Given the current implementation has no issues
beyond the cosmetic I'd rather not delay this series any longer, especially as
the cosmetic defect is largely pre-existing (vmf_insert_mixed{_mkwrite}() could
have equally had a separate upgrade interface).

--

Cheers,

David / dhildenb