Re: [PATCH] mm/khugepaged: don't map folios that are not uptodate in MADV_COLLAPSE

[Kiryl Shutsemau]

On Fri, Nov 06, 2026 at 01:29:31PM +0100, David Hildenbrand (Arm) wrote:
> > Separately from this fix: nothing on the mapping side catches this.
> > Every caller checks uptodate on its own (next_uptodate_folio(),
> > filemap_fault(), shmem_get_folio()), and khugepaged was the one that
> > went straight from filemap_lock_folio() to do_set_pmd().
> > 
> > Should do_set_pmd() and set_pte_range() assert it? Something like
> > 
> >         VM_WARN_ON_ONCE_FOLIO(folio->mapping && !folio_test_anon(folio) &&
> >                               !folio_test_uptodate(folio), folio);
> 
> I think so. But looking at these hard-to-trigger but in the end very nasty
> problems, should we actually reject the operation and do a pr_warn_once() or
> something?

Yes, rejecting sounds better.

Not in set_pte_range() as I suggested, though. Fault-around only gets
there with folios that next_uptodate_folio() checked under the folio
lock, and for CoW it maps the new anon folio after do_cow_fault() has
already copied from the page cache one.

Checking the folio ->fault() returned in finish_fault() covers PTE, PMD
and CoW; do_set_pmd() covers khugepaged. Both return VM_FAULT_SIGBUS.

Something like this, only compile-tested:

diff --git a/mm/memory.c b/mm/memory.c
index 6fe0b7dab927..ff2f2deff761 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -5674,6 +5674,22 @@ static vm_fault_t __do_fault(struct vm_fault *vmf)
        return ret;
 }

+/*
+ * A page cache folio that is not uptodate holds data that was never read in
+ * or cleared, and mapping it leaks stale memory to userspace. The callers are
+ * expected to check, so treat a hit as a kernel bug and refuse the mapping.
+ */
+static bool file_folio_not_uptodate(struct folio *folio)
+{
+       if (likely(folio_test_uptodate(folio)))
+               return false;
+       /* Driver pages from ->fault() never get PG_uptodate */
+       if (folio_test_anon(folio) || !folio->mapping)
+               return false;
+       WARN_ONCE(1, "refusing to map a page cache folio that is not uptodate\n");
+       return true;
+}
+
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
 static void deposit_prealloc_pte(struct vm_fault *vmf)
 {
@@ -5714,6 +5730,9 @@ vm_fault_t do_set_pmd(struct vm_fault *vmf, struct folio *folio, struct page *pa
                return ret;
        page = &folio->page;

+       if (file_folio_not_uptodate(folio))
+               return VM_FAULT_SIGBUS;
+
        /*
         * Just backoff if any subpage of a THP is corrupted otherwise
         * the corrupted page may mapped by PMD silently to escape the
@@ -5849,6 +5868,16 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
        unsigned long addr;
        bool needs_fallback = false;

+       /*
+        * Check the folio ->fault() returned. For CoW that is the source of the
+        * copy; DAX CoW has none.
+        */
+       if (vmf->page) {
+               folio = page_folio(vmf->page);
+               if (file_folio_not_uptodate(folio))
+                       return VM_FAULT_SIGBUS;
+       }
+
 fallback:
        addr = vmf->address;
-- 
  Kiryl Shutsemau / Kirill A. Shutemov