- * PFNMAP is faster than MIXEDMAP due to reduced page
- * administration. So use MIXEDMAP only if private VMA, where
- * we need to support COW.
+ * We'd like to use VM_PFNMAP on shared mappings, where
+ * (vma->vm_flags & VM_SHARED) != 0, for performance reasons,
+ * but for some reason VM_PFNMAP + x86 PAT + write-combine is very
+ * bad for performance. Until that has been sorted out, use
+ * VM_MIXEDMAP on all mappings. See freedesktop.org bug #75719