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-rw-r--r--mm/huge_memory.c18
1 files changed, 17 insertions, 1 deletions
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 9a6bd6c8d55a..5f3ad65c85de 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -783,6 +783,12 @@ struct page *follow_devmap_pmd(struct vm_area_struct *vma, unsigned long addr,
assert_spin_locked(pmd_lockptr(mm, pmd));
+ /*
+ * When we COW a devmap PMD entry, we split it into PTEs, so we should
+ * not be in this function with `flags & FOLL_COW` set.
+ */
+ WARN_ONCE(flags & FOLL_COW, "mm: In follow_devmap_pmd with FOLL_COW set");
+
if (flags & FOLL_WRITE && !pmd_write(*pmd))
return NULL;
@@ -1128,6 +1134,16 @@ out_unlock:
return ret;
}
+/*
+ * FOLL_FORCE can write to even unwritable pmd's, but only
+ * after we've gone through a COW cycle and they are dirty.
+ */
+static inline bool can_follow_write_pmd(pmd_t pmd, unsigned int flags)
+{
+ return pmd_write(pmd) ||
+ ((flags & FOLL_FORCE) && (flags & FOLL_COW) && pmd_dirty(pmd));
+}
+
struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
unsigned long addr,
pmd_t *pmd,
@@ -1138,7 +1154,7 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
assert_spin_locked(pmd_lockptr(mm, pmd));
- if (flags & FOLL_WRITE && !pmd_write(*pmd))
+ if (flags & FOLL_WRITE && !can_follow_write_pmd(*pmd, flags))
goto out;
/* Avoid dumping huge zero page */