[PATCH v8 09/23] mm/shmem: Allows file-back mem to be uffd wr-protected on thps

Peter Xu
2022-04-05T02:42:06Z

We don't have "huge" version of pte markers, instead when necessary we split
the thp.

However split the thp is not enough, because file-backed thp is handled totally
differently comparing to anonymous thps: rather than doing a real split, the
thp pmd will simply got cleared in __split_huge_pmd_locked().

That is not enough if e.g. when there is a thp covers range [0, 2M) but we want
to wr-protect small page resides in [4K, 8K) range, because after
__split_huge_pmd() returns, there will be a none pmd, and change_pmd_range()
will just skip it right after the split.

Here we leverage the previously introduced change_pmd_prepare() macro so that
we'll populate the pmd with a pgtable page after the pmd split (in which
process the pmd will be cleared for cases like shmem). Then change_pte_range()
will do all the rest for us by installing the uffd-wp pte marker at any none
pte that we'd like to wr-protect.

Signed-off-by: Peter Xu <peterx@redhat.com>
---
mm/mprotect.c | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/mm/mprotect.c b/mm/mprotect.c
index bd62d5938c6c..e0a567b66d07 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -333,8 +333,15 @@ static inline unsigned long change_pmd_range(struct vm_area_struct *vma,
}

if (is_swap_pmd(*pmd) || pmd_trans_huge(*pmd) || pmd_devmap(*pmd)) {
- if (next - addr != HPAGE_PMD_SIZE) {
+ if ((next - addr != HPAGE_PMD_SIZE) ||
+ uffd_wp_protect_file(vma, cp_flags)) {
__split_huge_pmd(vma, pmd, addr, false, NULL);
+ /*
+ * For file-backed, the pmd could have been
+ * cleared; make sure pmd populated if
+ * necessary, then fall-through to pte level.
+ */
+ change_pmd_prepare(vma, pmd, cp_flags);
} else {
int nr_ptes = change_huge_pmd(vma, pmd, addr,
newprot, cp_flags);
--
2.32.0