linux-kernel - [PATCH 12/23] shmem/userfaultfd: Allows file-back mem to be uffd wr-protected on thps

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Date:   Mon, 22 Mar 2021 20:49:01 -0400
From:   Peter Xu <peterx@...hat.com>
To:     linux-kernel@...r.kernel.org, linux-mm@...ck.org
Cc:     "Kirill A . Shutemov" <kirill@...temov.name>,
        Jerome Glisse <jglisse@...hat.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Hugh Dickins <hughd@...gle.com>, peterx@...hat.com,
        Nadav Amit <nadav.amit@...il.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>
Subject: [PATCH 12/23] shmem/userfaultfd: Allows file-back mem to be uffd wr-protected on thps

We don't have "huge" version of PTE_SWP_UFFD_WP_SPECIAL, instead when necessary
we split the thp if the huge page is uffd wr-protected previously.

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 dropped in __split_huge_pmd_locked().

That is definitely 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.

Here we leverage the previously introduced change_protection_prepare() macro so
that we'll populate the pmd with a pgtable page.  Then change_pte_range() will
do all the rest for us, e.g., install the uffd-wp swap special pte marker at
any pte that we'd like to wr-protect, under the protection of pgtable lock.

Signed-off-by: Peter Xu <peterx@...hat.com>
---
 mm/mprotect.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/mm/mprotect.c b/mm/mprotect.c
index 6b63e3544b47..51c954afa406 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -296,8 +296,16 @@ 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 wr-protecting a file-backed memory range */
+			    unlikely(!vma_is_anonymous(vma) &&
+				     (cp_flags & MM_CP_UFFD_WP))) {
 				__split_huge_pmd(vma, pmd, addr, false, NULL);
+				/*
+				 * For file-backed, the pmd could have been
+				 * gone; still provide a pte pgtable if needed.
+				 */
+				change_protection_prepare(vma, pmd, addr, cp_flags);
 			} else {
 				int nr_ptes = change_huge_pmd(vma, pmd, addr,
 							      newprot, cp_flags);
-- 
2.26.2