linux-kernel - [PATCH v6 09/23] mm/shmem: Allows file-back mem to be uffd wr-protected on thps

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Message-Id: <20211115080048.74584-1-peterx@redhat.com>
Date:   Mon, 15 Nov 2021 16:00:48 +0800
From:   Peter Xu <peterx@...hat.com>
To:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Cc:     Nadav Amit <nadav.amit@...il.com>, peterx@...hat.com,
        Alistair Popple <apopple@...dia.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Matthew Wilcox <willy@...radead.org>,
        Jerome Glisse <jglisse@...hat.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        David Hildenbrand <david@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Hugh Dickins <hughd@...gle.com>
Subject: [PATCH v6 09/23] mm/shmem: Allows file-back mem to be uffd wr-protected on thps

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@...hat.com>
---
 mm/mprotect.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/mm/mprotect.c b/mm/mprotect.c
index be837c4dbc64..0d4bf755cee8 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -319,8 +319,15 @@ 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