[PATCH v7 09/23] mm/shmem: Allows file-back mem to be uffd wr-protected on thps

From: Peter Xu <peterx@redhat.com>
To: linux-mm@kvack.org, linux-kernel@vger.kernel.org
Cc: peterx@redhat.com, Nadav Amit <nadav.amit@gmail.com>,
	Hugh Dickins <hughd@google.com>,
	David Hildenbrand <david@redhat.com>,
	Axel Rasmussen <axelrasmussen@google.com>,
	Matthew Wilcox <willy@infradead.org>,
	Alistair Popple <apopple@nvidia.com>,
	Mike Rapoport <rppt@linux.vnet.ibm.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Jerome Glisse <jglisse@redhat.com>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	"Kirill A . Shutemov" <kirill@shutemov.name>,
	Andrea Arcangeli <aarcange@redhat.com>
Subject: [PATCH v7 09/23] mm/shmem: Allows file-back mem to be uffd wr-protected on thps
Date: Fri,  4 Mar 2022 13:16:54 +0800	[thread overview]
Message-ID: <20220304051708.86193-10-peterx@redhat.com> (raw)
In-Reply-To: <20220304051708.86193-1-peterx@redhat.com>

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 4878b6b99df9..95b307d4766d 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -329,8 +329,15 @@ 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

