linux-kernel - [PATCH v8 03/23] mm: Check against orig_pte for finish

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Message-Id: <20220405014836.14077-1-peterx@redhat.com>
Date:   Mon,  4 Apr 2022 21:48:36 -0400
From:   Peter Xu <peterx@...hat.com>
To:     linux-kernel@...r.kernel.org, linux-mm@...ck.org
Cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        David Hildenbrand <david@...hat.com>,
        Hugh Dickins <hughd@...gle.com>,
        Jerome Glisse <jglisse@...hat.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Alistair Popple <apopple@...dia.com>, peterx@...hat.com
Subject: [PATCH v8 03/23] mm: Check against orig_pte for finish_fault()

We used to check against none pte in finish_fault(), with the assumption
that the orig_pte is always none pte.

This change prepares us to be able to call do_fault() on !none ptes.  For
example, we should allow that to happen for pte marker so that we can restore
information out of the pte markers.

Let's change the "pte_none" check into detecting changes since we fetched
orig_pte.  One trivial thing to take care of here is, when pmd==NULL for
the pgtable we may not initialize orig_pte at all in handle_pte_fault().

By default orig_pte will be all zeros however the problem is not all
architectures are using all-zeros for a none pte.  pte_clear() will be the
right thing to use here so that we'll always have a valid orig_pte value
for the whole handle_pte_fault() call.

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

diff --git a/mm/memory.c b/mm/memory.c
index 3f396241a7db..b1af996b09ca 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4241,7 +4241,7 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
 				      vmf->address, &vmf->ptl);
 	ret = 0;
 	/* Re-check under ptl */
-	if (likely(pte_none(*vmf->pte)))
+	if (likely(pte_same(*vmf->pte, vmf->orig_pte)))
 		do_set_pte(vmf, page, vmf->address);
 	else
 		ret = VM_FAULT_NOPAGE;
@@ -4709,6 +4709,13 @@ static vm_fault_t handle_pte_fault(struct vm_fault *vmf)
 		 * concurrent faults and from rmap lookups.
 		 */
 		vmf->pte = NULL;
+		/*
+		 * Always initialize orig_pte.  This matches with below
+		 * code to have orig_pte to be the none pte if pte==NULL.
+		 * This makes the rest code to be always safe to reference
+		 * it, e.g. in finish_fault() we'll detect pte changes.
+		 */
+		pte_clear(vmf->vma->vm_mm, vmf->address, &vmf->orig_pte);
 	} else {
 		/*
 		 * If a huge pmd materialized under us just retry later.  Use
-- 
2.32.0