From:   Chih-En Lin <shiyn.lin@gmail.com>
To:     Andrew Morton <akpm@linux-foundation.org>,
        Qi Zheng <zhengqi.arch@bytedance.com>,
        David Hildenbrand <david@redhat.com>,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
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        Nadav Amit <namit@vmware.com>, Barry Song <baohua@kernel.org>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
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        "Eric W. Biederman" <ebiederm@xmission.com>,
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        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        "Liam R. Howlett" <Liam.Howlett@Oracle.com>,
        Fenghua Yu <fenghua.yu@intel.com>,
        Andrei Vagin <avagin@gmail.com>,
        Barret Rhoden <brho@google.com>,
        Michal Hocko <mhocko@suse.com>,
        "Jason A. Donenfeld" <Jason@zx2c4.com>,
        Alexey Gladkov <legion@kernel.org>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-mm@kvack.org, linux-trace-kernel@vger.kernel.org,
        linux-perf-users@vger.kernel.org,
        Dinglan Peng <peng301@purdue.edu>,
        Pedro Fonseca <pfonseca@purdue.edu>,
        Jim Huang <jserv@ccns.ncku.edu.tw>,
        Huichun Feng <foxhoundsk.tw@gmail.com>,
        Chih-En Lin <shiyn.lin@gmail.com>
Subject: [PATCH v4 08/14] mm/gup: Trigger break COW PTE before calling follow_pfn_pte()
Date:   Tue,  7 Feb 2023 11:51:33 +0800
Message-Id: <20230207035139.272707-9-shiyn.lin@gmail.com>
In-Reply-To: <20230207035139.272707-1-shiyn.lin@gmail.com>
References: <20230207035139.272707-1-shiyn.lin@gmail.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

In most of cases, GUP will not modify the page table, excluding
follow_pfn_pte(). To deal with COW PTE, Trigger the break COW
PTE fault before calling follow_pfn_pte().

Signed-off-by: Chih-En Lin <shiyn.lin@gmail.com>
---
 mm/gup.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/mm/gup.c b/mm/gup.c
index f45a3a5be53a..e702c0800105 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -545,7 +545,8 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
 	if (WARN_ON_ONCE((flags & (FOLL_PIN | FOLL_GET)) ==
 			 (FOLL_PIN | FOLL_GET)))
 		return ERR_PTR(-EINVAL);
-	if (unlikely(pmd_bad(*pmd)))
+	/* COW-ed PTE has write protection which can trigger pmd_bad(). */
+	if (unlikely(pmd_write(*pmd) && pmd_bad(*pmd)))
 		return no_page_table(vma, flags);
 
 	ptep = pte_offset_map_lock(mm, pmd, address, &ptl);
@@ -588,6 +589,11 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
 		if (is_zero_pfn(pte_pfn(pte))) {
 			page = pte_page(pte);
 		} else {
+			if (test_bit(MMF_COW_PTE, &mm->flags) &&
+			    !pmd_write(*pmd)) {
+				page = ERR_PTR(-EMLINK);
+				goto out;
+			}
 			ret = follow_pfn_pte(vma, address, ptep, flags);
 			page = ERR_PTR(ret);
 			goto out;
-- 
2.34.1