linux-kernel - [PATCH v4 08/14] mm/gup: Trigger break COW PTE before calling follow_pfn

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Message-Id: <20230207035139.272707-9-shiyn.lin@gmail.com>
Date:   Tue,  7 Feb 2023 11:51:33 +0800
From:   Chih-En Lin <shiyn.lin@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Qi Zheng <zhengqi.arch@...edance.com>,
        David Hildenbrand <david@...hat.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        John Hubbard <jhubbard@...dia.com>,
        Nadav Amit <namit@...are.com>, Barry Song <baohua@...nel.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
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        Ingo Molnar <mingo@...hat.com>,
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        Mark Rutland <mark.rutland@....com>,
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        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Andrei Vagin <avagin@...il.com>,
        Barret Rhoden <brho@...gle.com>,
        Michal Hocko <mhocko@...e.com>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Alexey Gladkov <legion@...nel.org>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-trace-kernel@...r.kernel.org,
        linux-perf-users@...r.kernel.org,
        Dinglan Peng <peng301@...due.edu>,
        Pedro Fonseca <pfonseca@...due.edu>,
        Jim Huang <jserv@...s.ncku.edu.tw>,
        Huichun Feng <foxhoundsk.tw@...il.com>,
        Chih-En Lin <shiyn.lin@...il.com>
Subject: [PATCH v4 08/14] mm/gup: Trigger break COW PTE before calling follow_pfn_pte()

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@...il.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