From 2b683a4ff6ee091d5764ad867fbfae65f80cfefb Mon Sep 17 00:00:00 2001
From: Hugh Dickins <hughd@google.com>
Date: Thu, 8 Jun 2023 18:24:38 -0700
Subject: [PATCH] mm/userfaultfd: retry if pte_offset_map() fails
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Instead of worrying whether the pmd is stable, userfaultfd_must_wait()
call pte_offset_map() as before, but go back to try again if that fails.

Risk of endless loop?  It already broke out if pmd_none(), !pmd_present()
or pmd_trans_huge(), and pte_offset_map() would have cleared pmd_bad():
which leaves pmd_devmap().  Presumably pmd_devmap() is inappropriate in a
vma subject to userfaultfd (it would have been mistreated before), but add
a check just to avoid all possibility of endless loop there.

Link: https://lkml.kernel.org/r/54423f-3dff-fd8d-614a-632727cc4cfb@google.com
Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: Peter Xu <peterx@redhat.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: "Huang, Ying" <ying.huang@intel.com>
Cc: Ira Weiny <ira.weiny@intel.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Lorenzo Stoakes <lstoakes@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Naoya Horiguchi <naoya.horiguchi@nec.com>
Cc: Pavel Tatashin <pasha.tatashin@soleen.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Ralph Campbell <rcampbell@nvidia.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: SeongJae Park <sj@kernel.org>
Cc: Song Liu <song@kernel.org>
Cc: Steven Price <steven.price@arm.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Will Deacon <will@kernel.org>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Yu Zhao <yuzhao@google.com>
Cc: Zack Rusin <zackr@vmware.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---
 fs/userfaultfd.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index f7a0817b1ec0..ca83423f8d54 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -349,12 +349,13 @@ static inline bool userfaultfd_must_wait(struct userfaultfd_ctx *ctx,
 	if (!pud_present(*pud))
 		goto out;
 	pmd = pmd_offset(pud, address);
+again:
 	_pmd = pmdp_get_lockless(pmd);
 	if (pmd_none(_pmd))
 		goto out;
 
 	ret = false;
-	if (!pmd_present(_pmd))
+	if (!pmd_present(_pmd) || pmd_devmap(_pmd))
 		goto out;
 
 	if (pmd_trans_huge(_pmd)) {
@@ -363,11 +364,11 @@ static inline bool userfaultfd_must_wait(struct userfaultfd_ctx *ctx,
 		goto out;
 	}
 
-	/*
-	 * the pmd is stable (as in !pmd_trans_unstable) so we can re-read it
-	 * and use the standard pte_offset_map() instead of parsing _pmd.
-	 */
 	pte = pte_offset_map(pmd, address);
+	if (!pte) {
+		ret = true;
+		goto again;
+	}
 	/*
 	 * Lockless access: we're in a wait_event so it's ok if it
 	 * changes under us.  PTE markers should be handled the same as none