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Date:   Sun, 21 May 2023 22:06:32 -0700 (PDT)
From:   Hugh Dickins <hughd@google.com>
To:     Andrew Morton <akpm@linux-foundation.org>
cc:     Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@kernel.org>,
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        Song Liu <song@kernel.org>,
        Thomas Hellstrom <thomas.hellstrom@linux.intel.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: [PATCH 14/31] fs/userfaultfd: retry if pte_offset_map() fails
In-Reply-To: <68a97fbe-5c1e-7ac6-72c-7b9c6290b370@google.com>
Message-ID: <424274a4-7c13-e14-b380-428fc69a45c5@google.com>
References: <68a97fbe-5c1e-7ac6-72c-7b9c6290b370@google.com>
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Precedence: bulk

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.

Signed-off-by: Hugh Dickins <hughd@google.com>
---
 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
-- 
2.35.3