linux-kernel - [PATCH 2/2] userfaultfd: Don't BUG_ON() if khugepaged yanks our page table

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Message-Id: <20240812-uffd-thp-flip-fix-v1-2-4fc1db7ccdd0@google.com>
Date: Mon, 12 Aug 2024 18:42:17 +0200
From: Jann Horn <jannh@...gle.com>
To: Andrew Morton <akpm@...ux-foundation.org>, 
 Pavel Emelyanov <xemul@...allels.com>, 
 Andrea Arcangeli <aarcange@...hat.com>, Hugh Dickins <hughd@...gle.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
 Jann Horn <jannh@...gle.com>, stable@...r.kernel.org
Subject: [PATCH 2/2] userfaultfd: Don't BUG_ON() if khugepaged yanks our
 page table

Since khugepaged was changed to allow retracting page tables in file
mappings without holding the mmap lock, these BUG_ON()s are wrong - get rid
of them.

We could also remove the preceding "if (unlikely(...))" block, but then
we could reach pte_offset_map_lock() with transhuge pages not just for file
mappings but also for anonymous mappings - which would probably be fine but
I think is not necessarily expected.

Cc: stable@...r.kernel.org
Fixes: 1d65b771bc08 ("mm/khugepaged: retract_page_tables() without mmap or vma lock")
Signed-off-by: Jann Horn <jannh@...gle.com>
---
 mm/userfaultfd.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index ec3750467aa5..0dfa97db6feb 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -806,9 +806,10 @@ static __always_inline ssize_t mfill_atomic(struct userfaultfd_ctx *ctx,
 			err = -EFAULT;
 			break;
 		}
-
-		BUG_ON(pmd_none(*dst_pmd));
-		BUG_ON(pmd_trans_huge(*dst_pmd));
+		/*
+		 * For shmem mappings, khugepaged is allowed to remove page
+		 * tables under us; pte_offset_map_lock() will deal with that.
+		 */
 
 		err = mfill_atomic_pte(dst_pmd, dst_vma, dst_addr,
 				       src_addr, flags, &folio);

-- 
2.46.0.76.ge559c4bf1a-goog