linux-kernel - [PATCH v3 29/35] mm: prevent userfaults to be handled under per-vma lock

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Message-ID: <20230216051750.3125598-30-surenb@google.com>
Date:   Wed, 15 Feb 2023 21:17:44 -0800
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     akpm@...ux-foundation.org
Cc:     michel@...pinasse.org, jglisse@...gle.com, mhocko@...e.com,
        vbabka@...e.cz, hannes@...xchg.org, mgorman@...hsingularity.net,
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        peterz@...radead.org, ldufour@...ux.ibm.com, paulmck@...nel.org,
        mingo@...hat.com, will@...nel.org, luto@...nel.org,
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        soheil@...gle.com, leewalsh@...gle.com, posk@...gle.com,
        michalechner92@...glemail.com, linux-mm@...ck.org,
        linux-arm-kernel@...ts.infradead.org,
        linuxppc-dev@...ts.ozlabs.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, kernel-team@...roid.com,
        Suren Baghdasaryan <surenb@...gle.com>
Subject: [PATCH v3 29/35] mm: prevent userfaults to be handled under per-vma lock

Due to the possibility of handle_userfault dropping mmap_lock, avoid fault
handling under VMA lock and retry holding mmap_lock. This can be handled
more gracefully in the future.

Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
Suggested-by: Peter Xu <peterx@...hat.com>
---
 mm/memory.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/mm/memory.c b/mm/memory.c
index 555612d153ad..751aebc1b29f 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -5254,6 +5254,15 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
 	if (!vma_start_read(vma))
 		goto inval;
 
+	/*
+	 * Due to the possibility of userfault handler dropping mmap_lock, avoid
+	 * it for now and fall back to page fault handling under mmap_lock.
+	 */
+	if (userfaultfd_armed(vma)) {
+		vma_end_read(vma);
+		goto inval;
+	}
+
 	/* Check since vm_start/vm_end might change before we lock the VMA */
 	if (unlikely(address < vma->vm_start || address >= vma->vm_end)) {
 		vma_end_read(vma);
-- 
2.39.1