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Message-ID: <3b793737-2575-84c5-962e-6acfa529c8a0@nvidia.com>
Date:   Wed, 7 Dec 2022 15:54:34 -0800
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Subject: Re: [PATCH v2 05/10] mm/hugetlb: Make userfaultfd_huge_must_wait()
 safe to pmd unshare
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To:     Peter Xu <peterx@redhat.com>
CC:     <linux-mm@kvack.org>, <linux-kernel@vger.kernel.org>,
        Muchun Song <songmuchun@bytedance.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        "James Houghton" <jthoughton@google.com>,
        Jann Horn <jannh@google.com>, Rik van Riel <riel@surriel.com>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        "David Hildenbrand" <david@redhat.com>,
        Nadav Amit <nadav.amit@gmail.com>
References: <20221207203034.650899-1-peterx@redhat.com>
 <20221207203034.650899-6-peterx@redhat.com>
 <b3308387-464a-52ae-114b-34ab94e3b5c6@nvidia.com> <Y5ElX/Flm7lrHmCx@x1n>
From:   John Hubbard <jhubbard@nvidia.com>
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On 12/7/22 15:44, Peter Xu wrote:
> The part that matters in the comment is "need to be before setting current
> state".
> 
> 	blocking_state = userfaultfd_get_blocking_state(vmf->flags);
> 	if (is_vm_hugetlb_page(vma))
> 		hugetlb_vma_lock_read(vma);
> 	set_current_state(blocking_state);
> 
> down_read() can sleep and also modify the task state, we cannot take the
> lock after that point because otherwise the task state will be messed up.
> 

OK, how about:

	/*
	 * Take the vma lock now, in order to safely call
	 * userfaultfd_huge_must_wait() a little bit later. Because acquiring
	 * the (sleepable) vma lock potentially modifies the current task state,
	 * that must be before explicitly calling set_current_state().
	 */

Other than that, the patch looks good, so:

Reviewed-by: John Hubbard <jhubbard@nvidia.com>

thanks,
-- 
John Hubbard
NVIDIA