linux-kernel - Re: [PATCH v2 05/10] mm/hugetlb: Make userfaultfd_huge_must

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Message-ID: <3b793737-2575-84c5-962e-6acfa529c8a0@nvidia.com>
Date:   Wed, 7 Dec 2022 15:54:34 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Peter Xu <peterx@...hat.com>
CC:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        Muchun Song <songmuchun@...edance.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "James Houghton" <jthoughton@...gle.com>,
        Jann Horn <jannh@...gle.com>, Rik van Riel <riel@...riel.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        "David Hildenbrand" <david@...hat.com>,
        Nadav Amit <nadav.amit@...il.com>
Subject: Re: [PATCH v2 05/10] mm/hugetlb: Make userfaultfd_huge_must_wait()
 safe to pmd unshare

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@...dia.com>

thanks,
-- 
John Hubbard
NVIDIA