Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Content-Type: text/plain;
        charset=us-ascii
Mime-Version: 1.0 (Mac OS X Mail 16.0 \(3696.120.41.1.1\))
Subject: Re: [PATCH RFC 02/10] mm/hugetlb: Comment huge_pte_offset() for its
 locking requirements
From:   Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <20221030212929.335473-3-peterx@redhat.com>
Date:   Mon, 31 Oct 2022 22:46:52 -0700
Cc:     kernel list <linux-kernel@vger.kernel.org>, linux-mm@kvack.org,
        Andrew Morton <akpm@linux-foundation.org>,
        James Houghton <jthoughton@google.com>,
        Miaohe Lin <linmiaohe@huawei.com>,
        David Hildenbrand <david@redhat.com>,
        Muchun Song <songmuchun@bytedance.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Rik van Riel <riel@surriel.com>
Content-Transfer-Encoding: 7bit
Message-Id: <F09923E5-605F-476E-976F-C7EABDF64938@gmail.com>
References: <20221030212929.335473-1-peterx@redhat.com>
 <20221030212929.335473-3-peterx@redhat.com>
To:     Peter Xu <peterx@redhat.com>
Precedence: bulk

On Oct 30, 2022, at 2:29 PM, Peter Xu <peterx@redhat.com> wrote:

> huge_pte_offset() is potentially a pgtable walker, looking up pte_t* for a
> hugetlb address.
> 
> Normally, it's always safe to walk the pgtable as long as we're with the
> mmap lock held for either read or write, because that guarantees the
> pgtable pages will always be valid during the process.
> 
> But it's not true for hugetlbfs: hugetlbfs has the pmd sharing feature, it
> means that even with mmap lock held, the PUD pgtable page can still go away
> from under us if pmd unsharing is possible during the walk.
> 
> It's not always the case, e.g.:
> 
>  (1) If the mapping is private we're not prone to pmd sharing or
>      unsharing, so it's okay.
> 
>  (2) If we're with the hugetlb vma lock held for either read/write, it's
>      okay too because pmd unshare cannot happen at all.
> 
> Document all these explicitly for huge_pte_offset(), because it's really
> not that obvious.  This also tells all the callers on what it needs to
> guarantee huge_pte_offset() thread-safety.
> 
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
> arch/arm64/mm/hugetlbpage.c | 32 ++++++++++++++++++++++++++++++++

Please excuse my ignorant question - is there something specific for arm64
code here? Other archs seem to have similar code, no?