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;
Date:   Tue, 23 May 2023 18:57:17 -0700 (PDT)
From:   Hugh Dickins <hughd@google.com>
To:     Alistair Popple <apopple@nvidia.com>
cc:     Hugh Dickins <hughd@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@kernel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Matthew Wilcox <willy@infradead.org>,
        David Hildenbrand <david@redhat.com>,
        Suren Baghdasaryan <surenb@google.com>,
        Qi Zheng <zhengqi.arch@bytedance.com>,
        Yang Shi <shy828301@gmail.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Peter Xu <peterx@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Will Deacon <will@kernel.org>, Yu Zhao <yuzhao@google.com>,
        Ralph Campbell <rcampbell@nvidia.com>,
        Ira Weiny <ira.weiny@intel.com>,
        Steven Price <steven.price@arm.com>,
        SeongJae Park <sj@kernel.org>,
        Naoya Horiguchi <naoya.horiguchi@nec.com>,
        Christophe Leroy <christophe.leroy@csgroup.eu>,
        Zack Rusin <zackr@vmware.com>, Jason Gunthorpe <jgg@ziepe.ca>,
        Axel Rasmussen <axelrasmussen@google.com>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        Pasha Tatashin <pasha.tatashin@soleen.com>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Minchan Kim <minchan@kernel.org>,
        Christoph Hellwig <hch@infradead.org>,
        Song Liu <song@kernel.org>,
        Thomas Hellstrom <thomas.hellstrom@linux.intel.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH 02/31] mm/migrate: remove cruft from
 migration_entry_wait()s
In-Reply-To: <87bkib95uj.fsf@nvidia.com>
Message-ID: <8bc94098-8a57-b62f-a26c-7c73bf945b41@google.com>
References: <68a97fbe-5c1e-7ac6-72c-7b9c6290b370@google.com> <1659568-468a-6d36-c26-6a52a335ab59@google.com> <87bkib95uj.fsf@nvidia.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Precedence: bulk

On Tue, 23 May 2023, Alistair Popple wrote:
> 
> Hugh Dickins <hughd@google.com> writes:
> 
> > migration_entry_wait_on_locked() does not need to take a mapped pte
> > pointer, its callers can do the unmap first.  Annotate it with
> > __releases(ptl) to reduce sparse warnings.
> 
> Thanks Hugh, I debated doing something similar when I reworked this to
> not take a pageref. However I was unsure if the pte unmap/unlock
> ordering mattered as this reverses the order of operations (from unlock
> then unmap to unmap then unlock).
> 
> I never found any reason why that would be a problem though so please
> add:
> 
> Reviewed-by: Alistair Popple <apopple@nvidia.com>

Thanks Alistair.  Right, the ordering doesn't matter: you might briefly
think that because I'm going to put the rcu_read_unlock() into the
pte_unmap(), that there's something wrong with doing that early; but
you'd quickly realize that holding on to the spinlock is good enough.

Hugh