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:   Thu, 1 Jun 2023 22:35:10 -0700 (PDT)
From:   Hugh Dickins <hughd@google.com>
To:     Jason Gunthorpe <jgg@ziepe.ca>
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>,
        Alistair Popple <apopple@nvidia.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>,
        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>,
        Russell King <linux@armlinux.org.uk>,
        "David S. Miller" <davem@davemloft.net>,
        Michael Ellerman <mpe@ellerman.id.au>,
        "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
        Heiko Carstens <hca@linux.ibm.com>,
        Christian Borntraeger <borntraeger@linux.ibm.com>,
        Claudio Imbrenda <imbrenda@linux.ibm.com>,
        Alexander Gordeev <agordeev@linux.ibm.com>,
        Jann Horn <jannh@google.com>,
        linux-arm-kernel@lists.infradead.org, sparclinux@vger.kernel.org,
        linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH 02/12] mm/pgtable: add PAE safety to __pte_offset_map()
In-Reply-To: <ZHeg3oRljRn6wlLX@ziepe.ca>
Message-ID: <40349492-6f33-2a19-4a5-eabbe6b48aca@google.com>
References: <35e983f5-7ed3-b310-d949-9ae8b130cdab@google.com> <923480d5-35ab-7cac-79d0-343d16e29318@google.com> <ZHeg3oRljRn6wlLX@ziepe.ca>
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Precedence: bulk

On Wed, 31 May 2023, Jason Gunthorpe wrote:
> On Sun, May 28, 2023 at 11:16:16PM -0700, Hugh Dickins wrote:
> > There is a faint risk that __pte_offset_map(), on a 32-bit architecture
> > with a 64-bit pmd_t e.g. x86-32 with CONFIG_X86_PAE=y, would succeed on
> > a pmdval assembled from a pmd_low and a pmd_high which never belonged
> > together: their combination not pointing to a page table at all, perhaps
> > not even a valid pfn.  pmdp_get_lockless() is not enough to prevent that.
> > 
> > Guard against that (on such configs) by local_irq_save() blocking TLB
> > flush between present updates, as linux/pgtable.h suggests.  It's only
> > needed around the pmdp_get_lockless() in __pte_offset_map(): a race when
> > __pte_offset_map_lock() repeats the pmdp_get_lockless() after getting the
> > lock, would just send it back to __pte_offset_map() again.
> 
> What about the other places calling pmdp_get_lockless ? It seems like
> this is quietly making it part of the API that the caller must hold
> the IPIs off.

No, I'm making no judgment of other places where pmdp_get_lockless() is
used: examination might show that some need more care, but I'll just
assume that each is taking as much care as it needs.

But here where I'm making changes, I do see that we need this extra care.

> 
> And Jann had a note that this approach used by the lockless functions
> doesn't work anyhow:
> 
> https://lore.kernel.org/linux-mm/CAG48ez3h-mnp9ZFC10v+-BW_8NQvxbwBsMYJFP8JX31o0B17Pg@mail.gmail.com/

Thanks a lot for the link: I don't know why, but I never saw that mail
thread at all before.  I have not fully digested it yet, to be honest:
MADV_DONTNEED, doesn't flush TLB yet, etc - I'll have to get into the
right frame of mind for that.

> 
> Though we never fixed it, AFAIK..

I'm certainly depending very much on pmdp_get_lockless(): and hoping to
find its case is easier to defend than at the ptep_get_lockless() level.

Thanks,
Hugh