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;
Message-ID: <fec3f46e-a777-06e7-0ba0-a8cf169afa02@redhat.com>
Date:   Mon, 28 Nov 2022 14:52:57 +0100
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101
 Thunderbird/102.4.1
Content-Language: en-US
To:     Jann Horn <jannh@google.com>, security@kernel.org,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     Yang Shi <shy828301@gmail.com>, Peter Xu <peterx@redhat.com>,
        John Hubbard <jhubbard@nvidia.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
References: <20221125213714.4115729-1-jannh@google.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Subject: Re: [PATCH v3 1/3] mm/khugepaged: Take the right locks for page table
 retraction
In-Reply-To: <20221125213714.4115729-1-jannh@google.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk

On 25.11.22 22:37, Jann Horn wrote:
> pagetable walks on address ranges mapped by VMAs can be done under the mmap
> lock, the lock of an anon_vma attached to the VMA, or the lock of the VMA's
> address_space. Only one of these needs to be held, and it does not need to
> be held in exclusive mode.
> 
> Under those circumstances, the rules for concurrent access to page table
> entries are:
> 
>   - Terminal page table entries (entries that don't point to another page
>     table) can be arbitrarily changed under the page table lock, with the
>     exception that they always need to be consistent for
>     hardware page table walks and lockless_pages_from_mm().
>     This includes that they can be changed into non-terminal entries.
>   - Non-terminal page table entries (which point to another page table)
>     can not be modified; readers are allowed to READ_ONCE() an entry, verify
>     that it is non-terminal, and then assume that its value will stay as-is.
> 
> Retracting a page table involves modifying a non-terminal entry, so
> page-table-level locks are insufficient to protect against concurrent
> page table traversal; it requires taking all the higher-level locks under
> which it is possible to start a page walk in the relevant range in
> exclusive mode.
> 
> The collapse_huge_page() path for anonymous THP already follows this rule,
> but the shmem/file THP path was getting it wrong, making it possible for
> concurrent rmap-based operations to cause corruption.

This sounds sane and correct to me. No expert on file-THP, though.

For anon-THP it's the mmap lock and the rmap locks. I assume the only 
difference for file-THP is that the rmap lock is actually the mapping 
lock. Looking at rmap_walk_file(), that seems to be the case.


I wish at least PTE table removal could be done easier ... I already 
experimented some time ago with some ideas (e.g., lock in PMD table 
memmap) but it's all far from trivial and space in the memmap is rare.

-- 
Thanks,

David / dhildenb