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: <80f45fec-3e91-c7b3-7fb4-1aa9355c627a@redhat.com>
Date:   Fri, 19 May 2023 11:21:35 +0200
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101
 Thunderbird/102.10.0
Content-Language: en-US
To:     Peter Collingbourne <pcc@google.com>
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        =?UTF-8?B?UXVuLXdlaSBMaW4gKOael+e+pOW0tCk=?= 
        <Qun-wei.Lin@mediatek.com>, linux-arm-kernel@lists.infradead.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        "surenb@google.com" <surenb@google.com>,
        =?UTF-8?B?Q2hpbndlbiBDaGFuZyAo5by16Yym5paHKQ==?= 
        <chinwen.chang@mediatek.com>,
        "kasan-dev@googlegroups.com" <kasan-dev@googlegroups.com>,
        =?UTF-8?B?S3Vhbi1ZaW5nIExlZSAo5p2O5Yag56mOKQ==?= 
        <Kuan-Ying.Lee@mediatek.com>,
        =?UTF-8?B?Q2FzcGVyIExpICjmnY7kuK3mpq4p?= <casper.li@mediatek.com>,
        "gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>,
        vincenzo.frascino@arm.com,
        Alexandru Elisei <alexandru.elisei@arm.com>, will@kernel.org,
        eugenis@google.com, Steven Price <steven.price@arm.com>,
        stable@vger.kernel.org
References: <20230512235755.1589034-1-pcc@google.com>
 <20230512235755.1589034-2-pcc@google.com>
 <7471013e-4afb-e445-5985-2441155fc82c@redhat.com> <ZGJtJobLrBg3PtHm@arm.com>
 <ZGLC0T32sgVkG5kX@google.com>
 <851940cd-64f1-9e59-3de9-b50701a99281@redhat.com>
 <CAMn1gO79e+v3ceNY0YfwrYTvU1monKWmTedXsYjtucmM7s=MVA@mail.gmail.com>
 <c9f1fc7c-62a2-4768-7992-52e34ec36d0f@redhat.com>
 <CAMn1gO7t0S7CmeU=59Lq10N0WvrKebM=W91W7sa+SQoG13Uppw@mail.gmail.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Subject: Re: [PATCH 1/3] mm: Move arch_do_swap_page() call to before
 swap_free()
In-Reply-To: <CAMn1gO7t0S7CmeU=59Lq10N0WvrKebM=W91W7sa+SQoG13Uppw@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk

>> Sorry, I meant actual anonymous memory pages, not shmem. Like, anonymous
>> pages that are COW-shared due to fork() or KSM.
>>
>> How does MTE, in general, interact with that? Assume one process ends up
>> modifying the tags ... and the page is COW-shared with a different
>> process that should not observe these tag modifications.
> 
> Tag modifications cause write faults if the page is read-only, so for
> COW shared pages we would end up copying the page in the usual way,
> which on arm64 would copy the tags as well via the copy_highpage hook
> (see arch/arm64/mm/copypage.c).

Oh, that makes sense, thanks for pointing that out!

... and I can spot that KSM also checks the tag when de-duplicating: 
pages_identical() ends up calling memcmp_pages(), which knows how to 
deal with tags.

Interestingly, calc_checksum() does not seem to care about tags. But 
that simply implies that pages with the same content have same checksum, 
independent of the tag. And pages_identical() is the single source of truth.

-- 
Thanks,

David / dhildenb