linux-kernel - Re: [PATCH 1/3] mm: Move arch_do_swap_page() call to before swap

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Message-ID: <80f45fec-3e91-c7b3-7fb4-1aa9355c627a@redhat.com>
Date:   Fri, 19 May 2023 11:21:35 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Peter Collingbourne <pcc@...gle.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Qun-wei Lin (林群崴) 
        <Qun-wei.Lin@...iatek.com>, linux-arm-kernel@...ts.infradead.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        "surenb@...gle.com" <surenb@...gle.com>,
        Chinwen Chang (張錦文) 
        <chinwen.chang@...iatek.com>,
        "kasan-dev@...glegroups.com" <kasan-dev@...glegroups.com>,
        Kuan-Ying Lee (李冠穎) 
        <Kuan-Ying.Lee@...iatek.com>,
        Casper Li (李中榮) <casper.li@...iatek.com>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        vincenzo.frascino@....com,
        Alexandru Elisei <alexandru.elisei@....com>, will@...nel.org,
        eugenis@...gle.com, Steven Price <steven.price@....com>,
        stable@...r.kernel.org
Subject: Re: [PATCH 1/3] mm: Move arch_do_swap_page() call to before
 swap_free()

>> 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