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

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Date:   Wed, 17 May 2023 10:30:19 +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()

>> Would the idea be to fail swap_readpage() on the one that comes last,
>> simply retrying to lookup the page?
> 
> The idea would be that T2's arch_swap_readpage() could potentially not
> find tags if it ran after swap_free(), so T2 would produce a page
> without restored tags. But that wouldn't matter, because T1 reaching
> swap_free() means that T2 will follow the goto at [1] after waiting
> for T1 to unlock at [2], and T2's page will be discarded.

Ah, right.

> 
>> This might be a naive question, but how does MTE play along with shared
>> anonymous pages?
> 
> It should work fine. shmem_writepage() calls swap_writepage() which
> calls arch_prepare_to_swap() to write the tags. And
> shmem_swapin_folio() has a call to arch_swap_restore() to restore
> them.

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.

-- 
Thanks,

David / dhildenb