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 10:31:14 +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()


>>> Is there any other way of handling this? E.g. not release the metadata
>>> in arch_swap_invalidate_page() but later in set_pte_at() once it was
>>> restored. But then we may leak this metadata if there's no set_pte_at()
>>> (the process mapping the swap entry died).
>>
>> That was my immediate thought: do we really have to hook into
>> swap_range_free() at all?
> 
> As I alluded to in another reply, without the hook in
> swap_range_free() I think we would either end up with a race or an
> effective memory leak in the arch code that maintains the metadata for
> swapped out pages, as there would be no way for the arch-specific code
> to know when it is safe to free it after swapin.

Agreed, hooking swap_range_free() is actually cleaner (also considering 
COW-shared pages).

> 
>> And I also wondered why we have to do this
>> from set_pte_at() and not do this explicitly (maybe that's the other
>> arch_* callback on the swapin path).
> 
> I don't think it's necessary, as the set_pte_at() call sites for
> swapped in pages are known. I'd much rather do this via an explicit
> hook at those call sites, as the existing approach of implicit
> restoring seems too subtle and easy to be overlooked when refactoring,
> as we have seen with this bug. In the end we only have 3 call sites
> for the hook and hopefully the comments that I'm adding are sufficient
> to ensure that any new swapin code should end up with a call to the
> hook in the right place.


Agreed, much cleaner, thanks!

-- 
Thanks,

David / dhildenb