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Date:   Wed, 17 May 2023 10:34:41 +0200
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Subject: Re: [PATCH 1/3] mm: Move arch_do_swap_page() call to before
 swap_free()
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To:     Peter Collingbourne <pcc@google.com>
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
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From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
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On 16.05.23 04:35, Peter Collingbourne wrote:
> On Mon, May 15, 2023 at 05:16:09PM -0700, Peter Collingbourne wrote:
>> On Sat, May 13, 2023 at 05:29:53AM +0200, David Hildenbrand wrote:
>>> On 13.05.23 01:57, Peter Collingbourne wrote:
>>>> Commit c145e0b47c77 ("mm: streamline COW logic in do_swap_page()") moved
>>>> the call to swap_free() before the call to set_pte_at(), which meant that
>>>> the MTE tags could end up being freed before set_pte_at() had a chance
>>>> to restore them. One other possibility was to hook arch_do_swap_page(),
>>>> but this had a number of problems:
>>>>
>>>> - The call to the hook was also after swap_free().
>>>>
>>>> - The call to the hook was after the call to set_pte_at(), so there was a
>>>>     racy window where uninitialized metadata may be exposed to userspace.
>>>>     This likely also affects SPARC ADI, which implements this hook to
>>>>     restore tags.
>>>>
>>>> - As a result of commit 1eba86c096e3 ("mm: change page type prior to
>>>>     adding page table entry"), we were also passing the new PTE as the
>>>>     oldpte argument, preventing the hook from knowing the swap index.
>>>>
>>>> Fix all of these problems by moving the arch_do_swap_page() call before
>>>> the call to free_page(), and ensuring that we do not set orig_pte until
>>>> after the call.
>>>>
>>>> Signed-off-by: Peter Collingbourne <pcc@google.com>
>>>> Suggested-by: Catalin Marinas <catalin.marinas@arm.com>
>>>> Link: https://linux-review.googlesource.com/id/I6470efa669e8bd2f841049b8c61020c510678965
>>>> Cc: <stable@vger.kernel.org> # 6.1
>>>> Fixes: ca827d55ebaa ("mm, swap: Add infrastructure for saving page metadata on swap")
>>>> Fixes: 1eba86c096e3 ("mm: change page type prior to adding page table entry")
>>>
>>> I'm confused. You say c145e0b47c77 changed something (which was after above
>>> commits), indicate that it fixes two other commits, and indicate "6.1" as
>>> stable which does not apply to any of these commits.
>>
>> Sorry, the situation is indeed a bit confusing.
>>
>> - In order to make the arch_do_swap_page() hook suitable for fixing the
>>    bug introduced by c145e0b47c77, patch 1 addresses a number of issues,
>>    including fixing bugs introduced by ca827d55ebaa and 1eba86c096e3,
>>    but we haven't fixed the c145e0b47c77 bug yet, so there's no Fixes:
>>    tag for it yet.
>>
>> - Patch 2, relying on the fixes in patch 1, makes MTE install an
>>    arch_do_swap_page() hook (indirectly, by making arch_swap_restore()
>>    also hook arch_do_swap_page()), thereby fixing the c145e0b47c77 bug.
>>
>> - 6.1 is the first stable version in which all 3 commits in my Fixes: tags
>>    are present, so that is the version that I've indicated in my stable
>>    tag for this series. In theory patch 1 could be applied to older kernel
>>    versions, but it wouldn't fix any problems that we are facing with MTE
>>    (because it only fixes problems relating to the arch_do_swap_page()
>>    hook, which older kernel versions don't hook with MTE), and there are
>>    some merge conflicts if we go back further anyway. If the SPARC folks
>>    (the previous only user of this hook) want to fix these issues with ADI,
>>    they can propose their own backport.
>>
>>>> @@ -3959,7 +3960,6 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>>>>    	VM_BUG_ON(!folio_test_anon(folio) ||
>>>>    			(pte_write(pte) && !PageAnonExclusive(page)));
>>>>    	set_pte_at(vma->vm_mm, vmf->address, vmf->pte, pte);
>>>> -	arch_do_swap_page(vma->vm_mm, vma, vmf->address, pte, vmf->orig_pte);
>>>>    	folio_unlock(folio);
>>>>    	if (folio != swapcache && swapcache) {
>>>
>>>
>>> You are moving the folio_free_swap() call after the folio_ref_count(folio)
>>> == 1 check, which means that such (previously) swapped pages that are
>>> exclusive cannot be detected as exclusive.
>>
>> Ack. I will fix this in v2.
> 
> I gave this some thought and concluded that the added complexity needed
> to make this hook suitable for arm64 without breaking sparc probably
> isn't worth it in the end, and as I explained in patch 2, sparc ought
> to be moving away from this hook anyway. So in v2 I replaced patches 1
> and 2 with a patch that adds a direct call to the arch_swap_restore()
> hook before the call to swap_free().

As a side note, I recall that sparc code might be a bit fragile and 
eventually broken already (arch_unmap_one()):

https://lkml.kernel.org/r/d98bd1f9-e9b7-049c-7bde-3348b074eb18@redhat.com

-- 
Thanks,

David / dhildenb