linux-kernel - Re: [RFC PATCH v1 3/4] mm/memory: Use ptep_get_lockless_norecency() for orig

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Date: Tue, 26 Mar 2024 18:58:55 +0100
From: David Hildenbrand <david@...hat.com>
To: Ryan Roberts <ryan.roberts@....com>, Mark Rutland <mark.rutland@....com>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
 Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>,
 Adrian Hunter <adrian.hunter@...el.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Muchun Song <muchun.song@...ux.dev>
Cc: linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v1 3/4] mm/memory: Use ptep_get_lockless_norecency()
 for orig_pte

>>>>
>>>> vmf->orig_pte = ptep_get_lockless_norecency(vmf->pte)
>>>> /* not dirty */
>>>>
>>>> /* Now, thread 2 ends up setting the PTE dirty under PT lock. */
> 
> Ahh, this comment about thread 2 is not referring to the code immediately below
> it. It all makes much more sense now. :)

Sorry :)

> 
>>>>
>>>> spin_lock(vmf->ptl);
>>>> entry = vmf->orig_pte;
>>>> if (unlikely(!pte_same(ptep_get(vmf->pte), entry))) {
>>>>       ...
>>>> }
>>>> ...
>>>> entry = pte_mkyoung(entry);
>>>
>>> Do you mean pte_mkdirty() here? You're talking about dirty everywhere else.
>>
>> No, that is just thread 1 seeing "oh, nothing to do" and then goes ahead and
>> unconditionally does that in handle_pte_fault().
>>
>>>
>>>> if (ptep_set_access_flags(vmf->vma, ...)
>>>> ...
>>>> pte_unmap_unlock(vmf->pte, vmf->ptl);
>>>>
>>>>
>>>> Generic ptep_set_access_flags() will do another pte_same() check and realize
>>>> "hey, there was a change!" let's update the PTE!
>>>>
>>>> set_pte_at(vma->vm_mm, address, ptep, entry);
>>>
>>> This is called from the generic ptep_set_access_flags() in your example, right?
>>>
>>
>> Yes.
>>
>>>>
>>>> would overwrite the dirty bit set by thread 2.
>>>
>>> I'm not really sure what you are getting at... Is your concern that there is a
>>> race where the page could become dirty in the meantime and it now gets lost? I
>>> think that's why arm64 overrides ptep_set_access_flags(); since the hw can
>>> update access/dirty we have to deal with the races.
>>
>> My concern is that your patch can in subtle ways lead to use losing PTE dirty
>> bits on architectures that don't have the HW-managed dirty bit. They do exist ;)
> 
> But I think the example you give can already happen today? Thread 1 reads
> orig_pte = ptep_get_lockless(). So that's already racy, if thread 2 is going to
> set dirty just after the get, then thread 1 is going to set the PTE back to (a
> modified version of) orig_pte. Isn't it already broken?

No, because the pte_same() check under PTL would have detected it, and 
we would have backed out. And I think the problem comes to live when we 
convert pte_same()->pte_same_norecency(), because we fail to protect PTE 
access/dirty changes that happend under PTL from another thread.

But could be I am missing something :)

>> Arm64 should be fine in that regard.
>>
> 
> There is plenty of arm64 HW that doesn't do HW access/dirty update. But our
> ptep_set_access_flags() can always deal with a racing update, even if that
> update originates from SW.
> 
> Why do I have the feeling you're about to explain (very patiently) exactly why
> I'm wrong?... :)

heh ... or you'll tell me (vary patiently) why I am wrong :)

-- 
Cheers,

David / dhildenb