Re: [RFC V3 PATCH] arm64: mm: swap: save and restore mte tags for large folios

[David Hildenbrand <david@redhat.com>]

On 17.11.23 19:41, Barry Song wrote:
> On Fri, Nov 17, 2023 at 7:28 PM David Hildenbrand <david@redhat.com> wrote:
>>
>> On 17.11.23 01:15, Barry Song wrote:
>>> On Fri, Nov 17, 2023 at 7:47 AM Barry Song <21cnbao@gmail.com> wrote:
>>>>
>>>> On Thu, Nov 16, 2023 at 5:36 PM David Hildenbrand <david@redhat.com> wrote:
>>>>>
>>>>> On 15.11.23 21:49, Barry Song wrote:
>>>>>> On Wed, Nov 15, 2023 at 11:16 PM David Hildenbrand <david@redhat.com> wrote:
>>>>>>>
>>>>>>> On 14.11.23 02:43, Barry Song wrote:
>>>>>>>> This patch makes MTE tags saving and restoring support large folios,
>>>>>>>> then we don't need to split them into base pages for swapping out
>>>>>>>> on ARM64 SoCs with MTE.
>>>>>>>>
>>>>>>>> arch_prepare_to_swap() should take folio rather than page as parameter
>>>>>>>> because we support THP swap-out as a whole.
>>>>>>>>
>>>>>>>> Meanwhile, arch_swap_restore() should use page parameter rather than
>>>>>>>> folio as swap-in always works at the granularity of base pages right
>>>>>>>> now.
>>>>>>>
>>>>>>> ... but then we always have order-0 folios and can pass a folio, or what
>>>>>>> am I missing?
>>>>>>
>>>>>> Hi David,
>>>>>> you missed the discussion here:
>>>>>>
>>>>>> https://lore.kernel.org/lkml/CAGsJ_4yXjex8txgEGt7+WMKp4uDQTn-fR06ijv4Ac68MkhjMDw@mail.gmail.com/
>>>>>> https://lore.kernel.org/lkml/CAGsJ_4xmBAcApyK8NgVQeX_Znp5e8D4fbbhGguOkNzmh1Veocg@mail.gmail.com/
>>>>>
>>>>> Okay, so you want to handle the refault-from-swapcache case where you get a
>>>>> large folio.
>>>>>
>>>>> I was mislead by your "folio as swap-in always works at the granularity of
>>>>> base pages right now" comment.
>>>>>
>>>>> What you actually wanted to say is "While we always swap in small folios, we
>>>>> might refault large folios from the swapcache, and we only want to restore
>>>>> the tags for the page of the large folio we are faulting on."
>>>>>
>>>>> But, I do if we can't simply restore the tags for the whole thing at once
>>>>> at make the interface page-free?
>>>>>
>>>>> Let me elaborate:
>>>>>
>>>>> IIRC, if we have a large folio in the swapcache, the swap entries/offset are
>>>>> contiguous. If you know you are faulting on page[1] of the folio with a
>>>>> given swap offset, you can calculate the swap offset for page[0] simply by
>>>>> subtracting from the offset.
>>>>>
>>>>> See page_swap_entry() on how we perform this calculation.
>>>>>
>>>>>
>>>>> So you can simply pass the large folio and the swap entry corresponding
>>>>> to the first page of the large folio, and restore all tags at once.
>>>>>
>>>>> So the interface would be
>>>>>
>>>>> arch_prepare_to_swap(struct folio *folio);
>>>>> void arch_swap_restore(struct page *folio, swp_entry_t start_entry);
>>>>>
>>>>> I'm sorry if that was also already discussed.
>>>>
>>>> This has been discussed. Steven, Ryan and I all don't think this is a good
>>>> option. in case we have a large folio with 16 basepages, as do_swap_page
>>>> can only map one base page for each page fault, that means we have
>>>> to restore 16(tags we restore in each page fault) * 16(the times of page faults)
>>>> for this large folio.
>>>>
>>>> and still the worst thing is the page fault in the Nth PTE of large folio
>>>> might free swap entry as that swap has been in.
>>>> do_swap_page()
>>>> {
>>>>      /*
>>>>       * Remove the swap entry and conditionally try to free up the swapcache.
>>>>       * We're already holding a reference on the page but haven't mapped it
>>>>       * yet.
>>>>       */
>>>>       swap_free(entry);
>>>> }
>>>>
>>>> So in the page faults other than N, I mean 0~N-1 and N+1 to 15, you might access
>>>> a freed tag.
>>>
>>> And David, one more information is that to keep the parameter of
>>> arch_swap_restore() unchanged as folio,
>>> i actually tried an ugly approach in rfc v2:
>>>
>>> +void arch_swap_restore(swp_entry_t entry, struct folio *folio)
>>> +{
>>> + if (system_supports_mte()) {
>>> +      /*
>>> +       * We don't support large folios swap in as whole yet, but
>>> +       * we can hit a large folio which is still in swapcache
>>> +       * after those related processes' PTEs have been unmapped
>>> +       * but before the swapcache folio  is dropped, in this case,
>>> +       * we need to find the exact page which "entry" is mapping
>>> +       * to. If we are not hitting swapcache, this folio won't be
>>> +       * large
>>> +     */
>>> + struct page *page = folio_file_page(folio, swp_offset(entry));
>>> + mte_restore_tags(entry, page);
>>> + }
>>> +}
>>>
>>> And obviously everybody in the discussion hated it :-)
>>>
>>
>> I can relate :D
>>
>>> i feel the only way to keep API unchanged using folio is that we
>>> support restoring PTEs
>>> all together for the whole large folio and we support the swap-in of
>>> large folios. This is
>>> in my list to do, I will send a patchset based on Ryan's large anon
>>> folios series after a
>>> while. till that is really done, it seems using page rather than folio
>>> is a better choice.
>>
>> I think just restoring all tags and remembering for a large folio that
>> they have been restored might be the low hanging fruit. But as always,
>> devil is in the detail :)
> 
> Hi David,
> thanks for all your suggestions though my feeling is this is too complex and
> is not worth it for at least  three reasons.

Fair enough.

> 
> 1. In multi-thread and particularly multi-processes, we need some locks to
> protect and help know if one process is the first one to restore tags and if
> someone else is restoring tags when one process wants to restore. there
> is not this kind of fine-grained lock at all.

We surely always hold the folio lock on swapin/swapout, no? So when 
these functions are called.

So that might just work already -- unless I am missing something important.

> 
> 2. folios are not always large, in many cases, they have just one base page
> and there is no tail to remember. and it seems pretty ugly if we turn out have
> to use different ways to remember restoring state for small folios and
> large folios.

if (folio_test_large(folio)) {

} else {

}

Easy ;)

Seriously, it's not that complicated and/or ugly.

> 
> 3. there is nothing wrong to use page to restore tags since right now swap-in
> is page. restoring tags and swapping-in become harmonious with each other
> after this patch. I would argue what is really wrong is the current mainline.
> 
> If eventually we are able to do_swap_page() for the whole large folio, we
> move to folios for MTE tags as well. These two behaviours make a new
> harmonious picture again.
> 

Just making both functions consume folios is much cleaner and also more 
future proof.

Consuming now a page where we used to consume a folio is a step backwards.

-- 
Cheers,

David / dhildenb