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

[David Hildenbrand <david@redhat.com>]

On 17.11.23 00:47, Barry Song 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.

Can't you remember that it's already been restored? That seems like a 
reasonable thing to have.

For large folios we have plenty of page flags in tail pages available?

-- 
Cheers,

David / dhildenb