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

[David Hildenbrand <david@redhat.com>]

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 :)

-- 
Cheers,

David / dhildenb