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Message-ID: <dfb303b9-151b-49ec-b7ef-691c71bd5385@linux.dev>
Date: Fri, 19 Sep 2025 18:53:05 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: David Hildenbrand <david@...hat.com>
Cc: Qun-wei Lin (林群崴) <Qun-wei.Lin@...iatek.com>,
"catalin.marinas@....com" <catalin.marinas@....com>,
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<rppt@...nel.org>, "willy@...radead.org" <willy@...radead.org>,
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Subject: Re: [PATCH v5 2/6] mm: remap unused subpages to shared zeropage when
splitting isolated thp
On 2025/9/19 16:14, Lance Yang wrote:
>
>
> On 2025/9/19 15:55, David Hildenbrand wrote:
>>>> I think where possible we really only want to identify problematic
>>>> (tagged) pages and skip them. And we should either look into fixing KSM
>>>> as well or finding out why KSM is not affected.
>>>
>>> Yeah. Seems like we could introduce a new helper,
>>> folio_test_mte_tagged(struct
>>> folio *folio). By default, it would return false, and architectures like
>>> arm64
>>> can override it.
>>
>> If we add a new helper it should instead express the semantics that we
>> cannot deduplicate.
>
> Agreed.
>
>>
>> For THP, I recall that only some pages might be tagged. So likely we
>> want to check per page.
>
> Yes, a per-page check would be simpler.
>
>>
>>>
>>> Looking at the code, the PG_mte_tagged flag is not set for regular THP.
>>
>> I think it's supported for THP per page. Only for hugetlb we tag the
>> whole thing through the head page instead of individual pages.
>
> Right. That's exactly what I meant.
>
>>
>>> The MTE
>>> status actually comes from the VM_MTE flag in the VMA that maps it.
>>>
>>
>> During the rmap walk we could check the VMA flag, but there would be
>> no way to just stop the THP shrinker scanning this page early.
>>
>>> static inline bool folio_test_hugetlb_mte_tagged(struct folio *folio)
>>> {
>>> bool ret = test_bit(PG_mte_tagged, &folio->flags.f);
>>>
>>> VM_WARN_ON_ONCE(!folio_test_hugetlb(folio));
>>>
>>> /*
>>> * If the folio is tagged, ensure ordering with a likely subsequent
>>> * read of the tags.
>>> */
>>> if (ret)
>>> smp_rmb();
>>> return ret;
>>> }
>>>
>>> static inline bool page_mte_tagged(struct page *page)
>>> {
>>> bool ret = test_bit(PG_mte_tagged, &page->flags.f);
>>>
>>> VM_WARN_ON_ONCE(folio_test_hugetlb(page_folio(page)));
>>>
>>> /*
>>> * If the page is tagged, ensure ordering with a likely subsequent
>>> * read of the tags.
>>> */
>>> if (ret)
>>> smp_rmb();
>>> return ret;
>>> }
>>>
>>> contpte_set_ptes()
>>> __set_ptes()
>>> __set_ptes_anysz()
>>> __sync_cache_and_tags()
>>> mte_sync_tags()
>>> set_page_mte_tagged()
>>>
>>> Then, having the THP shrinker skip any folios that are identified as
>>> MTE-tagged.
>>
>> Likely we should just do something like (maybe we want better naming)
>>
>> #ifndef page_is_mergable
>> #define page_is_mergable(page) (true)
>> #endif
>
>
> Maybe something like page_is_optimizable()? Just a thought ;p
>
>>
>> And for arm64 have it be
>>
>> #define page_is_mergable(page) (!page_mte_tagged(page))
>>
>>
>> And then do
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 1f0813b956436..1cac9093918d6 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -4251,7 +4251,8 @@ static bool thp_underused(struct folio *folio)
>>
>> for (i = 0; i < folio_nr_pages(folio); i++) {
>> kaddr = kmap_local_folio(folio, i * PAGE_SIZE);
>> - if (!memchr_inv(kaddr, 0, PAGE_SIZE)) {
>> + if (page_is_mergable(folio_page(folio, i)) &&
>> + !memchr_inv(kaddr, 0, PAGE_SIZE)) {
>> num_zero_pages++;
>> if (num_zero_pages > khugepaged_max_ptes_none) {
>> kunmap_local(kaddr);
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index 946253c398072..476a9a9091bd3 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -306,6 +306,8 @@ static bool try_to_map_unused_to_zeropage(struct
>> page_vma_mapped_walk *pvmw,
>>
>> if (PageCompound(page))
>> return false;
>> + if (!page_is_mergable(page))
>> + return false;
>> VM_BUG_ON_PAGE(!PageAnon(page), page);
>> VM_BUG_ON_PAGE(!PageLocked(page), page);
>> VM_BUG_ON_PAGE(pte_present(ptep_get(pvmw->pte)), page);
>
> Looks good to me!
>
>>
>>
>> For KSM, similarly just bail out early. But still wondering if this is
>> already checked
>> somehow for KSM.
>
> +1 I'm looking for a machine to test it on.
Interestingly, it seems KSM is already skipping MTE-tagged pages. My test,
running on a v6.8.0 kernel inside QEMU (with MTE enabled), shows no merging
activity for those pages ...
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