linux-kernel - Re: [RFC PATCH 0/9] introduce PGTY_mgt_entry page

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Message-ID: <6d14e212-418e-4734-b256-b75c6fade25c@redhat.com>
Date: Thu, 24 Jul 2025 11:20:42 +0200
From: David Hildenbrand <david@...hat.com>
To: Huan Yang <link@...o.com>, Andrew Morton <akpm@...ux-foundation.org>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Rik van Riel
 <riel@...riel.com>, "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Vlastimil Babka <vbabka@...e.cz>, Harry Yoo <harry.yoo@...cle.com>,
 Xu Xin <xu.xin16@....com.cn>, Chengming Zhou <chengming.zhou@...ux.dev>,
 Mike Rapoport <rppt@...nel.org>, Suren Baghdasaryan <surenb@...gle.com>,
 Michal Hocko <mhocko@...e.com>, Zi Yan <ziy@...dia.com>,
 Matthew Brost <matthew.brost@...el.com>,
 Joshua Hahn <joshua.hahnjy@...il.com>, Rakie Kim <rakie.kim@...com>,
 Byungchul Park <byungchul@...com>, Gregory Price <gourry@...rry.net>,
 Ying Huang <ying.huang@...ux.alibaba.com>,
 Alistair Popple <apopple@...dia.com>,
 "Matthew Wilcox (Oracle)" <willy@...radead.org>,
 Christian Brauner <brauner@...nel.org>, Usama Arif <usamaarif642@...il.com>,
 Yu Zhao <yuzhao@...gle.com>, Baolin Wang <baolin.wang@...ux.alibaba.com>,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/9] introduce PGTY_mgt_entry page_type

On 24.07.25 11:12, David Hildenbrand wrote:
> On 24.07.25 11:09, Huan Yang wrote:
>>
>> 在 2025/7/24 16:59, David Hildenbrand 写道:
>>> On 24.07.25 10:44, Huan Yang wrote:
>>>> Summary
>>>> ==
>>>> This patchset reuses page_type to store migrate entry count during the
>>>> period from migrate entry setup to removal, enabling accelerated VMA
>>>> traversal when removing migrate entries, following a similar
>>>> principle to
>>>> early termination when folio is unmapped in try_to_migrate.
>>>
>>> I absolutely detest (ab)using page types for that, so no from my side
>>> unless I am missing something important.
>>>
>>>>
>>>> In my self-constructed test scenario, the migration time can be reduced
>>>
>>> How relevant is that in practice?
>>
>> IMO, any folio mapped < nr vma in mapping(anon_vma, addresss_space),
>> will benefit from this.
>>
>> So, all pages that have been COW-ed by child processes can be skipped.
> 
> For small anon folios, you could use the anon-exclusive marker to derive
> "there can only be a single mapping".
> 
> It's stored alongside the migration entry.
> 
> So once you restored that single migration entry, you can just stop the
> walk.

Essentially, something (untested) like this:

diff --git a/mm/migrate.c b/mm/migrate.c
index 425401b2d4e14..aa5bf96b1daee 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -421,6 +421,15 @@ static bool remove_migration_pte(struct folio *folio,

                 /* No need to invalidate - it was non-present before */
                 update_mmu_cache(vma, pvmw.address, pvmw.pte);
+
+               /*
+                * If the small anon folio is exclusive, here can be exactly one
+                * page mapping -- the one we just restored.
+                */
+               if (!folio_test_large(folio) && (rmap_flags & RMAP_EXCLUSIVE)) {
+                       page_vma_mapped_walk_done(&pvmw);
+                       break;
+               }
         }

         return true;

-- 
Cheers,

David / dhildenb