linux-kernel - Re: [PATCH v3 13/20] mm: Copy-on-Write (COW) reuse support for PTE-mapped THP

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Message-ID: <3cb85c4b-7e28-4a70-92da-1d835eaf467d@redhat.com>
Date: Tue, 22 Apr 2025 09:05:41 +0200
From: David Hildenbrand <david@...hat.com>
To: Kairui Song <ryncsn@...il.com>
Cc: linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
 cgroups@...r.kernel.org, linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
 linux-api@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>,
 "Matthew Wilcox (Oracle)" <willy@...radead.org>, Tejun Heo <tj@...nel.org>,
 Zefan Li <lizefan.x@...edance.com>, Johannes Weiner <hannes@...xchg.org>,
 Michal Koutný <mkoutny@...e.com>,
 Jonathan Corbet <corbet@....net>, Andy Lutomirski <luto@...nel.org>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 Muchun Song <muchun.song@...ux.dev>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH v3 13/20] mm: Copy-on-Write (COW) reuse support for
 PTE-mapped THP

On 22.04.25 04:52, Kairui Song wrote:
> On Sun, Apr 20, 2025 at 12:35 AM Kairui Song <ryncsn@...il.com> wrote:
>>
>> On Sun, Apr 20, 2025 at 12:32 AM David Hildenbrand <david@...hat.com> wrote:
>>>
>>> On 19.04.25 18:25, David Hildenbrand wrote:
>>>
>>> Oh, re-reading the condition 3 times, I realize that the sanity check is wrong ...
>>>
>>> diff --git a/mm/memory.c b/mm/memory.c
>>> index 037b6ce211f1f..a17eeef3f1f89 100644
>>> --- a/mm/memory.c
>>> +++ b/mm/memory.c
>>> @@ -3789,7 +3789,7 @@ static bool __wp_can_reuse_large_anon_folio(struct folio *folio,
>>>
>>>           /* Stabilize the mapcount vs. refcount and recheck. */
>>>           folio_lock_large_mapcount(folio);
>>> -       VM_WARN_ON_ONCE(folio_large_mapcount(folio) < folio_ref_count(folio));
>>> +       VM_WARN_ON_ONCE(folio_large_mapcount(folio) > folio_ref_count(folio));
>>
>> Ah, now it makes sense to me now :)
>>
>> Thanks for the quick response.
>>
>>>
>>>           if (folio_test_large_maybe_mapped_shared(folio))
>>>                   goto unlock;
>>>
>>> Our refcount must be at least the mapcount, that's what we want to assert.
>>>
>>> Can you test and send a fix patch if that makes it fly for you?
>>
>> Sure I'll keep the testing, I think it will just fix it, I have a few
>> WARN_ON_FOLIO reports all reporting mapcount is smaller than refcount.
> 
> Hi David,
> 
> I'm no longer seeing any warning after this, it fixed the problem well.

Cool, can you send a fix?

-- 
Cheers,

David / dhildenb