linux-kernel - Re: [GIT PULL] arm64 updates for 6.13-rc1

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Message-ID: <6aec1d44-4a89-4acf-a16b-4493626b93bb@os.amperecomputing.com>
Date: Mon, 2 Dec 2024 08:22:57 -0800
From: Yang Shi <yang@...amperecomputing.com>
To: David Hildenbrand <david@...hat.com>,
 Catalin Marinas <catalin.marinas@....com>
Cc: Sasha Levin <sashal@...nel.org>,
 Linus Torvalds <torvalds@...ux-foundation.org>, Will Deacon
 <will@...nel.org>, linux-arm-kernel@...ts.infradead.org,
 linux-kernel@...r.kernel.org
Subject: Re: [GIT PULL] arm64 updates for 6.13-rc1



On 11/28/24 1:56 AM, David Hildenbrand wrote:
> On 28.11.24 02:21, Yang Shi wrote:
>>>> diff --git a/arch/arm64/mm/copypage.c b/arch/arm64/mm/copypage.c
>>>> index 87b3f1a25535..ef303a2262c5 100644
>>>> --- a/arch/arm64/mm/copypage.c
>>>> +++ b/arch/arm64/mm/copypage.c
>>>> @@ -30,9 +30,9 @@ void copy_highpage(struct page *to, struct page 
>>>> *from)
>>>>         if (!system_supports_mte())
>>>>             return;
>>>> -    if (folio_test_hugetlb(src) &&
>>>> -        folio_test_hugetlb_mte_tagged(src)) {
>>>> -        if (!folio_try_hugetlb_mte_tagging(dst))
>>>> +    if (folio_test_hugetlb(src)) {
>>>> +        if (!folio_test_hugetlb_mte_tagged(src) ||
>>>> +            !folio_try_hugetlb_mte_tagging(dst))
>>>>                 return;
>>>>             /*
>>> I wonder why we had a 'return' here originally rather than a
>>> WARN_ON_ONCE() as we do further down for the page case. Do you seen any
>>> issue with the hunk below? Destination should be a new folio and not
>>> tagged yet:
>>
>> Yes, I did see problem. Because we copy tags for all sub pages then set
>> folio mte tagged when copying the data for the first subpage. The
>> warning will be triggered when we copy the second subpage.
>
> It's rather weird, though. We're instructed to copy a single page, yet 
> copy tags for all pages.
>
> This really only makes sense when called from folio_copy(), where we 
> are guaranteed to copy all pages.
>
> I'm starting to wonder if we should be able to hook into / overload 
> folio_copy() instead, to just handle the complete hugetlb copy 
> ourselves in one shot, and assume that copy_highpage() will never be 
> called for hugetlb pages (WARN and don't copy tags).

Actually folio_copy() is just called by migration. Copy huge page in CoW 
is more complicated and uses copy_user_highpage()->copy_highpage() 
instead of folio_copy(). It may start the page copy from any subpage. 
For example, if the CoW is triggered by accessing to the address in the 
middle of 2M. Kernel may copy the second half first then the first half 
to guarantee the accessed data in cache.