linux-kernel - Re: [PATCH 2/2] kpageflags: fix wrong KPF_THP on non-pmd-mappable compound pages

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Message-ID: <79910a02-7767-4f70-9248-319aba79fb45@redhat.com>
Date: Wed, 3 Jul 2024 12:11:26 +0200
From: David Hildenbrand <david@...hat.com>
To: ran xiaokai <ranxiaokai627@....com>
Cc: akpm@...ux-foundation.org, baohua@...nel.org,
 linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, peterx@...hat.com, ran.xiaokai@....com.cn,
 ryan.roberts@....com, svetly.todorov@...verge.com, vbabka@...e.cz,
 willy@...radead.org, ziy@...dia.com
Subject: Re: [PATCH 2/2] kpageflags: fix wrong KPF_THP on non-pmd-mappable
 compound pages

On 03.07.24 11:20, ran xiaokai wrote:
>> On 26.06.24 04:49, ran xiaokai wrote:
>>> From: Ran Xiaokai <ran.xiaokai@....com.cn>
>>>
>>> KPF_COMPOUND_HEAD and KPF_COMPOUND_TAIL are set on "common" compound
>>> pages, which means of any order, but KPF_THP should only be set
>>> when the folio is a 2M pmd mappable THP. Since commit 19eaf44954df
>>
>> "should only be set" -- who says that? :)
>>
>> The documentation only talks about "Contiguous pages which construct
>> transparent hugepages". Sure, when it was added there were only PMD ones.
>>
>>
>>> ("mm: thp: support allocation of anonymous multi-size THP"),
>>> multiple orders of folios can be allocated and mapped to userspace,
>>> so the folio_test_large() check is not sufficient here,
>>> replace it with folio_test_pmd_mappable() to fix this.
>>>
>>
>> A couple of points:
>>
>> 1) If I am not daydreaming, ever since we supported non-PMD THP in the
>>     pagecache (much longer than anon mTHP), we already indicate KPF_THP
>>     for them here. So this is not anon specific? Or am I getting the
>>     PG_lru check all wrong?
>>
>> 2) Anon THP are disabled as default. If some custom tool cannot deal
>>     with that "extension" we did with smaller THP, it shall be updated if
>>     it really has to special-case PMD-mapped THP, before enabled by the
>>     admin.
>>
>>
>> I think this interface does exactly what we want, as it is right now.
>> Unless there is *good* reason, we should keep it like that.
>>
>> So I suggest
>>
>> a) Extend the documentation to just state "THP of any size and any
>> mapping granularity" or sth like that.
>>
>> b) Maybe using folio_test_large_rmappable() instead of "(k & (1 <<
>>     PG_lru)) || is_anon", so even isolated-from-LRU THPs are indicated
>>     properly.
> 
> Hi, David,
> 
> The "is_anon" check was introduced to also include page vector cache
> pages, but now large folios are added to lru list directly, bypassed
> the pagevec cache. So the is_anon check seems unnecessary here.
> As now pagecache also supports large folios, the is_anon check seems
> unsufficient here.
> 
> Can i say that for userspace memory,
> folio_test_large_rmappable() == folio_test_large()?
> if that is true, except the "if ((k & (1 << PG_lru)) || is_anon)"
> check, we can also remove the folio_test_large() check,
> like this:
> 
> else if (folio_test_large_rmappable(folio)) {
>          u |= 1 << KPF_THP;
>      else if (is_huge_zero_folio(folio)) {
>          u |= 1 << KPF_ZERO_PAGE;
>          u |= 1 << KPF_THP;
>      }
> } else if (is_zero_pfn(page_to_pfn(page)))
> 
> This will also include the isolated folios.

You'll have to keep the folio_test_large() check, 
folio_test_large_rmappable() wants us to check that ahead of time.

Something like

...
else if (folio_test_large(folio) && folio_test_large_rmappable(folio)) {
	/* Note: we indicate any THPs here, not just PMD-sized ones */
	u |= 1 << KPF_THP;
} else if (is_huge_zero_folio(folio)) {
	u |= 1 << KPF_ZERO_PAGE;
	u |= 1 << KPF_THP
} else if (is_zero_pfn(page_to_pfn(page))) {
	u |= 1 << KPF_ZERO_PAGE;
}

Would likely work and keep the existing behavior (+ include isolated ones).

-- 
Cheers,

David / dhildenb