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Message-Id: <20240703092023.76749-1-ranxiaokai627@163.com>
Date: Wed, 3 Jul 2024 09:20:23 +0000
From: ran xiaokai <ranxiaokai627@....com>
To: david@...hat.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,
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ranxiaokai627@....com,
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Subject: Re: [PATCH 2/2] kpageflags: fix wrong KPF_THP on non-pmd-mappable compound pages
>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.
>c) Whoever is interested in getting the folio size, use this flag along
> with a scan for the KPF_COMPOUND_HEAD.
>
>
>I'll note that, scanning documentation, PAGE_IS_HUGE currently only
>applies to PMD *mapped* THP. It doesn't consider PTE-mapped THP at all
>(including PMD-ones). Likely, documentation should be updated to state
>"PMD-mapped THP or any hugetlb page".
>
>> Also kpageflags is not only for userspace memory but for all valid pfn
>> pages,including slab pages or drivers used pages, so the PG_lru and
>> is_anon check are unnecessary here.
>
>I'm completely confused about that statements. We do have KPF_OFFLINE,
>KPF_PGTABLE, KPF_SLAB, ... I'm missing something important.
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