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

[ran xiaokai <ranxiaokai627@163.com>]

>On 26.06.24 04:49, ran xiaokai wrote:
>> From: Ran Xiaokai <ran.xiaokai@zte.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.