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

[ran xiaokai]

> On Tue Jun 25, 2024 at 10:49 PM EDT, ran xiaokai wrote:
> > From: Ran Xiaokai <ran.xiaokai@xxxxxxxxxx>
> >
> > 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
> > ("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.
> >
> > 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.
> 
> But THP is userspace memory. slab pages or driver pages cannot be THP.

I see, the THP naming implies userspace memory. Not only compound order.
 
> >
> > Signed-off-by: Ran Xiaokai <ran.xiaokai@xxxxxxxxxx>
> > ---
> >  fs/proc/page.c | 14 ++++----------
> >  1 file changed, 4 insertions(+), 10 deletions(-)
> >
> > diff --git a/fs/proc/page.c b/fs/proc/page.c
> > index 2fb64bdb64eb..3e7b70449c2f 100644
> > --- a/fs/proc/page.c
> > +++ b/fs/proc/page.c
> > @@ -146,19 +146,13 @@ u64 stable_page_flags(const struct page *page)
> >  		u |= kpf_copy_bit(k, KPF_COMPOUND_HEAD, PG_head);
> >  	else
> >  		u |= 1 << KPF_COMPOUND_TAIL;
> > +
> Unnecessary new line.

yes, will fix.

> 
> >  	if (folio_test_hugetlb(folio))
> >  		u |= 1 << KPF_HUGE;
> > -	/*
> > -	 * We need to check PageLRU/PageAnon
> > -	 * to make sure a given page is a thp, not a non-huge compound page.
> > -	 */
> > -	else if (folio_test_large(folio)) {
> > -		if ((k & (1 << PG_lru)) || is_anon)
> > -			u |= 1 << KPF_THP;
> > -		else if (is_huge_zero_folio(folio)) {
> > +	else if (folio_test_pmd_mappable(folio)) {
> > +		u |= 1 << KPF_THP;
> 
> lru and anon check should stay.

thanks, will fix.

> 
> > +		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;
> >  
> 
> -- 
> Best Regards,
> Yan, Zi