Re: [PATCH 1/2] mm: pgtable: add missing flag and statistics for kernel PTE page

[Qi Zheng]

Hi Mike,

On 2024/2/4 18:58, Mike Rapoport wrote:
> On Thu, Feb 01, 2024 at 04:05:40PM +0800, Qi Zheng wrote:
> > For kernel PTE page, we do not need to allocate and initialize its split
> > ptlock, but as a page table page, it's still necessary to add PG_table
> > flag and NR_PAGETABLE statistics for it.
> >
> > Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
> > ---
> >   include/asm-generic/pgalloc.h |  7 ++++++-
> >   include/linux/mm.h            | 21 ++++++++++++++++-----
> >   2 files changed, 22 insertions(+), 6 deletions(-)
>
> This should also update the architectures that define
> __HAVE_ARCH_PTE_ALLOC_ONE_KERNEL, otherwise NR_PAGETABLE counts will get
> wrong.

Yes, this patchset only focuses on the generic implementation. For those
architectures that define __HAVE_ARCH_PTE_ALLOC_ONE_KERNEL, some reuse
the generic __pte_alloc_one_kernel(), but some have their own customized
implementations, which indeed need to be fixed.

I wasn't familiar with those architectures and didn't investigate why
they couldn't reuse the generic __pte_alloc_one_kernel(), so I didn't
fix them. It would be better if there are maintainers corresponding to
the architecture who can help fix it. After all, they have a better
understanding of the historical background and have a testing
environment. ;)

> Another related thing is that many architectures have custom allocations
> for early page tables and these would also benefit form NR_PAGETABLE
> accounting.

Indeed, this is also a point that can be optimized.

Thanks.

>   
> > diff --git a/include/asm-generic/pgalloc.h b/include/asm-generic/pgalloc.h
> > index 879e5f8aa5e9..908bd9140ac2 100644
> > --- a/include/asm-generic/pgalloc.h
> > +++ b/include/asm-generic/pgalloc.h
> > @@ -23,6 +23,8 @@ static inline pte_t *__pte_alloc_one_kernel(struct mm_struct *mm)
> >   
> >   	if (!ptdesc)
> >   		return NULL;
> > +
> > +	__pagetable_pte_ctor(ptdesc);
> >   	return ptdesc_address(ptdesc);
> >   }
> >   
> > @@ -46,7 +48,10 @@ static inline pte_t *pte_alloc_one_kernel(struct mm_struct *mm)
> >    */
> >   static inline void pte_free_kernel(struct mm_struct *mm, pte_t *pte)
> >   {
> > -	pagetable_free(virt_to_ptdesc(pte));
> > +	struct ptdesc *ptdesc = virt_to_ptdesc(pte);
> > +
> > +	__pagetable_pte_dtor(ptdesc);
> > +	pagetable_free(ptdesc);
> >   }
> >   
> >   /**
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index e442fd0efdd9..e37db032764e 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -2922,26 +2922,37 @@ static inline bool ptlock_init(struct ptdesc *ptdesc) { return true; }
> >   static inline void ptlock_free(struct ptdesc *ptdesc) {}
> >   #endif /* USE_SPLIT_PTE_PTLOCKS */
> >   
> > -static inline bool pagetable_pte_ctor(struct ptdesc *ptdesc)
> > +static inline void __pagetable_pte_ctor(struct ptdesc *ptdesc)
> >   {
> >   	struct folio *folio = ptdesc_folio(ptdesc);
> >   
> > -	if (!ptlock_init(ptdesc))
> > -		return false;
> >   	__folio_set_pgtable(folio);
> >   	lruvec_stat_add_folio(folio, NR_PAGETABLE);
> > +}
> > +
> > +static inline bool pagetable_pte_ctor(struct ptdesc *ptdesc)
> > +{
> > +	if (!ptlock_init(ptdesc))
> > +		return false;
> > +
> > +	__pagetable_pte_ctor(ptdesc);
> >   	return true;
> >   }
> >   
> > -static inline void pagetable_pte_dtor(struct ptdesc *ptdesc)
> > +static inline void __pagetable_pte_dtor(struct ptdesc *ptdesc)
> >   {
> >   	struct folio *folio = ptdesc_folio(ptdesc);
> >   
> > -	ptlock_free(ptdesc);
> >   	__folio_clear_pgtable(folio);
> >   	lruvec_stat_sub_folio(folio, NR_PAGETABLE);
> >   }
> >   
> > +static inline void pagetable_pte_dtor(struct ptdesc *ptdesc)
> > +{
> > +	ptlock_free(ptdesc);
> > +	__pagetable_pte_dtor(ptdesc);
> > +}
> > +
> >   pte_t *__pte_offset_map(pmd_t *pmd, unsigned long addr, pmd_t *pmdvalp);
> >   static inline pte_t *pte_offset_map(pmd_t *pmd, unsigned long addr)
> >   {
> > --
> > 2.30.2