linux-kernel - Re: [PATCH v2 6/6] mm: Introduce ctor/dtor at PGD level

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Message-ID: <20250122074954.8685-A-hca@linux.ibm.com>
Date: Wed, 22 Jan 2025 08:49:54 +0100
From: Heiko Carstens <hca@...ux.ibm.com>
To: Alexander Gordeev <agordeev@...ux.ibm.com>
Cc: Kevin Brodsky <kevin.brodsky@....com>, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Mike Rapoport (IBM)" <rppt@...nel.org>,
        Ryan Roberts <ryan.roberts@....com>,
        Thomas Gleixner <tglx@...utronix.de>, Will Deacon <will@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Qi Zheng <zhengqi.arch@...edance.com>, linux-alpha@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-csky@...r.kernel.org, linux-hexagon@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-m68k@...ts.linux-m68k.org,
        linux-mips@...r.kernel.org, linux-openrisc@...r.kernel.org,
        linux-parisc@...r.kernel.org, linux-riscv@...ts.infradead.org,
        linux-s390@...r.kernel.org, linux-snps-arc@...ts.infradead.org,
        linux-um@...ts.infradead.org, loongarch@...ts.linux.dev,
        x86@...nel.org
Subject: Re: [PATCH v2 6/6] mm: Introduce ctor/dtor at PGD level

On Tue, Jan 21, 2025 at 05:37:33PM +0100, Alexander Gordeev wrote:
> On Fri, Jan 03, 2025 at 06:44:15PM +0000, Kevin Brodsky wrote:
> 
> Hi Kevin,
> ...
> > diff --git a/arch/s390/include/asm/pgalloc.h b/arch/s390/include/asm/pgalloc.h
> > index 5fced6d3c36b..b19b6ed2ab53 100644
> > --- a/arch/s390/include/asm/pgalloc.h
> > +++ b/arch/s390/include/asm/pgalloc.h
> > @@ -130,11 +130,18 @@ static inline void pud_populate(struct mm_struct *mm, pud_t *pud, pmd_t *pmd)
> >  
> >  static inline pgd_t *pgd_alloc(struct mm_struct *mm)
> >  {
> > -	return (pgd_t *) crst_table_alloc(mm);
> > +	unsigned long *table = crst_table_alloc(mm);
> > +
> > +	if (!table)
> > +		return NULL;
> 
> I do not know status of this series, but FWIW, this call is missed:
> 
> 	crst_table_init(table, _REGION1_ENTRY_EMPTY); 

Why is that missing?

A pgd table can be a Region1, Region2, or Region3 table. The only caller of
this function is mm_init() via mm_alloc_pgd(); and right after mm_alloc_pgd()
there is a call to init_new_context() which will initialize the pgd correctly.

I guess what really gets odd, and might be broken (haven't checked yet) is
what happens on dynamic upgrade of page table levels (->crst_table_upgrade()).

With that a pgd may become a pud, and with that we get an imbalance with
the ctor/dtor calls for the various page table levels when they get freed
again. Plus, at first glance, it looks also broken that we have open-coded
crst_alloc() calls instead of using the "proper" page table allocation API
within crst_table_upgrade(), which again would cause an imbalance.