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

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Message-ID: <Z5D7TSR3AttkC0Jf@li-008a6a4c-3549-11b2-a85c-c5cc2836eea2.ibm.com>
Date: Wed, 22 Jan 2025 15:06:05 +0100
From: Alexander Gordeev <agordeev@...ux.ibm.com>
To: Heiko Carstens <hca@...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 Wed, Jan 22, 2025 at 08:49:54AM +0100, Heiko Carstens wrote:
> > >  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?

Because the follow-up pagetable_pgd_ctor() is called against uninitialized
page table, while other pagetable_pXd_ctor() variants are called against
initialized one. I could imagine complications as result of that.

Whether Region1 table is the right choice is a big question though, as you
noticed below.

> 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.

init_new_context() is in a way a constructor as well, so whole thing looks odd
to me. But I do not immedeately see a better way :(

> 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()).

Hmm, that is a good point.

> 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

The ctor/dtor mismatch should not be a problem, as pagetable_pgd|p4d|pud_ctor()
are the same and there is one pagetable_dtor() for all top levels as of now.
But if it ever comes to separate implementations, then we are in the world
of pain.

> 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.

This is a good point too.

Many thanks!