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

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Message-ID: <Z5FgSn0y/c9pItED@li-008a6a4c-3549-11b2-a85c-c5cc2836eea2.ibm.com>
Date: Wed, 22 Jan 2025 22:16:58 +0100
From: Alexander Gordeev <agordeev@...ux.ibm.com>
To: Heiko Carstens <hca@...ux.ibm.com>, Kevin Brodsky <kevin.brodsky@....com>
Cc: 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 03:06:05PM +0100, Alexander Gordeev wrote:

Hi Kevin,

> 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.

As discussed with Heiko, we do not want to add the extra crst_table_init() call
at least due to performance impact. So please ignore my comment.

> > 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.

The below bits are seems to be missed. We will test it and send a patch.

diff --git a/arch/s390/mm/pgalloc.c b/arch/s390/mm/pgalloc.c
index a4e761902093..d33f55b7ee98 100644
--- a/arch/s390/mm/pgalloc.c
+++ b/arch/s390/mm/pgalloc.c
@@ -88,12 +88,14 @@ int crst_table_upgrade(struct mm_struct *mm, unsigned long end)
 		if (unlikely(!p4d))
 			goto err_p4d;
 		crst_table_init(p4d, _REGION2_ENTRY_EMPTY);
+		pagetable_p4d_ctor(virt_to_ptdesc(p4d));
 	}
 	if (end > _REGION1_SIZE) {
 		pgd = crst_table_alloc(mm);
 		if (unlikely(!pgd))
 			goto err_pgd;
 		crst_table_init(pgd, _REGION1_ENTRY_EMPTY);
+		pagetable_pgd_ctor(virt_to_ptdesc(pgd));
 	}
 
 	spin_lock_bh(&mm->page_table_lock);
@@ -130,6 +132,7 @@ int crst_table_upgrade(struct mm_struct *mm, unsigned long end)
 	return 0;
 
 err_pgd:
+	pagetable_dtor(virt_to_ptdesc(p4d));
 	crst_table_free(mm, p4d);
 err_p4d:
 	return -ENOMEM;

Thanks!