linux-kernel - Re: [PATCHv5 15/16] x86/mm: Introduce kernel_ident_mapping

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Date: Mon, 8 Jan 2024 16:35:36 +0300
From: "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>
To: "Huang, Kai" <kai.huang@...el.com>
Cc: "kexec@...ts.infradead.org" <kexec@...ts.infradead.org>,
	"linux-coco@...ts.linux.dev" <linux-coco@...ts.linux.dev>,
	"ashish.kalra@....com" <ashish.kalra@....com>,
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
	"thomas.lendacky@....com" <thomas.lendacky@....com>,
	"Hunter, Adrian" <adrian.hunter@...el.com>,
	"Reshetova, Elena" <elena.reshetova@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"seanjc@...gle.com" <seanjc@...gle.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"bhe@...hat.com" <bhe@...hat.com>,
	"Nakajima, Jun" <jun.nakajima@...el.com>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
	"bp@...en8.de" <bp@...en8.de>,
	"rafael@...nel.org" <rafael@...nel.org>,
	"sathyanarayanan.kuppuswamy@...ux.intel.com" <sathyanarayanan.kuppuswamy@...ux.intel.com>,
	"x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCHv5 15/16] x86/mm: Introduce kernel_ident_mapping_free()

On Mon, Jan 08, 2024 at 01:13:18PM +0000, Huang, Kai wrote:
> On Mon, 2024-01-08 at 13:17 +0300, kirill.shutemov@...ux.intel.com wrote:
> > On Mon, Jan 08, 2024 at 03:30:21AM +0000, Huang, Kai wrote:
> > > On Mon, 2024-01-08 at 03:13 +0000, Huang, Kai wrote:
> > > > On Sat, 2023-12-23 at 02:52 +0300, Kirill A. Shutemov wrote:
> > > > > The helper complements kernel_ident_mapping_init(): it frees the
> > > > > identity mapping that was previously allocated. It will be used in the
> > > > > error path to free a partially allocated mapping or if the mapping is no
> > > > > longer needed.
> > > > > 
> > > > > The caller provides a struct x86_mapping_info with the free_pgd_page()
> > > > > callback hooked up and the pgd_t to free.
> > > > > 
> > > > > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> > > > > ---
> > > > >  arch/x86/include/asm/init.h |  3 ++
> > > > >  arch/x86/mm/ident_map.c     | 73 +++++++++++++++++++++++++++++++++++++
> > > > >  2 files changed, 76 insertions(+)
> > > > > 
> > > > > diff --git a/arch/x86/include/asm/init.h b/arch/x86/include/asm/init.h
> > > > > index cc9ccf61b6bd..14d72727d7ee 100644
> > > > > --- a/arch/x86/include/asm/init.h
> > > > > +++ b/arch/x86/include/asm/init.h
> > > > > @@ -6,6 +6,7 @@
> > > > >  
> > > > >  struct x86_mapping_info {
> > > > >  	void *(*alloc_pgt_page)(void *); /* allocate buf for page table */
> > > > > +	void (*free_pgt_page)(void *, void *); /* free buf for page table */
> > > > >  	void *context;			 /* context for alloc_pgt_page */
> > > > >  	unsigned long page_flag;	 /* page flag for PMD or PUD entry */
> > > > >  	unsigned long offset;		 /* ident mapping offset */
> > > > > @@ -16,4 +17,6 @@ struct x86_mapping_info {
> > > > >  int kernel_ident_mapping_init(struct x86_mapping_info *info, pgd_t *pgd_page,
> > > > >  				unsigned long pstart, unsigned long pend);
> > > > >  
> > > > > +void kernel_ident_mapping_free(struct x86_mapping_info *info, pgd_t *pgd);
> > > > 
> > > > Maybe range-based free function can provide more flexibility (e.g., you can
> > > > directly call the free function to cleanup in kernel_ident_mapping_init()
> > > > internally when something goes wrong), but I guess this is sufficient for
> > > > current use case (and perhaps the majority use cases).
> > > > 
> > > > Reviewed-by: Kai Huang <kai.huang@...el.com>
> > > > 
> > > 
> > > Another argument of range-based free function is, theoretically you can build
> > > the identical mapping table using different x86_mapping_info on different
> > > ranges, thus it makes less sense to use one 'struct x86_mapping_info *info' to
> > > free the entire page table, albeit in this implementation only the
> > > 'free_pgt_page()' callback is used. 
> > 
> > The interface can be changed if there will be need for such behaviour.
> > This kind of future-proofing rarely helpful.
> > 
> 
> Do you want to just pass the 'free_pgt_page' function pointer to
> kernel_ident_mapping_free(), instead of 'struct x86_mapping_info *info'?  As
> mentioned above conceptually the page table can be built from multiple
> x86_mapping_info for multiple ranges.

I don't think we have such cases in kernel. Let's not overcomplicate
things. I see value in keeping interface symmetric.

We can always change things according to needs.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov