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

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Date: Mon, 8 Jan 2024 03:30:21 +0000
From: "Huang, Kai" <kai.huang@...el.com>
To: "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>, "mingo@...hat.com"
	<mingo@...hat.com>, "dave.hansen@...ux.intel.com"
	<dave.hansen@...ux.intel.com>, "x86@...nel.org" <x86@...nel.org>,
	"bp@...en8.de" <bp@...en8.de>
CC: "kexec@...ts.infradead.org" <kexec@...ts.infradead.org>, "Edgecombe, Rick
 P" <rick.p.edgecombe@...el.com>, "Reshetova, Elena"
	<elena.reshetova@...el.com>, "rafael@...nel.org" <rafael@...nel.org>,
	"Nakajima, Jun" <jun.nakajima@...el.com>, "peterz@...radead.org"
	<peterz@...radead.org>, "sathyanarayanan.kuppuswamy@...ux.intel.com"
	<sathyanarayanan.kuppuswamy@...ux.intel.com>, "Hunter, Adrian"
	<adrian.hunter@...el.com>, "thomas.lendacky@....com"
	<thomas.lendacky@....com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "ashish.kalra@....com"
	<ashish.kalra@....com>, "seanjc@...gle.com" <seanjc@...gle.com>,
	"bhe@...hat.com" <bhe@...hat.com>, "linux-coco@...ts.linux.dev"
	<linux-coco@...ts.linux.dev>
Subject: Re: [PATCHv5 15/16] x86/mm: Introduce kernel_ident_mapping_free()

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.