Re: [PATCHv7 08/14] x86/mm: Reserve unaccepted memory bitmap

[Kirill A. Shutemov]

On Thu, Dec 01, 2022 at 11:37:10AM +0200, Mike Rapoport wrote:
> On Wed, Nov 30, 2022 at 04:28:40AM +0300, Kirill A. Shutemov wrote:
> > On Tue, Jul 26, 2022 at 11:07:14AM +0200, Borislav Petkov wrote:
> > > On Tue, Jun 14, 2022 at 03:02:25PM +0300, Kirill A. Shutemov wrote:
> > > > diff --git a/arch/x86/kernel/e820.c b/arch/x86/kernel/e820.c
> > > > index f267205f2d5a..22d1fe48dcba 100644
> > > > --- a/arch/x86/kernel/e820.c
> > > > +++ b/arch/x86/kernel/e820.c
> > > > @@ -1316,6 +1316,16 @@ void __init e820__memblock_setup(void)
> > > >  	int i;
> > > >  	u64 end;
> > > >  
> > > > +	/* Mark unaccepted memory bitmap reserved */
> > > > +	if (boot_params.unaccepted_memory) {
> > > > +		unsigned long size;
> > > > +
> > > > +		/* One bit per 2MB */
> > > > +		size = DIV_ROUND_UP(e820__end_of_ram_pfn() * PAGE_SIZE,
> > > > +				    PMD_SIZE * BITS_PER_BYTE);
> > > > +		memblock_reserve(boot_params.unaccepted_memory, size);
> > > > +	}
> > > > +
> > > 
> > > Hmm, I don't like how this is dropped right in the middle of a unrelated
> > > function.
> > > 
> > > You're adding arch/x86/mm/unaccepted_memory.c later. Why don't you put
> > > that chunk in a function there which is called by early_reserve_memory()
> > > which does exactly what you want - reserve memory early, before memblock
> > > allocations?
> > 
> > early_reserve_memory() specifically called before e820__memory_setup()
> > (see comment in setup_arch()), so we don't have e820_table finalized and
> > we need it to get correct RAM size from e820__end_of_ram_pfn().
> > 
> > I guess we can hide the chunk in a function in unaccepted_memory.c and
> > call it from here, but it would require #ifdeffery in a header file as the
> > .c is only compiled for CONFIG_UNACCEPTED_MEMORY=y.
> > 
> > Looks like an overkill to me, no?
> 
> Agree. Can we just extend the comment to explain why we reserve the bitmap
> at e820__memblock_setup() rather than in early_reserve_memory(), pretty
> much with the explanation above?

Okay, I will do this:

diff --git a/arch/x86/kernel/e820.c b/arch/x86/kernel/e820.c
index 49b5164a4cba..62068956bb76 100644
--- a/arch/x86/kernel/e820.c
+++ b/arch/x86/kernel/e820.c
@@ -1316,7 +1316,14 @@ void __init e820__memblock_setup(void)
 	int i;
 	u64 end;
 
-	/* Mark unaccepted memory bitmap reserved */
+	/*
+	 * Mark unaccepted memory bitmap reserved.
+	 *
+	 * This kind of reservation usually done from early_reserve_memory(),
+	 * but early_reserve_memory() called before e820__memory_setup(), so
+	 * e820_table is not finalized and e820__end_of_ram_pfn() cannot be
+	 * used to get correct RAM size.
+	 */
 	if (boot_params.unaccepted_memory) {
 		unsigned long size;
 
-- 
  Kiryl Shutsemau / Kirill A. Shutemov