linux-kernel - Re: [PATCH] x86/mm: Fix boot with some memory above MAXMEM

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Date:   Mon, 25 May 2020 18:08:20 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Tony Luck <tony.luck@...el.com>, x86@...nel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Dave Hansen <dave.hansen@...el.com>, stable@...r.kernel.org
Subject: Re: [PATCH] x86/mm: Fix boot with some memory above MAXMEM

On Mon, May 25, 2020 at 05:59:43PM +0300, Mike Rapoport wrote:
> On Mon, May 25, 2020 at 07:49:02AM +0300, Kirill A. Shutemov wrote:
> > On Mon, May 11, 2020 at 10:17:21PM +0300, Kirill A. Shutemov wrote:
> > > A 5-level paging capable machine can have memory above 46-bit in the
> > > physical address space. This memory is only addressable in the 5-level
> > > paging mode: we don't have enough virtual address space to create direct
> > > mapping for such memory in the 4-level paging mode.
> > > 
> > > Currently, we fail boot completely: NULL pointer dereference in
> > > subsection_map_init().
> > > 
> > > Skip creating a memblock for such memory instead and notify user that
> > > some memory is not addressable.
> > > 
> > > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> > > Reviewed-by: Dave Hansen <dave.hansen@...el.com>
> > > Cc: stable@...r.kernel.org # v4.14
> > > ---
> > 
> > Gentle ping.
> > 
> > It's not urgent, but it's a bug fix. Please consider applying.
> > 
> > > Tested with a hacked QEMU: https://gist.github.com/kiryl/d45eb54110944ff95e544972d8bdac1d
> > > 
> > > ---
> > >  arch/x86/kernel/e820.c | 19 +++++++++++++++++--
> > >  1 file changed, 17 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/arch/x86/kernel/e820.c b/arch/x86/kernel/e820.c
> > > index c5399e80c59c..d320d37d0f95 100644
> > > --- a/arch/x86/kernel/e820.c
> > > +++ b/arch/x86/kernel/e820.c
> > > @@ -1280,8 +1280,8 @@ void __init e820__memory_setup(void)
> > >  
> > >  void __init e820__memblock_setup(void)
> > >  {
> > > +	u64 size, end, not_addressable = 0;
> > >  	int i;
> > > -	u64 end;
> > >  
> > >  	/*
> > >  	 * The bootstrap memblock region count maximum is 128 entries
> > > @@ -1307,7 +1307,22 @@ void __init e820__memblock_setup(void)
> > >  		if (entry->type != E820_TYPE_RAM && entry->type != E820_TYPE_RESERVED_KERN)
> > >  			continue;
> > >  
> > > -		memblock_add(entry->addr, entry->size);
> > > +		if (entry->addr >= MAXMEM) {
> > > +			not_addressable += entry->size;
> > > +			continue;
> > > +		}
> > > +
> > > +		end = min_t(u64, end, MAXMEM - 1);
> > > +		size = end - entry->addr;
> > > +		not_addressable += entry->size - size;
> > > +		memblock_add(entry->addr, size);
> > > +	}
> > > +
> > > +	if (not_addressable) {
> > > +		pr_err("%lldGB of physical memory is not addressable in the paging mode\n",
> > > +		       not_addressable >> 30);
> > > +		if (!pgtable_l5_enabled())
> > > +			pr_err("Consider enabling 5-level paging\n");
> 
> Could this happen at all when l5 is enabled?
> Does it mean we need kmap() for 64-bit?

It's future-profing. Who knows what paging modes we would have in the
future.

-- 
 Kirill A. Shutemov