Re: [PATCH v3 1/2] x86/setup: don't remove E820_TYPE_RAM for pfn 0

[Mike Rapoport <rppt@kernel.org>]

On Wed, Jan 13, 2021 at 01:56:45PM +0100, David Hildenbrand wrote:
> On 11.01.21 20:40, Mike Rapoport wrote:
> > From: Mike Rapoport <rppt@linux.ibm.com>
> > 
> > The first 4Kb of memory is a BIOS owned area and to avoid its allocation
> > for the kernel it was not listed in e820 tables as memory. As the result,
> > pfn 0 was never recognised by the generic memory management and it is not a
> > part of neither node 0 nor ZONE_DMA.
> > 
> > If set_pfnblock_flags_mask() would be ever called for the pageblock
> > corresponding to the first 2Mbytes of memory, having pfn 0 outside of
> > ZONE_DMA would trigger
> > 
> > 	VM_BUG_ON_PAGE(!zone_spans_pfn(page_zone(page), pfn), page);
> > 
> > Along with reserving the first 4Kb in e820 tables, several first pages are
> > reserved with memblock in several places during setup_arch(). These
> > reservations are enough to ensure the kernel does not touch the BIOS area
> > and it is not necessary to remove E820_TYPE_RAM for pfn 0.
> > 
> > Remove the update of e820 table that changes the type of pfn 0 and move the
> > comment describing why it was done to trim_low_memory_range() that reserves
> > the beginning of the memory.
> > 
> > Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
> > ---
> >  arch/x86/kernel/setup.c | 20 +++++++++-----------
> >  1 file changed, 9 insertions(+), 11 deletions(-)
> > 
> > diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
> > index 740f3bdb3f61..3412c4595efd 100644
> > --- a/arch/x86/kernel/setup.c
> > +++ b/arch/x86/kernel/setup.c
> > @@ -660,17 +660,6 @@ static void __init trim_platform_memory_ranges(void)
> >  
> >  static void __init trim_bios_range(void)
> >  {
> > -	/*
> > -	 * A special case is the first 4Kb of memory;
> > -	 * This is a BIOS owned area, not kernel ram, but generally
> > -	 * not listed as such in the E820 table.
> > -	 *
> > -	 * This typically reserves additional memory (64KiB by default)
> > -	 * since some BIOSes are known to corrupt low memory.  See the
> > -	 * Kconfig help text for X86_RESERVE_LOW.
> > -	 */
> > -	e820__range_update(0, PAGE_SIZE, E820_TYPE_RAM, E820_TYPE_RESERVED);
> > -
> >  	/*
> >  	 * special case: Some BIOSes report the PC BIOS
> >  	 * area (640Kb -> 1Mb) as RAM even though it is not.
> > @@ -728,6 +717,15 @@ early_param("reservelow", parse_reservelow);
> >  
> >  static void __init trim_low_memory_range(void)
> >  {
> > +	/*
> > +	 * A special case is the first 4Kb of memory;
> > +	 * This is a BIOS owned area, not kernel ram, but generally
> > +	 * not listed as such in the E820 table.
> > +	 *
> > +	 * This typically reserves additional memory (64KiB by default)
> > +	 * since some BIOSes are known to corrupt low memory.  See the
> > +	 * Kconfig help text for X86_RESERVE_LOW.
> > +	 */
> >  	memblock_reserve(0, ALIGN(reserve_low, PAGE_SIZE));
> >  }
> >  	
> > 
> 
> The only somewhat-confusing thing is that in-between
> e820__memblock_setup() and trim_low_memory_range(), we already have
> memblock allocations. So [0..4095] might look like ordinary memory until
> we reserve it later on.
> 
> E.g., reserve_real_mode() does a
> 
> mem = memblock_find_in_range(0, 1<<20, size, PAGE_SIZE);
> ...
> memblock_reserve(mem, size);
> set_real_mode_mem(mem);
> 
> which looks kind of suspicious to me. Most probably I am missing
> something, just wanted to point that out. We might want to do such
> trimming/adjustments before any kind of allocations.

You are right and it looks suspicious, but the first page is reserved at
the very beginning of x86::setup_arch() and, moreover, memblock never
allocates it (look at memblock::memblock_find_in_range_node()).

As for the range 0x1000 <-> reserve_low, we are unlikely to allocate it in
the default top-down mode. The bottom-up mode was only allocating memory
above the kernel so this would also prevent allocation of the lowest
memory, at least until the recent changes for CMA allocation:

https://lore.kernel.org/lkml/20201217201214.3414100-1-guro@fb.com

That said, we'd better consolidate all the trim_some_memory() and move it
closer to the beginning of setup_arch().
I'm going to take a look at it in the next few days.
 
> -- 
> Thanks,
> 
> David / dhildenb
> 

-- 
Sincerely yours,
Mike.