Subject: Re: [PATCH mm-unstable] arch, mm: consolidate empty_zero_page

From: Mike Rapoport <[email-masked]>
Date: Tue, 27 Jan 2026 20:11:19 +0200

Hi,

On Tue, Jan 27, 2026 at 05:02:39PM +0100, Andreas Larsson wrote:
> On 2026-01-24 10:56, Mike Rapoport wrote:
>
> > Every architecture defines empty_zero_page that way or another, but for the
> > most of them it is always a page aligned page in BSS and most definitions
> > of ZERO_PAGE do virt_to_page(empty_zero_page).
> 
> Running this in an LDOM on an UltraSparc T4 sparc64, the entire LDOM
> hangs after a while during boot.
> 
> > diff --git a/arch/sparc/mm/init_64.c b/arch/sparc/mm/init_64.c
> > index c2d19c9a9244..2bd99944176d 100644
> > --- a/arch/sparc/mm/init_64.c
> > +++ b/arch/sparc/mm/init_64.c
> > @@ -177,9 +177,6 @@ extern unsigned long sparc_ramdisk_image64;
> >  extern unsigned int sparc_ramdisk_image;
> >  extern unsigned int sparc_ramdisk_size;
> >  
> > -struct page *mem_map_zero __read_mostly;
> > -EXPORT_SYMBOL(mem_map_zero);
> > -
> >  unsigned int sparc64_highest_unlocked_tlb_ent __read_mostly;
> >  
> >  unsigned long sparc64_kern_pri_context __read_mostly;
> > @@ -2506,18 +2503,6 @@ void __init mem_init(void)
> >  	 */
> >  	register_page_bootmem_info();
> >  
> > -	/*
> > -	 * Set up the zero page, mark it reserved, so that page count
> > -	 * is not manipulated when freeing the page from user ptes.
> > -	 */
> > -	mem_map_zero = alloc_pages(GFP_KERNEL|__GFP_ZERO, 0);
> > -	if (mem_map_zero == NULL) {
> > -		prom_printf("paging_init: Cannot alloc zero page.\n");
> > -		prom_halt();
> > -	}
> > -	mark_page_reserved(mem_map_zero);
> > -
> > -
> >  	if (tlb_type == cheetah || tlb_type == cheetah_plus)
> >  		cheetah_ecache_flush_init();
> >  }
> 
> This just removes the mark_page_reserved(mem_map_zero) without 
> replacing it with something corresponding to that. Perhaps part
> of the problem?

I don't think so, empty_zero_page is in BSS now an it's reserved as a part
of the kernel image.

I suspect that virt_to_page() does not work BSS symbols on sparc64. Can you
please try with this patch:

diff --git a/arch/sparc/include/asm/pgtable_64.h b/arch/sparc/include/asm/pgtable_64.h
index 74ede706fb32..0578c5172d4e 100644
--- a/arch/sparc/include/asm/pgtable_64.h
+++ b/arch/sparc/include/asm/pgtable_64.h
@@ -22,6 +22,7 @@
 #include <asm/adi.h>
 #include <asm/page.h>
 #include <asm/processor.h>
+#include <asm/vaddrs.h>
 
 /* The kernel image occupies 0x4000000 to 0x6000000 (4MB --> 96MB).
  * The page copy blockops can use 0x6000000 to 0x8000000.
@@ -210,6 +211,11 @@ extern unsigned long _PAGE_CACHE;
 extern unsigned long pg_iobits;
 extern unsigned long _PAGE_ALL_SZ_BITS;
 
+extern unsigned long kern_base;
+#define ZERO_PAGE(vaddr)						   \
+	(virt_to_page(empty_zero_page + ((unsigned long)__va(kern_base)) - \
+		      ((unsigned long)KERNBASE)))
+
 /* PFNs are real physical page numbers.  However, mem_map only begins to record
  * per-page information starting at pfn_base.  This is to handle systems where
  * the first physical page in the machine is at some huge physical address,
diff --git a/arch/sparc/mm/init_64.c b/arch/sparc/mm/init_64.c
index 2bd99944176d..d2d724ba4f83 100644
--- a/arch/sparc/mm/init_64.c
+++ b/arch/sparc/mm/init_64.c
@@ -170,6 +170,8 @@ static void __init read_obp_memory(const char *property,
 
 /* Kernel physical address base and size in bytes.  */
 unsigned long kern_base __read_mostly;
+EXPORT_SYMBOL(kern_base);
+
 unsigned long kern_size __read_mostly;
 
 /* Initial ramdisk setup */
 
> Cheers,
> Andreas

-- 
Sincerely yours,
Mike.