Re: [PATCH 4.14 023/159] mm/sparsemem: Allocate mem_section at runtime for CONFIG_SPARSEMEM_EXTREME=y

[Omar Sandoval <osandov@osandov.com>]

On Fri, Jan 12, 2018 at 08:55:49AM +0800, Dave Young wrote:
> On 01/10/18 at 02:16pm, Kirill A. Shutemov wrote:
> > On Wed, Jan 10, 2018 at 03:08:04AM +0000, Dave Young wrote:
> > > On Tue, Jan 09, 2018 at 12:05:52PM +0300, Kirill A. Shutemov wrote:
> > > > On Tue, Jan 09, 2018 at 03:24:40PM +0800, Dave Young wrote:
> > > > > On 01/09/18 at 01:41pm, Baoquan He wrote:
> > > > > > On 01/09/18 at 09:09am, Dave Young wrote:
> > > > > > 
> > > > > > > As for the macro name, VMCOREINFO_SYMBOL_ARRAY sounds better.
> > > > 
> > > > Yep, that's better.
> > > > 
> > > > > > I still think using vmcoreinfo_append_str is better. Unless we replace
> > > > > > all array variables with the newly added macro.
> > > > > > 
> > > > > > vmcoreinfo_append_str("SYMBOL(mem_section)=%lx\n",
> > > > > >                                 (unsigned long)mem_section);
> > > > > 
> > > > > I have no strong opinion, either change all array uses or just introduce
> > > > > the macro and start to use it from now on if we have similar array
> > > > > symbols.
> > > > 
> > > > Do you need some action on my side or will you folks take care about this?
> > > 
> > > I think Baoquan was suggesting to update all array users in current
> > > code, if you can check every VMCOREINFO_SYMBOL and update all the arrays
> > > he will be happy. But if can not do it easily I'm fine with a
> > > VMCOREINFO_SYMBOL_ARRAY changes only now, we kdump people can do it
> > > later as well. 
> > 
> > It seems it's the only array we have there. swapper_pg_dir is a potential
> > candidate, but it's 'unsigned long' on arm.
> > 
> > Below it patch with corrected macro name.
> > 
> > Please, consider applying.
> > 
> > From 70f3a84b97f2de98d1364f7b10b7a42a1d8e9968 Mon Sep 17 00:00:00 2001
> > From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
> > Date: Tue, 9 Jan 2018 02:55:47 +0300
> > Subject: [PATCH] kdump: Write a correct address of mem_section into vmcoreinfo
> > 
> > Depending on configuration mem_section can now be an array or a pointer
> > to an array allocated dynamically. In most cases, we can continue to refer
> > to it as 'mem_section' regardless of what it is.
> > 
> > But there's one exception: '&mem_section' means "address of the array" if
> > mem_section is an array, but if mem_section is a pointer, it would mean
> > "address of the pointer".
> > 
> > We've stepped onto this in kdump code. VMCOREINFO_SYMBOL(mem_section)
> > writes down address of pointer into vmcoreinfo, not array as we wanted.
> > 
> > Let's introduce VMCOREINFO_SYMBOL_ARRAY() that would handle the
> > situation correctly for both cases.
> > 
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> > Fixes: 83e3c48729d9 ("mm/sparsemem: Allocate mem_section at runtime for CONFIG_SPARSEMEM_EXTREME=y")
> > ---
> >  include/linux/crash_core.h | 2 ++
> >  kernel/crash_core.c        | 2 +-
> >  2 files changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/include/linux/crash_core.h b/include/linux/crash_core.h
> > index 06097ef30449..b511f6d24b42 100644
> > --- a/include/linux/crash_core.h
> > +++ b/include/linux/crash_core.h
> > @@ -42,6 +42,8 @@ phys_addr_t paddr_vmcoreinfo_note(void);
> >  	vmcoreinfo_append_str("PAGESIZE=%ld\n", value)
> >  #define VMCOREINFO_SYMBOL(name) \
> >  	vmcoreinfo_append_str("SYMBOL(%s)=%lx\n", #name, (unsigned long)&name)
> > +#define VMCOREINFO_SYMBOL_ARRAY(name) \
> > +	vmcoreinfo_append_str("SYMBOL(%s)=%lx\n", #name, (unsigned long)name)
> >  #define VMCOREINFO_SIZE(name) \
> >  	vmcoreinfo_append_str("SIZE(%s)=%lu\n", #name, \
> >  			      (unsigned long)sizeof(name))
> > diff --git a/kernel/crash_core.c b/kernel/crash_core.c
> > index b3663896278e..4f63597c824d 100644
> > --- a/kernel/crash_core.c
> > +++ b/kernel/crash_core.c
> > @@ -410,7 +410,7 @@ static int __init crash_save_vmcoreinfo_init(void)
> >  	VMCOREINFO_SYMBOL(contig_page_data);
> >  #endif
> >  #ifdef CONFIG_SPARSEMEM
> > -	VMCOREINFO_SYMBOL(mem_section);
> > +	VMCOREINFO_SYMBOL_ARRAY(mem_section);
> >  	VMCOREINFO_LENGTH(mem_section, NR_SECTION_ROOTS);
> >  	VMCOREINFO_STRUCT_SIZE(mem_section);
> >  	VMCOREINFO_OFFSET(mem_section, section_mem_map);
> > -- 
> >  Kirill A. Shutemov
> 
> 
> Acked-by: Dave Young <dyoung@redhat.com>
> 
> If stable kernel took the mem section commits, then should also cc
> stable.  Andrew, can you help to make this in 4.15?
> 
> Thanks
> Dave

Hm, this fix means that the vmlinux symbol table and vmcoreinfo have
different values for mem_section. That seems... odd. I had to patch
makedumpfile to fix the case of an explicit vmlinux being passed on the
command line (which I realized I don't need to do, but it should still
work):