Re: [PATCH] um/mm: get max_low_pfn from memblock

[Mike Rapoport <rppt@kernel.org>]

On Fri, Jun 14, 2024 at 09:31:59AM +0200, David Hildenbrand wrote:
> On 14.06.24 03:58, Wei Yang wrote:
> > Current calculation of max_low_pfn is introduced in commit af84eab20891
> > ("[PATCH] uml: fix LVM crash"). It is intended to set max_low_pfn to the
> > same value as max_pfn.
> > 
> > But I am not sure why the max_pfn is set to totalram_pages, which
> > represents the number of usable pages in system instead of an absolute
> > page frame number. (The change history stops there.)
> > 
> > While we can get the maximum page frame number from memblock, this looks
> > more reasonable than setting to totalram_pages.
> > 
> > Also this would help changing totalram_pages accounting, since we plan
> > to move the accounting into __free_pages_core(). With this change,
> > totalram_pages may not represent the total usable pages at this point,
> > since some pages would be deferred initialized.
> 
> Question is if deferred page init is even a thing on UM. But it certainly looks odd+suspiciously wrong to rely on totalram_pages().
 
As long as there is no HIGHMEM, 

max_pfn = max_low_pfn = PFN_DOWN(memblock_end_of_DRAM())

should be ok.
 
> > Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
> > CC: Jason Lunz <lunz@falooley.org>
> > CC: Jeff Dike <jdike@linux.intel.com>
> > Cc: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
> > Cc: Alasdair G Kergon <agk@redhat.com>
> > Cc: Jens Axboe <jens.axboe@oracle.com>
> > CC: Andrew Morton <akpm@linux-foundation.org>
> > CC: Mike Rapoport (IBM) <rppt@kernel.org>
> > CC: David Hildenbrand <david@redhat.com>
> > 
> > ---
> > A simple UML bootup test looks good.
> > ---
> >   arch/um/kernel/mem.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/arch/um/kernel/mem.c b/arch/um/kernel/mem.c
> > index ca91accd64fc..ca682879e28f 100644
> > --- a/arch/um/kernel/mem.c
> > +++ b/arch/um/kernel/mem.c
> > @@ -73,7 +73,7 @@ void __init mem_init(void)
> >   	/* this will put all low memory onto the freelists */
> >   	memblock_free_all();
> > -	max_low_pfn = totalram_pages();
> > +	max_low_pfn = PFN_DOWN(memblock_end_of_DRAM());

This assignment seem redundant as there is already

	max_low_pfn = min_low_pfn + (map_size >> PAGE_SHIFT);

in setup_physmem

> >   	max_pfn = max_low_pfn;
> >   	kmalloc_ok = 1;
> >   }
> 
> Matches what a bunch of other archs do.
> 
> Acked-by: David Hildenbrand <david@redhat.com>
> 
> 
> Randomly staring at other users:
> 
> arch/loongarch/kernel/numa.c:   max_low_pfn = PHYS_PFN(memblock_end_of_DRAM());
> arch/loongarch/kernel/setup.c:  max_low_pfn = PFN_PHYS(memblock_end_of_DRAM());
> 
> What? the latter cannot possibly be right, no? Looks odd at least.
> Only applies to CONFIG_OF_EARLY_FLATTREE. CCing loongarch maintainer.
> 
> -- 
> Cheers,
> 
> David / dhildenb
> 

-- 
Sincerely yours,
Mike.