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

[David Hildenbrand <david@redhat.com>]

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().

> 
> 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());
>   	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