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

[Wei Yang <richard.weiyang@gmail.com>]

On Fri, Jun 14, 2024 at 10:51:32AM +0300, Mike Rapoport wrote:
>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
>

Looks right, I added some log and shows they are the same.

Thanks

>> >   	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.

-- 
Wei Yang
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