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@xxxxxxxxx>
> > CC: Jason Lunz <lunz@xxxxxxxxxxxx>
> > CC: Jeff Dike <jdike@xxxxxxxxxxxxxxx>
> > Cc: Paolo 'Blaisorblade' Giarrusso <blaisorblade@xxxxxxxx>
> > Cc: Alasdair G Kergon <agk@xxxxxxxxxx>
> > Cc: Jens Axboe <jens.axboe@xxxxxxxxxx>
> > CC: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > CC: Mike Rapoport (IBM) <rppt@xxxxxxxxxx>
> > CC: David Hildenbrand <david@xxxxxxxxxx>
> >
> > ---
> > 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@xxxxxxxxxx>
>
>
> 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.
