Re: [PATCH v2 2/8] s390x/mm: Implement arch_remove_memory()

[Dan Williams]

On Tue, May 7, 2019 at 1:47 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 07.05.19 22:46, Dan Williams wrote:
> > On Tue, May 7, 2019 at 11:38 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
> >>
> >> Will come in handy when wanting to handle errors after
> >> arch_add_memory().
> >>
> >> Cc: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
> >> Cc: Heiko Carstens <heiko.carstens@xxxxxxxxxx>
> >> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> >> Cc: Michal Hocko <mhocko@xxxxxxxx>
> >> Cc: Mike Rapoport <rppt@xxxxxxxxxxxxxxxxxx>
> >> Cc: David Hildenbrand <david@xxxxxxxxxx>
> >> Cc: Vasily Gorbik <gor@xxxxxxxxxxxxx>
> >> Cc: Oscar Salvador <osalvador@xxxxxxxx>
> >> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> >> ---
> >>  arch/s390/mm/init.c | 13 +++++++------
> >>  1 file changed, 7 insertions(+), 6 deletions(-)
> >>
> >> diff --git a/arch/s390/mm/init.c b/arch/s390/mm/init.c
> >> index 31b1071315d7..1e0cbae69f12 100644
> >> --- a/arch/s390/mm/init.c
> >> +++ b/arch/s390/mm/init.c
> >> @@ -237,12 +237,13 @@ int arch_add_memory(int nid, u64 start, u64 size,
> >>  void arch_remove_memory(int nid, u64 start, u64 size,
> >>                         struct vmem_altmap *altmap)
> >>  {
> >> -       /*
> >> -        * There is no hardware or firmware interface which could trigger a
> >> -        * hot memory remove on s390. So there is nothing that needs to be
> >> -        * implemented.
> >> -        */
> >> -       BUG();
> >> +       unsigned long start_pfn = start >> PAGE_SHIFT;
> >> +       unsigned long nr_pages = size >> PAGE_SHIFT;
> >> +       struct zone *zone;
> >> +
> >> +       zone = page_zone(pfn_to_page(start_pfn));
> >
> > Does s390 actually support passing in an altmap? If 'yes', I think it
> > also needs the vmem_altmap_offset() fixup like x86-64:
> >
> >         /* With altmap the first mapped page is offset from @start */
> >         if (altmap)
> >                 page += vmem_altmap_offset(altmap);
> >
> > ...but I suspect it does not support altmap since
> > arch/s390/mm/vmem.c::vmemmap_populate() does not arrange for 'struct
> > page' capacity to be allocated out of an altmap defined page pool.
> >
> > I think it would be enough to disallow any arch_add_memory() on s390
> > where @altmap is non-NULL. At least until s390 gains ZONE_DEVICE
> > support and can enable the pmem use case.
> >
>
> As far as I know, it doesn't yet, however I guess this could change once
> virtio-pmem is supported?

I would expect and request virtio-pmem remain a non-starter on s390
until s390 gains ZONE_DEVICE support. As it stands virtio-pmem is just
another flavor of the general pmem driver and the pmem driver
currently only exports ZONE_DEVICE pfns tagged by the PTE_DEVMAP
pte-flag and PFN_DEV+PFN_MAP pfn_t-flags.

A hamstrung version of DAX (CONFIG_FS_DAX_LIMITED) is enabled for the
s390/dcssblk driver, but that requires that the driver indicate this
limited use case via the PTE_SPECIAL pte-flag and PFN_SPECIAL
pfn_t-flag. I otherwise do not want to see CONFIG_FS_DAX_LIMITED
spread outside of the s390/dcssblk use case.