[PATCH v1 5/5] s390/sparsemem: reduce section size to 128 MiB

[David Hildenbrand]

Ever since commit 421c175c4d609 ("[S390] Add support for memory hot-add.")
we've been using a section size of 256 MiB on s390x and 32 MiB on s390.
Before that, we were using a section size of 32 MiB on both
architectures.

Likely the reason was that we'd expect a storage increment size of
256 MiB under z/VM back then. As we didn't support memory blocks spanning
multiple memory sections, we would have had to handle having multiple
memory blocks for a single storage increment, which complicates things.
Although that issue reappeared with even bigger storage increment sizes
later, nowadays we have memory blocks that can span multiple memory
sections and we avoid any such issue completely.

Now that we have a new mechanism to expose additional memory to a VM --
virtio-mem -- reduce the section size to 128 MiB to allow for more
flexibility and reduce the metadata overhead when dealing with hot(un)plug
granularity smaller than 256 MiB.

128 MiB has been used by x86-64 since the very beginning. arm64 with 4k
base pages switched to 128 MiB as well: it's just big enough on these
architectures to allows for using a huge page (2 MiB) in the vmemmap in
sane setups with sizeof(struct page) == 64 bytes and a huge page mapping
in the direct mapping, while still allowing for small hot(un)plug
granularity.

For s390x, we could even switch to a 64 MiB section size, as our huge page
size is 1 MiB: but the smaller the section size, the more sections we'll
have to manage especially on bigger machines. Making it consistent with
x86-64 and arm64 feels like te right thing for now.

Note that the smallest memory hot(un)plug granularity is also limited by
the memory block size, determined by extracting the memory increment
size from SCLP. Under QEMU/KVM, implementing virtio-mem, we expose 0;
therefore, we'll end up with a memory block size of 128 MiB with a
128 MiB section size.

Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
 arch/s390/include/asm/sparsemem.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/s390/include/asm/sparsemem.h b/arch/s390/include/asm/sparsemem.h
index c549893602ea..ff628c50afac 100644
--- a/arch/s390/include/asm/sparsemem.h
+++ b/arch/s390/include/asm/sparsemem.h
@@ -2,7 +2,7 @@
 #ifndef _ASM_S390_SPARSEMEM_H
 #define _ASM_S390_SPARSEMEM_H
 
-#define SECTION_SIZE_BITS	28
+#define SECTION_SIZE_BITS	27
 #define MAX_PHYSMEM_BITS	CONFIG_MAX_PHYSMEM_BITS
 
 #endif /* _ASM_S390_SPARSEMEM_H */
-- 
2.46.0