[PATCH v2 2/3] powerpc/mm/hash: Avoid multiple HPT resize-ups on memory hotplug

[Leonardo Bras]

Every time a memory hotplug happens, and the memory limit crosses a 2^n
value, it may be necessary to perform HPT resizing-up, which can take
some time (over 100ms in my tests).

It usually is not an issue, but it can take some time if a lot of memory
is added to a guest with little starting memory:
Adding 256G to a 2GB guest, for example will require 8 HPT resizes.

Perform an HPT resize before memory hotplug, updating HPT to its
final size (considering a successful hotplug), taking the number of
HPT resizes to at most one per memory hotplug action.

Signed-off-by: Leonardo Bras <leobras.c@xxxxxxxxx>
---
 arch/powerpc/include/asm/book3s/64/hash.h     |  2 ++
 arch/powerpc/mm/book3s64/hash_utils.c         | 20 +++++++++++++++++++
 .../platforms/pseries/hotplug-memory.c        |  9 +++++++++
 3 files changed, 31 insertions(+)

diff --git a/arch/powerpc/include/asm/book3s/64/hash.h b/arch/powerpc/include/asm/book3s/64/hash.h
index d959b0195ad9..fad4af8b8543 100644
--- a/arch/powerpc/include/asm/book3s/64/hash.h
+++ b/arch/powerpc/include/asm/book3s/64/hash.h
@@ -255,6 +255,8 @@ int hash__create_section_mapping(unsigned long start, unsigned long end,
 				 int nid, pgprot_t prot);
 int hash__remove_section_mapping(unsigned long start, unsigned long end);
 
+void hash_batch_expand_prepare(unsigned long newsize);
+
 #endif /* !__ASSEMBLY__ */
 #endif /* __KERNEL__ */
 #endif /* _ASM_POWERPC_BOOK3S_64_HASH_H */
diff --git a/arch/powerpc/mm/book3s64/hash_utils.c b/arch/powerpc/mm/book3s64/hash_utils.c
index 608e4ed397a9..3fa395b3fe57 100644
--- a/arch/powerpc/mm/book3s64/hash_utils.c
+++ b/arch/powerpc/mm/book3s64/hash_utils.c
@@ -859,6 +859,26 @@ int hash__remove_section_mapping(unsigned long start, unsigned long end)
 
 	return rc;
 }
+
+void hash_batch_expand_prepare(unsigned long newsize)
+{
+	const u64 starting_size = ppc64_pft_size;
+
+	/*
+	 * Resizing-up HPT should never fail, but there are some cases system starts with higher
+	 * SHIFT than required, and we go through the funny case of resizing HPT down while
+	 * adding memory
+	 */
+
+	while (resize_hpt_for_hotplug(newsize, false) == -ENOSPC) {
+		newsize *= 2;
+		pr_warn("Hash collision while resizing HPT\n");
+
+		/* Do not try to resize to the starting size, or bigger value */
+		if (htab_shift_for_mem_size(newsize) >= starting_size)
+			break;
+	}
+}
 #endif /* CONFIG_MEMORY_HOTPLUG */
 
 static void __init hash_init_partition_table(phys_addr_t hash_table,
diff --git a/arch/powerpc/platforms/pseries/hotplug-memory.c b/arch/powerpc/platforms/pseries/hotplug-memory.c
index 8377f1f7c78e..48b2cfe4ce69 100644
--- a/arch/powerpc/platforms/pseries/hotplug-memory.c
+++ b/arch/powerpc/platforms/pseries/hotplug-memory.c
@@ -13,6 +13,7 @@
 #include <linux/memory.h>
 #include <linux/memory_hotplug.h>
 #include <linux/slab.h>
+#include <linux/pgtable.h>
 
 #include <asm/firmware.h>
 #include <asm/machdep.h>
@@ -671,6 +672,10 @@ static int dlpar_memory_add_by_count(u32 lmbs_to_add)
 	if (lmbs_available < lmbs_to_add)
 		return -EINVAL;
 
+	if (!radix_enabled())
+		hash_batch_expand_prepare(memblock_phys_mem_size() +
+						 lmbs_to_add * drmem_lmb_size());
+
 	for_each_drmem_lmb(lmb) {
 		if (lmb->flags & DRCONF_MEM_ASSIGNED)
 			continue;
@@ -788,6 +793,10 @@ static int dlpar_memory_add_by_ic(u32 lmbs_to_add, u32 drc_index)
 	if (lmbs_available < lmbs_to_add)
 		return -EINVAL;
 
+	if (!radix_enabled())
+		hash_batch_expand_prepare(memblock_phys_mem_size() +
+					  lmbs_to_add * drmem_lmb_size());
+
 	for_each_drmem_lmb_in_range(lmb, start_lmb, end_lmb) {
 		if (lmb->flags & DRCONF_MEM_ASSIGNED)
 			continue;
-- 
2.30.2