Re: [PATCH v2 4/7] s390/physmem_info: query diag500(STORAGE LIMIT) to support QEMU/KVM memory devices

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On 17.10.24 09:36, Alexander Gordeev wrote:
On Mon, Oct 14, 2024 at 04:46:16PM +0200, David Hildenbrand wrote:

Hi David!

Hi Alexander!


@@ -157,7 +189,9 @@ unsigned long detect_max_physmem_end(void)
  {
  	unsigned long max_physmem_end = 0;
- if (!sclp_early_get_memsize(&max_physmem_end)) {
+	if (!diag500_storage_limit(&max_physmem_end)) {
+		physmem_info.info_source = MEM_DETECT_DIAG500_STOR_LIMIT;
+	} else if (!sclp_early_get_memsize(&max_physmem_end)) {
  		physmem_info.info_source = MEM_DETECT_SCLP_READ_INFO;
  	} else {
  		max_physmem_end = search_mem_end();
@@ -170,11 +204,17 @@ void detect_physmem_online_ranges(unsigned long max_physmem_end)
  {
  	if (!sclp_early_read_storage_info()) {
  		physmem_info.info_source = MEM_DETECT_SCLP_STOR_INFO;
+		return;
  	} else if (!diag260()) {
  		physmem_info.info_source = MEM_DETECT_DIAG260;
-	} else if (max_physmem_end) {
-		add_physmem_online_range(0, max_physmem_end);
+		return;
+	} else if (physmem_info.info_source == MEM_DETECT_DIAG500_STOR_LIMIT) {
+		max_physmem_end = 0;
+		if (!sclp_early_get_memsize(&max_physmem_end))
+			physmem_info.info_source = MEM_DETECT_SCLP_READ_INFO;

Why search_mem_end() is not tried in case sclp_early_get_memsize() failed?

Patch #3 documents that:

+    The storage limit does not indicate currently usable storage, it may
+    include holes, standby storage and areas reserved for other means, such
+    as memory hotplug or virtio-mem devices. Other interfaces for detecting
+    actually usable storage, such as SCLP, must be used in conjunction with
+    this subfunction.

If SCLP would fail, something would be seriously wrong and we should just crash
instead of trying to fallback to the legacy way of scanning.


  	}
+	if (max_physmem_end)
+		add_physmem_online_range(0, max_physmem_end);
  }
void physmem_set_usable_limit(unsigned long limit)
diff --git a/arch/s390/include/asm/physmem_info.h b/arch/s390/include/asm/physmem_info.h
index f45cfc8bc233..51b68a43e195 100644
--- a/arch/s390/include/asm/physmem_info.h
+++ b/arch/s390/include/asm/physmem_info.h
@@ -9,6 +9,7 @@ enum physmem_info_source {
  	MEM_DETECT_NONE = 0,
  	MEM_DETECT_SCLP_STOR_INFO,
  	MEM_DETECT_DIAG260,
+	MEM_DETECT_DIAG500_STOR_LIMIT,
  	MEM_DETECT_SCLP_READ_INFO,
  	MEM_DETECT_BIN_SEARCH
  };
@@ -107,6 +108,8 @@ static inline const char *get_physmem_info_source(void)
  		return "sclp storage info";
  	case MEM_DETECT_DIAG260:
  		return "diag260";
+	case MEM_DETECT_DIAG500_STOR_LIMIT:
+		return "diag500 storage limit";

AFAIU you want to always override MEM_DETECT_DIAG500_STOR_LIMIT method
with an online memory detection method. In that case this code is dead.

Not in the above case, pathological case above where something went wrong
during sclp_early_get_memsize(). In that scenario, die_oom() would indicate
that there are no memory ranges but that "diag500 storage limit" worked.

Does that make sense?

Thanks for the review!

--
Cheers,

David / dhildenb





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