[PATCH] x86/mce: Dynamically size space for machine check records

[Tony Luck]

Systems with a large number of CPUs may generate a large
number of machine check records when things go seriously
wrong. But Linux has a fixed buffer that can only capture
a few dozen errors.

Allocate space based on the number of CPUs (with a minimum
value based on the historical fixed buffer that could store
80 records).

Signed-off-by: Tony Luck <tony.luck@xxxxxxxxx>
---

Discussion earlier concluded with the realization that it is
safe to dynamically allocate the mce_evt_pool at boot time.
So here's a patch to do that. Scaling algorithm here is a
simple linear "4 records per possible CPU" with a minimum
of 80 to match the legacy behavior. I'm open to other
suggestions.

Note that I threw in a "+1" to the return from ilog2() when
calling gen_pool_create(). From reading code, and running
some tests, it appears that the min_alloc_order argument
needs to be large enough to allocate one of the mce_evt_llist
structures.

Some other gen_pool users in Linux may also need this "+1".

 arch/x86/kernel/cpu/mce/genpool.c | 22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kernel/cpu/mce/genpool.c b/arch/x86/kernel/cpu/mce/genpool.c
index fbe8b61c3413..a1f0a8f29cf5 100644
--- a/arch/x86/kernel/cpu/mce/genpool.c
+++ b/arch/x86/kernel/cpu/mce/genpool.c
@@ -16,14 +16,13 @@
  * used to save error information organized in a lock-less list.
  *
  * This memory pool is only to be used to save MCE records in MCE context.
- * MCE events are rare, so a fixed size memory pool should be enough. Use
- * 2 pages to save MCE events for now (~80 MCE records at most).
+ * MCE events are rare, so a fixed size memory pool should be enough.
+ * Allocate on a sliding scale based on number of CPUs.
  */
-#define MCE_POOLSZ	(2 * PAGE_SIZE)
+#define MCE_MIN_ENTRIES	80
 
 static struct gen_pool *mce_evt_pool;
 static LLIST_HEAD(mce_event_llist);
-static char gen_pool_buf[MCE_POOLSZ];
 
 /*
  * Compare the record "t" with each of the records on list "l" to see if
@@ -118,14 +117,25 @@ int mce_gen_pool_add(struct mce *mce)
 
 static int mce_gen_pool_create(void)
 {
+	int mce_numrecords, mce_poolsz;
 	struct gen_pool *tmpp;
 	int ret = -ENOMEM;
+	void *mce_pool;
+	int order;
 
-	tmpp = gen_pool_create(ilog2(sizeof(struct mce_evt_llist)), -1);
+	order = ilog2(sizeof(struct mce_evt_llist)) + 1;
+	tmpp = gen_pool_create(order, -1);
 	if (!tmpp)
 		goto out;
 
-	ret = gen_pool_add(tmpp, (unsigned long)gen_pool_buf, MCE_POOLSZ, -1);
+	mce_numrecords = max(80, num_possible_cpus() * 4);
+	mce_poolsz = mce_numrecords * (1 << order);
+	mce_pool = kmalloc(mce_poolsz, GFP_KERNEL);
+	if (!mce_pool) {
+		gen_pool_destroy(tmpp);
+		goto out;
+	}
+	ret = gen_pool_add(tmpp, (unsigned long)mce_pool, mce_poolsz, -1);
 	if (ret) {
 		gen_pool_destroy(tmpp);
 		goto out;
-- 
2.43.0