Re: [RFC PATCH] mm/slub: enable debugging memory wasting of kmalloc
From: Robin Murphy
Date: Thu Jun 30 2022 - 04:52:32 EST
On 2022-06-30 03:47, Andrew Morton wrote:
On Thu, 30 Jun 2022 10:38:44 +0800 Feng Tang <feng.tang@xxxxxxxxx> wrote:
Hi Andrew,
Thanks for the review!
On Wed, Jun 29, 2022 at 07:30:06PM -0700, Andrew Morton wrote:
On Thu, 30 Jun 2022 09:47:15 +0800 Feng Tang <feng.tang@xxxxxxxxx> wrote:
kmalloc's API family is critical for mm, with one shortcoming that
its object size is fixed to be power of 2. When user requests memory
for '2^n + 1' bytes, actually 2^(n+1) bytes will be allocated, so
in worst case, there is around 50% memory space waste.
We've met a kernel boot OOM panic, and from the dumped slab info:
[ 26.062145] kmalloc-2k 814056KB 814056KB
>From debug we found there are huge number of 'struct iova_magazine',
whose size is 1032 bytes (1024 + 8), so each allocation will waste
1016 bytes. Though the issue is solved by giving the right(bigger)
size of RAM, it is still better to optimize the size (either use
a kmalloc friendly size or create a dedicated slab for it).
Well that's nice, and additional visibility is presumably a good thing.
But what the heck is going on with iova_magazine? Is anyone looking at
moderating its impact?
Yes, I have a very simple patch at hand
--- a/drivers/iommu/iova.c
+++ b/drivers/iommu/iova.c
@@ -614,7 +614,7 @@ EXPORT_SYMBOL_GPL(reserve_iova);
* dynamic size tuning described in the paper.
*/
-#define IOVA_MAG_SIZE 128
+#define IOVA_MAG_SIZE 127
Well OK. Would benefit from a comment explaining the reasoning.
But we still have eleventy squillion of these things in flight. Why?
They're storage for a per-CPU caching scheme - for n CPUs, there should
currently be (2n + 32) * 6 in flight, since there's one set for each of
6 sizes. The 32 really should be n or 2n as well since it's needlessly
large for small systems and a bottleneck for large ones, but it needs
some unpicking to allow for dynamic allocations.
Thanks,
Robin.