Re: [RFC PATCH] mm/slub: enable debugging memory wasting of kmalloc

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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.




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