Re: [PATCH v2 1/1] mm: Always release pages to the buddy allocator in memblock_free_late().

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On 2023-01-05 02:48, Ingo Molnar wrote:
* Aaron Thompson <dev@xxxxxxxxxx> wrote:

For example, on an Amazon EC2 t3.micro VM (1 GB) booting via EFI:

v6.2-rc2:
  # grep -E 'Node|spanned|present|managed' /proc/zoneinfo
  Node 0, zone      DMA
          spanned  4095
          present  3999
          managed  3840
  Node 0, zone    DMA32
          spanned  246652
          present  245868
          managed  178867

v6.2-rc2 + patch:
  # grep -E 'Node|spanned|present|managed' /proc/zoneinfo
  Node 0, zone      DMA
          spanned  4095
          present  3999
          managed  3840
  Node 0, zone    DMA32
          spanned  246652
          present  245868
          managed  222816   # +43,949 pages

[ Note the annotation I added to the output - might be useful in the
changelog too. ]

So this patch adds around +17% of RAM to this 1 GB virtual system? That
looks rather significant ...

Thanks,

	Ingo

It is significant, but I wouldn't describe it as being added. I would say that the system is currently losing 17% of RAM due to a bug, and this patch fixes that bug.

The actual numbers depend on the mappings given by the EFI, so they're largely out of our control. As an example, similar VMs that I run with the OVMF EFI lose about 3%. I couldn't say for sure which is the outlier, but my point is that the specific values are not really the focus, this is just an example that shows that the issue can be encountered in the wild with real impact. I know I'll be happy to get that memory back, whether it is 3% or 17% :)

Thanks,
-- Aaron



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