linux-kernel - Re: [PATCH v12] mm: report per-page metadata information

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Date: Sat, 18 May 2024 17:47:16 -0700 (PDT)
From: David Rientjes <rientjes@...gle.com>
To: Sourav Panda <souravpanda@...gle.com>
cc: corbet@....net, gregkh@...uxfoundation.org, rafael@...nel.org, 
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Subject: Re: [PATCH v12] mm: report per-page metadata information

On Sun, 12 May 2024, Sourav Panda wrote:

> Today, we do not have any observability of per-page metadata
> and how much it takes away from the machine capacity. Thus,
> we want to describe the amount of memory that is going towards
> per-page metadata, which can vary depending on build
> configuration, machine architecture, and system use.
> 
> This patch adds 2 fields to /proc/vmstat that can used as shown
> below:
> 
> Accounting per-page metadata allocated by boot-allocator:
> 	/proc/vmstat:nr_memmap_boot * PAGE_SIZE
> 
> Accounting per-page metadata allocated by buddy-allocator:
> 	/proc/vmstat:nr_memmap * PAGE_SIZE
> 
> Accounting total Perpage metadata allocated on the machine:
> 	(/proc/vmstat:nr_memmap_boot +
> 	 /proc/vmstat:nr_memmap) * PAGE_SIZE
> 
> Utility for userspace:
> 
> Observability: Describe the amount of memory overhead that is
> going to per-page metadata on the system at any given time since
> this overhead is not currently observable.
> 
> Debugging: Tracking the changes or absolute value in struct pages
> can help detect anomalies as they can be correlated with other
> metrics in the machine (e.g., memtotal, number of huge pages,
> etc).
> 
> page_ext overheads: Some kernel features such as page_owner
> page_table_check that use page_ext can be optionally enabled via
> kernel parameters. Having the total per-page metadata information
> helps users precisely measure impact. Furthermore, page-metadata
> metrics will reflect the amount of struct pages reliquished
> (or overhead reduced) when hugetlbfs pages are reserved which
> will vary depending on whether hugetlb vmemmap optimization is
> enabled or not.
> 
> For background and results see:
> lore.kernel.org/all/20240220214558.3377482-1-souravpanda@...gle.com
> 
> Signed-off-by: Sourav Panda <souravpanda@...gle.com>
> Reviewed-by: Pasha Tatashin <pasha.tatashin@...een.com>

Acked-by: David Rientjes <rientjes@...gle.com>