Re: [PATCH v3 4/4] mm: don't account memmap per-node
From: Alison Schofield
Date: Thu Aug 08 2024 - 14:55:37 EST
On Thu, Aug 08, 2024 at 03:42:37PM +0000, Pasha Tatashin wrote:
> Currently, when memory is hot-plugged or hot-removed the accounting is
> done based on the assumption that memmap is allocated from the same node
> as the hot-plugged/hot-removed memory, which is not always the case.
>
> In addition, there are challenges with keeping the node id of the memory
> that is being remove to the time when memmap accounting is actually
> performed: since this is done after remove_pfn_range_from_zone(), and
> also after remove_memory_block_devices(). Meaning that we cannot use
> pgdat nor walking though memblocks to get the nid.
>
How about directly include the failing cases and user visible impacts as
reported in the Tags appended below.
--Alison
> Given all of that, account the memmap overhead system wide instead.
>
> For this we are going to be using global atomic counters, but given that
> memmap size is rarely modified, and normally is only modified either
> during early boot when there is only one CPU, or under a hotplug global
> mutex lock, therefore there is no need for per-cpu optimizations.
>
> Reported-by: Yi Zhang <yi.zhang@xxxxxxxxxx>
> Closes: https://lore.kernel.org/linux-cxl/CAHj4cs9Ax1=CoJkgBGP_+sNu6-6=6v=_L-ZBZY0bVLD3wUWZQg@xxxxxxxxxxxxxx
> Reported-by: Alison Schofield <alison.schofield@xxxxxxxxx>
> Closes: https://lore.kernel.org/linux-mm/Zq0tPd2h6alFz8XF@aschofie-mobl2/#t
>
> Fixes: 15995a352474 ("mm: report per-page metadata information")
> Signed-off-by: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>
> ---
snip
>