linux-kernel - Re: [PATCH 0/3] mm: use memmap_on

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Message-ID: <29c9b998-f453-59f2-5084-9b4482b489cf@redhat.com>
Date:   Fri, 16 Jun 2023 09:44:50 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Vishal Verma <vishal.l.verma@...el.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <lenb@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oscar Salvador <osalvador@...e.de>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Jiang <dave.jiang@...el.com>
Cc:     linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, nvdimm@...ts.linux.dev,
        linux-cxl@...r.kernel.org, Huang Ying <ying.huang@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH 0/3] mm: use memmap_on_memory semantics for dax/kmem

On 16.06.23 00:00, Vishal Verma wrote:
> The dax/kmem driver can potentially hot-add large amounts of memory
> originating from CXL memory expanders, or NVDIMMs, or other 'device
> memories'. There is a chance there isn't enough regular system memory
> available to fit ythe memmap for this new memory. It's therefore
> desirable, if all other conditions are met, for the kmem managed memory
> to place its memmap on the newly added memory itself.
> 
> Arrange for this by first allowing for a module parameter override for
> the mhp_supports_memmap_on_memory() test using a flag, adjusting the
> only other caller of this interface in dirvers/acpi/acpi_memoryhotplug.c,
> exporting the symbol so it can be called by kmem.c, and finally changing
> the kmem driver to add_memory() in chunks of memory_block_size_bytes().

1) Why is the override a requirement here? Just let the admin configure 
it then then add conditional support for kmem.

2) I recall that there are cases where we don't want the memmap to land 
on slow memory (which online_movable would achieve). Just imagine the 
slow PMEM case. So this might need another configuration knob on the 
kmem side.


I recall some discussions on doing that chunk handling internally (so 
kmem can just perform one add_memory() and we'd split that up internally).

-- 
Cheers,

David / dhildenb