Re: [PATCH] Revert "powerpc/powernv: Increase memory block size to 1GB on radix"

[Michael Ellerman <mpe@ellerman.id.au>]

Nicholas Piggin <npiggin@gmail.com> writes:

> On Mon, 30 Apr 2018 18:34:32 +1000
> Balbir Singh <bsingharora@gmail.com> wrote:
>
>> This reverts commit 53ecde0b9126ff140abe3aefd7f0ec64d6fa36b0.
>> 
>> The commit above changed the memblock size to 1GiB, which did some
>> nice things like create fewer TLB entries for mapping memory at
>> the time of hotplug. The downside is that it changes the granularity
>> at which memory can be hot-plugged and hot-unplugged. The implication
>> is that if we had less than a 1GiB to hot-plug/hot-unplug that
>> would not be possible.
>> 
>> The reason we had this fix was to resolve an issue where we did not
>> split mappings on hot-unplug, leaving a TLB entry that spanned the
>> region that was unplugged. This is now fixed by 4dd5f8a99e79 which
>> splits the page table, removing the MMU mappings for the hot-unplugged
>> region correctly.
>> 
>> This trade-offs performance as we increase the number of TLB's to
>> map 1GiB of memory, mapped in via hot-plug, but gives us the ability
>> to handle smaller memory sizes.
>
> Why does powernv have any memory block size override at all now?

I dunno, history says:

    powerpc/powernv: Set memory_block_size_bytes to 256MB
    
    powerpc sets a low SECTION_SIZE_BITS to accomodate small pseries
    boxes. We default to 16MB memory blocks, and boxes with a lot
    of memory end up with enormous numbers of sysfs memory nodes.
    
    Set a more reasonable default for powernv of 256MB.
    
    Signed-off-by: Anton Blanchard <anton@samba.org>


It might make more sense to go back to a single definition for powerpc
and have it return a sensible size based on the total available memory -
though I guess memory hotplugged after boot might make that difficult.

Because we definitely have pseries machines with a lot of memory these
days too.

cheers