Re: [PATCH v2 2/5] mm,memory_hotplug: Introduce MHP_VMEMMAP_FLAGS

[Dan Williams]

On Tue, Jun 25, 2019 at 12:53 AM Oscar Salvador <osalvador@xxxxxxx> wrote:
>
> This patch introduces MHP_MEMMAP_DEVICE and MHP_MEMMAP_MEMBLOCK flags,
> and prepares the callers that add memory to take a "flags" parameter.
> This "flags" parameter will be evaluated later on in Patch#3
> to init mhp_restrictions struct.
>
> The callers are:
>
> add_memory
> __add_memory
> add_memory_resource
>
> Unfortunately, we do not have a single entry point to add memory, as depending
> on the requisites of the caller, they want to hook up in different places,
> (e.g: Xen reserve_additional_memory()), so we have to spread the parameter
> in the three callers.
>
> The flags are either MHP_MEMMAP_DEVICE or MHP_MEMMAP_MEMBLOCK, and only differ
> in the way they allocate vmemmap pages within the memory blocks.
>
> MHP_MEMMAP_MEMBLOCK:
>         - With this flag, we will allocate vmemmap pages in each memory block.
>           This means that if we hot-add a range that spans multiple memory blocks,
>           we will use the beginning of each memory block for the vmemmap pages.
>           This strategy is good for cases where the caller wants the flexiblity
>           to hot-remove memory in a different granularity than when it was added.
>
>           E.g:
>                 We allocate a range (x,y], that spans 3 memory blocks, and given
>                 memory block size = 128MB.
>                 [memblock#0  ]
>                 [0 - 511 pfns      ] - vmemmaps for section#0
>                 [512 - 32767 pfns  ] - normal memory
>
>                 [memblock#1 ]
>                 [32768 - 33279 pfns] - vmemmaps for section#1
>                 [33280 - 65535 pfns] - normal memory
>
>                 [memblock#2 ]
>                 [65536 - 66047 pfns] - vmemmap for section#2
>                 [66048 - 98304 pfns] - normal memory
>
> MHP_MEMMAP_DEVICE:
>         - With this flag, we will store all vmemmap pages at the beginning of
>           hot-added memory.
>
>           E.g:
>                 We allocate a range (x,y], that spans 3 memory blocks, and given
>                 memory block size = 128MB.
>                 [memblock #0 ]
>                 [0 - 1533 pfns    ] - vmemmap for section#{0-2}
>                 [1534 - 98304 pfns] - normal memory
>
> When using larger memory blocks (1GB or 2GB), the principle is the same.
>
> Of course, MHP_MEMMAP_DEVICE is nicer when it comes to have a large contigous
> area, while MHP_MEMMAP_MEMBLOCK allows us to have flexibility when removing the
> memory.

Concept and patch looks good to me, but I don't quite like the
proliferation of the _DEVICE naming, in theory it need not necessarily
be ZONE_DEVICE that is the only user of that flag. I also think it
might be useful to assign a flag for the default 'allocate from RAM'
case, just so the code is explicit. So, how about:

MHP_MEMMAP_PAGE_ALLOC
MHP_MEMMAP_MEMBLOCK
MHP_MEMMAP_RESERVED

...for the 3 cases?

Other than that, feel free to add:

Reviewed-by: Dan Williams <dan.j.williams@xxxxxxxxx>