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

Oscar Salvador
2019-07-25T09:28:04Z

On Wed, Jul 24, 2019 at 01:11:52PM -0700, Dan Williams wrote:
> On Tue, Jun 25, 2019 at 12:53 AM Oscar Salvador <osalvador@suse.de> 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:

Well, MHP_MEMMAP_DEVICE is not tied to ZONE_DEVICE.
MHP_MEMMAP_DEVICE was chosen to make a difference between:

* allocate memmap pages for the whole memory-device
* allocate memmap pages on each memoryblock that this memory-device spans

>
> 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@intel.com>

--
Oscar Salvador
SUSE L3