linux-kernel - Re: [PATCH v3 1/5] mm,memory_hotplug: Introduce MHP_MEMMAP_ON

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Date:   Fri, 26 Jul 2019 11:29:59 +0200
From:   Oscar Salvador <osalvador@...e.de>
To:     David Hildenbrand <david@...hat.com>
Cc:     akpm@...ux-foundation.org, dan.j.williams@...el.com,
        pasha.tatashin@...een.com, mhocko@...e.com,
        anshuman.khandual@....com, Jonathan.Cameron@...wei.com,
        vbabka@...e.cz, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/5] mm,memory_hotplug: Introduce MHP_MEMMAP_ON_MEMORY

On Fri, Jul 26, 2019 at 10:34:47AM +0200, David Hildenbrand wrote:
> > Want to add 384MB (3 sections, 3 memory-blocks)
> > e.g:
> > 
> > 	add_memory(0x1000, size_memory_block);
> > 	add_memory(0x2000, size_memory_block);
> > 	add_memory(0x3000, size_memory_block);
> > 
> > 	[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
> 
> I wouldn't even care about documenting this right now. We have no user
> so far, so spending 50% of the description on this topic isn't really
> needed IMHO :)

Fair enough, I could drop it.
Was just trying to be extra clear.

> 
> > 
> > or
> > 	add_memory(0x1000, size_memory_block * 3);
> > 
> > 	[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, per whole-range granularity is nicer when it comes to have a large
> > contigous area, while per memory-block granularity allows us to have flexibility
> > when removing the memory.
> 
> E.g., in my virtio-mem I am currently adding all memory blocks
> separately either way (to guranatee that remove_memory() works cleanly -
> see __release_memory_resource()), and to control the amount of
> not-offlined memory blocks (e.g., to make user space is actually
> onlining them). As it's just a prototype, this might change of course in
> the future.

What is virtio-mem for? Did it that raised from a need?
Is it something you could try this patch on?

> >  /*
> > + * We want memmap (struct page array) to be allocated from the hotadded range.
> > + * To do so, there are two possible ways depending on what the caller wants.
> > + * 1) Allocate memmap pages whole hot-added range.
> > + *    Here the caller will only call any add_memory() variant with the whole
> > + *    memory address.
> > + * 2) Allocate memmap pages per memblock
> > + *    Here, the caller will call any add_memory() variant per memblock
> > + *    granularity.
> > + * The former implies that we will use the beginning of the hot-added range
> > + * to store the memmap pages of the whole range, while the latter implies
> > + * that we will use the beginning of each memblock to store its own memmap
> > + * pages.
> 
> Can you make this documentation only state how MHP_MEMMAP_ON_MEMORY
> works? (IOW, shrink it heavily to what we actually implement)

Sure.

> Apart from the requested description/documentation changes
> 
> Reviewed-by: David Hildenbrand <david@...hat.com>

Thanks for having a look David ;-)
> 
> -- 
> 
> Thanks,
> 
> David / dhildenb

-- 
Oscar Salvador
SUSE L3