linux-kernel - Re: [RFC PATCH 2/3] mm/memory_hotplug: Create __shrink_pages and move it to offline

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Date:   Thu, 16 Aug 2018 16:58:49 +0200
From:   Oscar Salvador <osalvador@...hadventures.net>
To:     Jerome Glisse <jglisse@...hat.com>
Cc:     Michal Hocko <mhocko@...nel.org>, akpm@...ux-foundation.org,
        dan.j.williams@...el.com, Pavel.Tatashin@...rosoft.com,
        david@...hat.com, yasu.isimatu@...il.com, logang@...tatee.com,
        dave.jiang@...el.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Oscar Salvador <osalvador@...e.de>
Subject: Re: [RFC PATCH 2/3] mm/memory_hotplug: Create __shrink_pages and
 move it to offline_pages

On Thu, Aug 09, 2018 at 12:58:21PM -0400, Jerome Glisse wrote:
> I agree, i never thought about that before. Looking at existing resource
> management i think the simplest solution would be to use a refcount on the
> resources instead of the IORESOURCE_BUSY flags.
> 
> So when you release resource as part of hotremove you would only dec the
> refcount and a resource is not busy only when refcount is zero.
> 
> Just the idea i had in mind. Right now i am working on other thing, Oscar
> is this something you would like to work on ? Feel free to come up with
> something better than my first idea :)

So, I thought a bit about this.
First I talked a bit with Jerome about the refcount idea.
The problem with reconverting this to refcount is that it is too intrusive,
and I think it is not really needed.

I then thought about defining a new flag, something like

#define IORESOURCE_NO_HOTREMOVE	xxx

but we ran out of bits for the flag field.

I then thought about doing something like:

struct resource {
        resource_size_t start;
        resource_size_t end;
        const char *name;
        unsigned long flags;
        unsigned long desc;
        struct resource *parent, *sibling, *child;
#ifdef CONFIG_MEMORY_HOTREMOVE
        bool device_managed;
#endif
};

but it is just too awful, not needed, and bytes consuming.

The only idea I had left is:

register_memory_resource(), which defines a new resource for the added memory-chunk
is only called from add_memory().
This function is only being hit when we add memory-chunks.

HMM/devm gets the resources their own way, calling devm_request_mem_region().

So resources that are requested from HMM/devm, have the following flags:

 (IORESOURCE_MEM|IORESOURCE_BUSY)

while resources that are requested via mem-hotplug have:

 (IORESOURCE_SYSTEM_RAM | IORESOURCE_BUSY)

IORESOURCE_SYSTEM_RAM = (IORESOURCE_MEM|IORESOURCE_SYSRAM)

release_mem_region_adjustable() is only being called from hot-remove path, so
unless I am mistaken, all resources hitting that path should match IORESOURCE_SYSTEM_RAM.

That leaves me with the idea that we could check for the resource->flags to contain IORESOURCE_SYSRAM,
as I think it is only being set for memory-chunks that are added via memory-hot-add path.

In case it is not, we know that that resource belongs to HMM/devm, so we can back off since
they take care of releasing the resource via devm_release_mem_region.

I am working on a RFC v2 containing this, but, Jerome, could you confirm above assumption, please?

Of course, ideas/suggestions are also welcome.

Thanks
-- 
Oscar Salvador
SUSE L3