linux-kernel - Re: [HMM v13 01/18] mm/memory/hotplug: convert device parameter bool to set of flags

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Date:   Mon, 21 Nov 2016 12:27:15 +0530
From:   Anshuman Khandual <khandual@...ux.vnet.ibm.com>
To:     Jerome Glisse <jglisse@...hat.com>,
        Balbir Singh <bsingharora@...il.com>
Cc:     akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, John Hubbard <jhubbard@...dia.com>,
        Russell King <linux@...linux.org.uk>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Rich Felker <dalias@...c.org>,
        Chris Metcalf <cmetcalf@...lanox.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [HMM v13 01/18] mm/memory/hotplug: convert device parameter bool
 to set of flags

On 11/21/2016 10:23 AM, Jerome Glisse wrote:
> On Mon, Nov 21, 2016 at 11:44:36AM +1100, Balbir Singh wrote:
>>
>>
>> On 19/11/16 05:18, Jérôme Glisse wrote:
>>> Only usefull for arch where we support ZONE_DEVICE and where we want to
>>> also support un-addressable device memory. We need struct page for such
>>> un-addressable memory. But we should avoid populating the kernel linear
>>> mapping for the physical address range because there is no real memory
>>> or anything behind those physical address.
>>>
>>> Hence we need more flags than just knowing if it is device memory or not.
>>>
>>
>>
>> Isn't it better to add a wrapper to arch_add/remove_memory and do those
>> checks inside and then call arch_add/remove_memory to reduce the churn.
>> If you need selectively enable MEMORY_UNADDRESSABLE that can be done with
>> _ARCH_HAS_FEATURE
> 
> The flag parameter can be use by other new features and thus i thought the
> churn was fine. But i do not mind either way, whatever people like best.

Right, once we get the device memory classification right, these flags
can be used in more places.

> 
> [...]
> 
>>> -extern int arch_add_memory(int nid, u64 start, u64 size, bool for_device);
>>> +
>>> +/*
>>> + * For device memory we want more informations than just knowing it is device
>> 				     information
>>> + * memory. We want to know if we can migrate it (ie it is not storage memory
>>> + * use by DAX). Is it addressable by the CPU ? Some device memory like GPU
>>> + * memory can not be access by CPU but we still want struct page so that we
>> 			accessed
>>> + * can use it like regular memory.
>>
>> Can you please add some details on why -- migration needs them for example?
> 
> I am not sure what you mean ? DAX ie persistent memory device is intended to be
> use for filesystem or persistent storage. Hence memory migration does not apply
> to it (it would go against its purpose).

Why ? It can still be used for compaction, HW errors etc where we need to
move between persistent storage areas. The source and destination can be
persistent storage memory.

> 
> So i want to extend ZONE_DEVICE to be more then just DAX/persistent memory. For
> that i need to differentatiate between device memory that can be migrated and
> should be more or less treated like regular memory (with struct page). This is
> what the MEMORY_MOVABLE flag is for.

ZONE_DEVICE right now also supports struct page for the addressable memory,
(whether inside it's own range or in system RAM) with this we are extending
it to cover un-addressable memory with struct pages. Yes the differentiation
is required.

> 
> Finaly in my case the device memory is not accessible by the CPU so i need yet
> another flag. In the end i am extending ZONE_DEVICE to be use for 3 differents
> type of memory.
> 
> Is this the kind of explanation you are looking for ?