linux-ext4 - Re: [PATCH v6 01/10] mm: add zone device coherent type memory support

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Date:   Fri, 11 Feb 2022 12:07:11 -0500
From:   Felix Kuehling <felix.kuehling@....com>
To:     David Hildenbrand <david@...hat.com>,
        Alex Sierra <alex.sierra@....com>, akpm@...ux-foundation.org,
        linux-mm@...ck.org, rcampbell@...dia.com,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org
Cc:     amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        hch@....de, jgg@...dia.com, jglisse@...hat.com, apopple@...dia.com,
        willy@...radead.org
Subject: Re: [PATCH v6 01/10] mm: add zone device coherent type memory support


Am 2022-02-11 um 11:39 schrieb David Hildenbrand:
> On 11.02.22 17:15, David Hildenbrand wrote:
>> On 01.02.22 16:48, Alex Sierra wrote:
>>> Device memory that is cache coherent from device and CPU point of view.
>>> This is used on platforms that have an advanced system bus (like CAPI
>>> or CXL). Any page of a process can be migrated to such memory. However,
>>> no one should be allowed to pin such memory so that it can always be
>>> evicted.
>>>
>>> Signed-off-by: Alex Sierra <alex.sierra@....com>
>>> Acked-by: Felix Kuehling <Felix.Kuehling@....com>
>>> Reviewed-by: Alistair Popple <apopple@...dia.com>
>> So, I'm currently messing with PageAnon() pages and CoW semantics ...
>> all these PageAnon() ZONE_DEVICE variants don't necessarily make my life
>> easier but I'm not sure yet if they make my life harder. I hope you can
>> help me understand some of that stuff.
>>
>> 1) What are expected CoW semantics for DEVICE_COHERENT?
>>
>> I assume we'll share them just like other PageAnon() pages during fork()
>> readable, and the first sharer writing to them receives an "ordinary"
>> !ZONE_DEVICE copy.
>>
>> So this would be just like DEVICE_EXCLUSIVE CoW handling I assume, just
>> that we don't have to go through the loop of restoring a device
>> exclusive entry?
>>
>> 2) How are these pages freed to clear/invalidate PageAnon() ?
>>
>> I assume for PageAnon() ZONE_DEVICE pages we'll always for via
>> free_devmap_managed_page(), correct?
>>
>>
>> 3) FOLL_PIN
>>
>> While you write "no one should be allowed to pin such memory", patch #2
>> only blocks FOLL_LONGTERM. So I assume we allow ordinary FOLL_PIN and
>> you might want to be a bit more precise?
>>
>>
>> ... I'm pretty sure we cannot FOLL_PIN DEVICE_PRIVATE pages, but can we
>> FILL_PIN DEVICE_EXCLUSIVE pages? I strongly assume so?
>>
>>
>> Thanks for any information.
>>
> (digging a bit more, I realized that device exclusive pages are not
> actually/necessarily ZONE_DEVICE pages -- so I assume DEVICE_COHERENT
> will be the actual first PageAnon() ZONE_DEVICE pages that can be
> present in a page table.)

I think DEVICE_GENERIC pages can also be mapped in the page table. In 
fact, the first version of our patches attempted to add migration 
support to DEVICE_GENERIC. But we were convinced to create a new 
ZONE_DEVICE page type for our use case instead.

Regards,
   Felix


>