linux-ext4 - Re: [PATCH v1 03/14] mm: add iomem vma selection for memory migration

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Date:   Thu, 26 Aug 2021 18:27:31 -0400
From:   Felix Kuehling <felix.kuehling@....com>
To:     "Sierra Guiza, Alejandro (Alex)" <alex.sierra@....com>,
        Christoph Hellwig <hch@....de>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        rcampbell@...dia.com, linux-ext4@...r.kernel.org,
        linux-xfs@...r.kernel.org, amd-gfx@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org, jgg@...dia.com, jglisse@...hat.com
Subject: Re: [PATCH v1 03/14] mm: add iomem vma selection for memory migration

Am 2021-08-25 um 2:24 p.m. schrieb Sierra Guiza, Alejandro (Alex):
>
> On 8/25/2021 2:46 AM, Christoph Hellwig wrote:
>> On Tue, Aug 24, 2021 at 10:48:17PM -0500, Alex Sierra wrote:
>>>           } else {
>>> -            if (!(migrate->flags & MIGRATE_VMA_SELECT_SYSTEM))
>>> +            if (!(migrate->flags & MIGRATE_VMA_SELECT_SYSTEM) &&
>>> +                !(migrate->flags & MIGRATE_VMA_SELECT_IOMEM))
>>>                   goto next;
>>>               pfn = pte_pfn(pte);
>>>               if (is_zero_pfn(pfn)) {
>> .. also how is this going to work for the device public memory?  That
>> should be pte_special() an thus fail vm_normal_page.
> Perhaps we're missing something, as we're not doing any special
> marking for the device public pfn/entries.
> pfn_valid return true, pte_special return false and pfn_t_devmap
> return false on these pages. Same as system pages.
> That's the reason vm_normal_page returns the page correctly through
> pfn_to_page helper.

Hi Christoph,

I think we're missing something here. As far as I can tell, all the work
we did first with DEVICE_GENERIC and now DEVICE_PUBLIC always used
normal pages. Are we missing something in our driver code that would
make these PTEs special? I don't understand how that can be, because
driver code is not really involved in updating the CPU mappings. Maybe
it's something we need to do in the migration helpers.

Thanks,
  Felix


>
> Regards,
> Alex S.