linux-ext4 - Re: [PATCH v8 06/15] mm: remove the vma check in migrate_vma

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Date:   Thu, 14 Jul 2022 15:31:32 +1000
From:   Alistair Popple <apopple@...dia.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Alex Sierra <alex.sierra@....com>, jgg@...dia.com,
        Felix.Kuehling@....com, 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,
        hch@....de, jglisse@...hat.com, willy@...radead.org,
        akpm@...ux-foundation.org
Subject: Re: [PATCH v8 06/15] mm: remove the vma check in migrate_vma_setup()


David Hildenbrand <david@...hat.com> writes:

> On 07.07.22 21:03, Alex Sierra wrote:
>> From: Alistair Popple <apopple@...dia.com>
>>
>> migrate_vma_setup() checks that a valid vma is passed so that the page
>> tables can be walked to find the pfns associated with a given address
>> range. However in some cases the pfns are already known, such as when
>> migrating device coherent pages during pin_user_pages() meaning a valid
>> vma isn't required.
>
> As raised in my other reply, without a VMA ... it feels odd to use a
> "migrate_vma" API. For an internal (mm/migrate_device.c) use case it is
> ok I guess, but it certainly adds a bit of confusion. For example,
> because migrate_vma_setup() will undo ref+lock not obtained by it.
>
> I guess the interesting point is that
>
> a) Besides migrate_vma_pages() and migrate_vma_setup(), the ->vma is unused.
>
> b) migrate_vma_setup() does collect+unmap+cleanup if unmap failed.
>
> c) With our source page in our hands, we cannot be processing a hole in
> a VMA.
>
>
>
> Not sure if it's better. but I would
>
> a) Enforce in migrate_vma_setup() that there is a VMA. Code outside of
> mm/migrate_device.c shouldn't be doing some hacks like this.
>
> b) Don't call migrate_vma_setup() from migrate_device_page(), but
> directly migrate_vma_unmap() and add a comment.
>
>
> That will leave a single change to this patch (migrate_vma_pages()). But
> is that even required? Because ....
>
>> @@ -685,7 +685,7 @@ void migrate_vma_pages(struct migrate_vma *migrate)
>>  			continue;
>>  		}
>>
>> -		if (!page) {
>> +		if (!page && migrate->vma) {
>
> How could we ever have !page in case of migrate_device_page()?

Oh good point. This patch was originally part of a larger series I was
working on at the time but you're right - for migrate_device_page() we
should never hit this case. I will respin the next patch (number 7 in
this series) to include this.

> Instead, I think a VM_BUG_ON(migrate->vma); should hold and you can just
> simplify.
>
>>  			if (!(migrate->src[i] & MIGRATE_PFN_MIGRATE))
>>  				continue;
>>  			if (!notified) {