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

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Date:   Mon, 11 Jul 2022 15:52:58 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Alex Sierra <alex.sierra@....com>, jgg@...dia.com
Cc:     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, apopple@...dia.com,
        willy@...radead.org, akpm@...ux-foundation.org
Subject: Re: [PATCH v8 06/15] mm: remove the vma check in migrate_vma_setup()

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()?

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) {
-- 
Thanks,

David / dhildenb