linux-kernel - Re: [PATCH 01/10] mm: turn migrate

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Date:   Sat, 17 Aug 2019 13:31:28 +0200
From:   Christoph Hellwig <hch@....de>
To:     Jason Gunthorpe <jgg@...lanox.com>
Cc:     Christoph Hellwig <hch@....de>,
        Jérôme Glisse <jglisse@...hat.com>,
        Ben Skeggs <bskeggs@...hat.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Bharata B Rao <bharata@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "nouveau@...ts.freedesktop.org" <nouveau@...ts.freedesktop.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 01/10] mm: turn migrate_vma upside down

On Fri, Aug 16, 2019 at 05:11:07PM +0000, Jason Gunthorpe wrote:
> -	if (args->cpages)
> -		migrate_vma_prepare(args);
> -	if (args->cpages)
> -		migrate_vma_unmap(args);
> +	if (!args->cpages)
> +		return 0;
> +
> +	migrate_vma_prepare(args);
> +	migrate_vma_unmap(args);

I don't think this is ok.  Both migrate_vma_prepare and migrate_vma_unmap
can reduce args->cpages, including possibly to 0.