linux-kernel - Re: [PATCH 9/9] mm: remove the MIGRATE_PFN

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Date:   Tue, 30 Jul 2019 07:46:33 +0200
From:   Christoph Hellwig <hch@....de>
To:     Jerome Glisse <jglisse@...hat.com>
Cc:     Christoph Hellwig <hch@....de>, Jason Gunthorpe <jgg@...lanox.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,
        nouveau@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 9/9] mm: remove the MIGRATE_PFN_WRITE flag

On Mon, Jul 29, 2019 at 07:30:44PM -0400, Jerome Glisse wrote:
> On Mon, Jul 29, 2019 at 05:28:43PM +0300, Christoph Hellwig wrote:
> > The MIGRATE_PFN_WRITE is only used locally in migrate_vma_collect_pmd,
> > where it can be replaced with a simple boolean local variable.
> > 
> > Signed-off-by: Christoph Hellwig <hch@....de>
> 
> NAK that flag is useful, for instance a anonymous vma might have
> some of its page read only even if the vma has write permission.
> 
> It seems that the code in nouveau is wrong (probably lost that
> in various rebase/rework) as this flag should be use to decide
> wether to map the device memory with write permission or not.
> 
> I am traveling right now, i will investigate what happened to
> nouveau code.

We can add it back when needed pretty easily.  Much of this has bitrotted
way to fast, and the pending ppc kvmhmm code doesn't need it either.