linux-kernel - Re: [PATCH] Initialise mmu_notifier

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Message-ID: <20190103143908.GQ6310@bombadil.infradead.org>
Date:   Thu, 3 Jan 2019 06:39:08 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Jerome Glisse <jglisse@...hat.com>
Cc:     John Hubbard <jhubbard@...dia.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-xfs@...r.kernel.org, linux-kernel@...r.kernel.org,
        Christian König <christian.koenig@....com>,
        Jan Kara <jack@...e.cz>
Subject: Re: [PATCH] Initialise mmu_notifier_range correctly

On Thu, Jan 03, 2019 at 09:29:59AM -0500, Jerome Glisse wrote:
> On Wed, Jan 02, 2019 at 08:18:33PM -0800, Matthew Wilcox wrote:
> > On Wed, Jan 02, 2019 at 07:32:08PM -0800, John Hubbard wrote:
> > > Having the range struct declared in separate places from the mmu_notifier_range_init()
> > > calls is not great. But I'm not sure I see a way to make it significantly cleaner, given
> > > that __follow_pte_pmd uses the range pointer as a way to decide to issue the mmn calls.
> > 
> > Yeah, I don't think there's anything we can do.  But I started reviewing
> > the comments, and they don't make sense together:
> > 
> >                 /*
> >                  * Note because we provide range to follow_pte_pmd it will
> >                  * call mmu_notifier_invalidate_range_start() on our behalf
> >                  * before taking any lock.
> >                  */
> >                 if (follow_pte_pmd(vma->vm_mm, address, &range,
> >                                    &ptep, &pmdp, &ptl))
> >                         continue;
> > 
> >                 /*
> >                  * No need to call mmu_notifier_invalidate_range() as we are
> >                  * downgrading page table protection not changing it to point
> >                  * to a new page.
> >                  *
> >                  * See Documentation/vm/mmu_notifier.rst
> >                  */
> > 
> > So if we don't call mmu_notifier_invalidate_range, why are we calling
> > mmu_notifier_invalidate_range_start and mmu_notifier_invalidate_range_end?
> > ie, why not this ...
> 
> Thus comments looks wrong to me ... we need to call
> mmu_notifier_invalidate_range() those are use by
> IOMMU. I might be to blame for those comments thought.

Yes, you're to blame for both of them.

a4d1a88525138 (Jérôme Glisse     2017-08-31 17:17:26 -0400  791)                 * Note because we provide start/end to follow_pte_pmd it will
a4d1a88525138 (Jérôme Glisse     2017-08-31 17:17:26 -0400  792)                 * call mmu_notifier_invalidate_range_start() on our behalf
a4d1a88525138 (Jérôme Glisse     2017-08-31 17:17:26 -0400  793)                 * before taking any lock.

0f10851ea475e (Jérôme Glisse     2017-11-15 17:34:07 -0800  794)                 * No need to call mmu_notifier_invalidate_range() as we are
0f10851ea475e (Jérôme Glisse     2017-11-15 17:34:07 -0800  795)                 * downgrading page table protection not changing it to point
0f10851ea475e (Jérôme Glisse     2017-11-15 17:34:07 -0800  796)                 * to a new page.