linux-kernel - Re: [PATCH] Initialise mmu_notifier

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Date:   Thu, 3 Jan 2019 09:59:51 -0500
From:   Jerome Glisse <jglisse@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
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 06:39:08AM -0800, Matthew Wilcox wrote:
> 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.
> 

I remember now we do not need to call invalidate range because
invalidate_range_end() does call invalidate_range so it is fine.
Comments should be better thought. So existing code is fine.

Cheers,
Jérôme