linux-kernel - Re: [PATCH v5 1/2] mm/mmu_notifier: make interval notifier updates safe

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Date:   Thu, 16 Jan 2020 10:13:11 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Ralph Campbell <rcampbell@...dia.com>
Cc:     "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        Jerome Glisse <jglisse@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        Christoph Hellwig <hch@....de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Shuah Khan <shuah@...nel.org>
Subject: Re: [PATCH v5 1/2] mm/mmu_notifier: make interval notifier updates
 safe

On Wed, Jan 15, 2020 at 02:04:47PM -0800, Ralph Campbell wrote:

> But I see your point if this sequence is done outside of the invalidate
> callback. In that case, if the driver shrank the interval, an invalidate
> callback for the right hand side could be missed before the insertion of
> the new interval for the right hand side.
> I'll explain this in the comments for nouveau_svmm_do_unmap() and
> dmirror_do_unmap().

Yes, this is why I'm not sure this is a good API for the core to
expose.

Batch manipulations is a resonable thing, but it should be forced to
work under safe conditions, ie while holding the required 'inv_begin'
on the interval tree, and the batching APIs should assert this
requirement.

Jason