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Subject: Re: [PATCH 1/2] mm/hmm: hmm_range_fault() NULL pointer bug
To:     Christoph Hellwig <hch@lst.de>
CC:     <linux-mm@kvack.org>, <linux-kernel@vger.kernel.org>,
        <amd-gfx@lists.freedesktop.org>, <dri-devel@lists.freedesktop.org>,
        <nouveau@lists.freedesktop.org>,
        =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= <jglisse@redhat.com>,
        Jason Gunthorpe <jgg@mellanox.com>,
        Andrew Morton <akpm@linux-foundation.org>
References: <20190823221753.2514-1-rcampbell@nvidia.com>
 <20190823221753.2514-2-rcampbell@nvidia.com> <20190824223754.GA21891@lst.de>
From:   Ralph Campbell <rcampbell@nvidia.com>
Message-ID: <e2ecc1a7-0d2f-5957-e6cb-b3c86c085d80@nvidia.com>
Date:   Mon, 26 Aug 2019 11:02:12 -0700
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On 8/24/19 3:37 PM, Christoph Hellwig wrote:
> On Fri, Aug 23, 2019 at 03:17:52PM -0700, Ralph Campbell wrote:
>> Although hmm_range_fault() calls find_vma() to make sure that a vma exists
>> before calling walk_page_range(), hmm_vma_walk_hole() can still be called
>> with walk->vma == NULL if the start and end address are not contained
>> within the vma range.
> 
> Should we convert to walk_vma_range instead?  Or keep walk_page_range
> but drop searching the vma ourselves?
> 
> Except for that the patch looks good to me:
> 
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> 

I think keeping the call to walk_page_range() makes sense.
Jason is hoping to be able to snapshot a range with & without vmas
and have the pfns[] filled with empty/valid entries as appropriate.

I plan to repost my patch changing hmm_range_fault() to use
walk.test_walk which will remove the call to find_vma().
Jason had some concerns about testing it so that's why I have
been working on some HMM self tests before resending it.