linux-kernel - Re: [PATCH v3 3/3] mm/hmm/test: add self tests for HMM

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Message-ID: <20191031124200.GJ22766@mellanox.com>
Date:   Thu, 31 Oct 2019 12:42:03 +0000
From:   Jason Gunthorpe <jgg@...lanox.com>
To:     Ralph Campbell <rcampbell@...dia.com>
CC:     Jerome Glisse <jglisse@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        Christoph Hellwig <hch@....de>,
        "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>
Subject: Re: [PATCH v3 3/3] mm/hmm/test: add self tests for HMM

On Wed, Oct 30, 2019 at 05:14:30PM -0700, Ralph Campbell wrote:

> > Well, that is good, is it also under drivers/char? It kind feels like
> > it should not be there...
> 
> I think most of the test modules live in lib/ but I wasn't sure that
> was the right place for the HMM test driver.
> If you think that is better, I can easily move it.

It would be good to get the various test people involved in this, I
really don't know.
 
> > > > It seems especially over-complicated to use a full page table layout
> > > > for this, wouldn't something simple like an xarray be good enough for
> > > > test purposes?
> > > 
> > > Possibly. A page table is really just a lookup table from virtual address
> > > to pfn/page. Part of the rationale was to mimic what a real device
> > > might do.
> > 
> > Well, but the details of the page table layout don't see really
> > important to this testing, IMHO.
> 
> One problem with XArray is that on 32-bit machines the value would
> need to be u64 to hold a pfn which won't fit in a ULONG_MAX.
> I guess we could make the driver 64-bit only.

Why would a 32 bit machine need a 64 bit pfn?

Jason