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Subject: Re: [PATCH] staging: gasket: Convert get_user_pages*() -->
 pin_user_pages*()
To:     Dan Carpenter <dan.carpenter@oracle.com>,
        Souptick Joarder <jrdr.linux@gmail.com>
CC:     <devel@driverdev.osuosl.org>, <gregkh@linuxfoundation.org>,
        <linux-kernel@vger.kernel.org>, <rspringer@google.com>,
        <toddpoynor@google.com>
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 <20200528110408.GJ30374@kadam> <20200529115330.GO30374@kadam>
From:   John Hubbard <jhubbard@nvidia.com>
Message-ID: <bd1064bf-65ae-2c5f-0dad-b52ae12c2ece@nvidia.com>
Date:   Fri, 29 May 2020 13:28:41 -0700
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On 2020-05-29 04:53, Dan Carpenter wrote:
...
> What are the runtime implications of this patch?  I'm still not clear on
> that honestly.

Instead of incrementing each page's refcount by 1 (with get_user_pages()),
pin_user_pages*() will increment by GUP_PIN_COUNTING_BIAS, which is 1024.
That by itself should not have any performance impact, of course, but
there's a couple more things:

For compound pages of more than 2 page size, it will also increment
a separate struct page's field, via hpage_pincount_add().

And finally, it will update /proc/vmstat counters on pin and unpin, via
the optimized mod_node_page_state() call.

So it's expected to be very light. And, for DMA (as opposed to DIO)
situations, the DMA setup time is inevitably much greater than any of
the above overheads, so I expect that this patch will be completely
invisible from a performance point of view.

It would be a "nice to have", though, if anyone were able to do a
performance comparison on the gasket driver for this patch, and/or
basic runtime verification, since I'm sure it's a specialized setup.


thanks,
-- 
John Hubbard
NVIDIA