linux-kernel - Re: [PATCH] staging: gasket: Convert get_user_pages*() --> pin_user

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Message-ID: <20200528110408.GJ30374@kadam>
Date:   Thu, 28 May 2020 14:04:08 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Souptick Joarder <jrdr.linux@...il.com>
Cc:     rspringer@...gle.com, toddpoynor@...gle.com, benchan@...omium.org,
        gregkh@...uxfoundation.org, devel@...verdev.osuosl.org,
        John Hubbard <jhubbard@...dia.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] staging: gasket: Convert get_user_pages*() -->
 pin_user_pages*()

On Thu, May 28, 2020 at 02:32:42AM +0530, Souptick Joarder wrote:
> This code was using get_user_pages_fast(), in a "Case 2" scenario
> (DMA/RDMA), using the categorization from [1]. That means that it's
> time to convert the get_user_pages_fast() + put_page() calls to
> pin_user_pages_fast() + unpin_user_page() calls.

You are saying that the page is used for DIO and not DMA, but it sure
looks to me like it is used for DMA.

   503                          /* Map the page into DMA space. */
   504                          ptes[i].dma_addr =
   505                                  dma_map_page(pg_tbl->device, page, 0, PAGE_SIZE,
   506                                               DMA_BIDIRECTIONAL);

To be honest, that starting paragraph was confusing.  At first I thought
you were saying gasket was an RDMA driver. :P  I shouldn't have to read
a different document to understand the commit message.  It should be
summarized enough and the other documentation is supplemental.

"In 2019 we introduced pin_user_pages() and now we are converting
get_user_pages() to the new API as appropriate".

> 
> There is some helpful background in [2]: basically, this is a small
> part of fixing a long-standing disconnect between pinning pages, and
> file systems' use of those pages.

What is the impact of this patch on runtime?

> 
> [1] Documentation/core-api/pin_user_pages.rst
> 
> [2] "Explicit pinning of user-space pages":
> 	https://lwn.net/Articles/807108/
> 
> Signed-off-by: Souptick Joarder <jrdr.linux@...il.com>
> Cc: John Hubbard <jhubbard@...dia.com>
> 
> Hi,
> 
> I'm compile tested this, but unable to run-time test, so any testing
> help is much appriciated.
> ---

The "Hi" part of patch should have been under the "---" cut off line so
this will definitely need to be resent.

regards,
dan carpenter