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

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Date:   Fri, 29 May 2020 11:46:39 +0530
From:   Souptick Joarder <jrdr.linux@...il.com>
To:     Dan Carpenter <dan.carpenter@...cle.com>,
        John Hubbard <jhubbard@...dia.com>
Cc:     Rob Springer <rspringer@...gle.com>,
        Todd Poynor <toddpoynor@...gle.com>, benchan@...omium.org,
        Greg KH <gregkh@...uxfoundation.org>,
        "open list:ANDROID DRIVERS" <devel@...verdev.osuosl.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] staging: gasket: Convert get_user_pages*() --> pin_user_pages*()

On Thu, May 28, 2020 at 4:34 PM Dan Carpenter <dan.carpenter@...cle.com> wrote:
>
> 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.

No, I was referring to "Case 2" scenario in change log which means  it is
used for DMA, not DIO.

>
>    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".

As all other similar conversion have similar change logs, so I was trying
to maintain the same. John might have a different opinion on this.

John, Any further opinion ??

>
> >
> > 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?

I don't have the hardware to validate the runtime impact and will
wait if someone is going to validate it for runtime impact.

>
> >
> > [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.

Sorry about it.
Will wait for feedback from John before resend it :)

>
> regards,
> dan carpenter
>