linux-kernel - Re: [linux-next RFC v2] mm/gup.c: Convert to use get_user_{page|pages}_fast

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Date:   Sun, 24 May 2020 18:06:55 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Souptick Joarder <jrdr.linux@...il.com>, <paulus@...abs.org>,
        <mpe@...erman.id.au>, <benh@...nel.crashing.org>,
        <akpm@...ux-foundation.org>, <peterz@...radead.org>,
        <mingo@...hat.com>, <acme@...nel.org>, <mark.rutland@....com>,
        <alexander.shishkin@...ux.intel.com>, <jolsa@...hat.com>,
        <namhyung@...nel.org>, <pbonzini@...hat.com>,
        <sfr@...b.auug.org.au>, <rppt@...ux.ibm.com>,
        <aneesh.kumar@...ux.ibm.com>, <msuchanek@...e.de>
CC:     <kvm-ppc@...r.kernel.org>, <linuxppc-dev@...ts.ozlabs.org>,
        <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
        <kvm@...r.kernel.org>, Matthew Wilcox <willy@...radead.org>
Subject: Re: [linux-next RFC v2] mm/gup.c: Convert to use
 get_user_{page|pages}_fast_only()

On 2020-05-23 21:27, Souptick Joarder wrote:
> API __get_user_pages_fast() renamed to get_user_pages_fast_only()
> to align with pin_user_pages_fast_only().
> 
> As part of this we will get rid of write parameter. Instead caller
> will pass FOLL_WRITE to get_user_pages_fast_only(). This will not
> change any existing functionality of the API.
> 
> All the callers are changed to pass FOLL_WRITE.

This looks good. A few nits below, but with those fixed, feel free to
add:

     Reviewed-by: John Hubbard <jhubbard@...dia.com>

> 
> There are few places where 1 is passed to 2nd parameter of
> __get_user_pages_fast() and return value is checked for 1
> like [1]. Those are replaced with new inline
> get_user_page_fast_only().
> 
> [1] if (__get_user_pages_fast(hva, 1, 1, &page) == 1)
> 

We try to avoid talking *too* much about the previous version of
the code. Just enough. So, instead of the above two paragraphs,
I'd compress it down to:

Also: introduce get_user_page_fast_only(), and use it in a few
places that hard-code nr_pages to 1.

...
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 93d93bd..8d4597f 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1817,10 +1817,16 @@ extern int mprotect_fixup(struct vm_area_struct *vma,
>   /*
>    * doesn't attempt to fault and will return short.
>    */
> -int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
> -			  struct page **pages);
> +int get_user_pages_fast_only(unsigned long start, int nr_pages,
> +			unsigned int gup_flags, struct page **pages);

Silly nit:

Can you please leave the original indentation in place? I don't normally
comment about this, but I like the original indentation better, and it matches
the pin_user_pages_fast() below, too.

...
> @@ -2786,8 +2792,8 @@ static int internal_get_user_pages_fast(unsigned long start, int nr_pages,
>    * If the architecture does not support this function, simply return with no
>    * pages pinned.
>    */
> -int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
> -			  struct page **pages)
> +int get_user_pages_fast_only(unsigned long start, int nr_pages,
> +			unsigned int gup_flags, struct page **pages)


Same thing here: you've changed the original indentation, which was (arguably, but
to my mind anyway) more readable, and for no reason. It still would have fit within
80 cols.

I'm sure it's a perfect 50/50 mix of people who prefer either indentation style, and
so for brand new code, I'll remain silent, as long as it is consistent with either
itself and/or the surrounding code. But changing it back and forth is a bit 
aggravating, and best avoided. :)


thanks,
-- 
John Hubbard
NVIDIA