linux-kernel - Re: [PATCH] mm/gup.c: Handle error at earliest for incorrect nr

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Message-ID: <c25cf212-8684-a107-bc9c-c7067dea7447@nvidia.com>
Date:   Fri, 1 May 2020 14:33:45 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Souptick Joarder <jrdr.linux@...il.com>,
        <akpm@...ux-foundation.org>
CC:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm/gup.c: Handle error at earliest for incorrect nr_pages
 value

On 2020-04-30 13:11, Souptick Joarder wrote:
> As per documentation, pin_user_pages_fast() & get_user_pages_fast()
> will return 0, if nr_pages <= 0. But this can be figure out only after
> going inside the internal_get_user_pages_fast().
> 
> This can be handled early. Adding a check for the same.


Please, no. For precisely the reasons that Andrew gave: you are
attempting to optimize for a case that doesn't matter other than
for error handling, and which is already handled adequately. And
as he also pointed out, it very slightly UN-optimizes the path that
we *do* care about. So why are you still advocating for this?

If you want to change the gup/pup API so that asking for zero pages
means -EINVAL, then fine, go for it. That's a large thing, and a
tree-wide audit, but if you feel it's worth pursuing then it's at
least consistent.

But this patch here needs to be abandoned.


thanks,
-- 
John Hubbard
NVIDIA

> 
> Signed-off-by: Souptick Joarder <jrdr.linux@...il.com>
> ---
>   mm/gup.c | 4 ++++
>   1 file changed, 4 insertions(+)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index 50681f0..a13aaa6 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -2817,6 +2817,8 @@ int get_user_pages_fast(unsigned long start, int nr_pages,
>   	 */
>   	if (WARN_ON_ONCE(gup_flags & FOLL_PIN))
>   		return -EINVAL;
> +	if (nr_pages <= 0)
> +		return 0;
>   
>   	/*
>   	 * The caller may or may not have explicitly set FOLL_GET; either way is
> @@ -2854,6 +2856,8 @@ int pin_user_pages_fast(unsigned long start, int nr_pages,
>   	/* FOLL_GET and FOLL_PIN are mutually exclusive. */
>   	if (WARN_ON_ONCE(gup_flags & FOLL_GET))
>   		return -EINVAL;
> +	if (nr_pages <= 0)
> +		return 0;
>   
>   	gup_flags |= FOLL_PIN;
>   	return internal_get_user_pages_fast(start, nr_pages, gup_flags, pages);
>