linux-kernel - Re: [linux-next RFC] mm/gup.c: Convert to use get_user_pages_fast

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Message-ID: <20200523172519.GA17206@bombadil.infradead.org>
Date:   Sat, 23 May 2020 10:25:19 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Souptick Joarder <jrdr.linux@...il.com>
Cc:     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, namhyung@...nel.org,
        pbonzini@...hat.com, sfr@...b.auug.org.au, rppt@...ux.ibm.com,
        msuchanek@...e.de, aneesh.kumar@...ux.ibm.com,
        kvm-ppc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        kvm@...r.kernel.org, John Hubbard <jhubbard@...dia.com>
Subject: Re: [linux-next RFC] mm/gup.c: Convert to use
 get_user_pages_fast_only()

On Sat, May 23, 2020 at 10:11:12PM +0530, Souptick Joarder wrote:
> Renaming the API __get_user_pages_fast() to get_user_pages_
> fast_only() to align with pin_user_pages_fast_only().

Please don't split a function name across lines.  That messes
up people who are grepping for the function name in the changelog.

> 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.
> 
> Updated the documentation of the API.

Everything you have done here is an improvement, and I'd be happy to
see it go in (after fixing the bug I note below).

But in reading through it, I noticed almost every user ...

> -	if (__get_user_pages_fast(hva, 1, 1, &page) == 1) {
> +	if (get_user_pages_fast_only(hva, 1, FOLL_WRITE, &page) == 1) {

passes '1' as the second parameter.  So do we want to add:

static inline bool get_user_page_fast_only(unsigned long addr,
		unsigned int gup_flags, struct page **pagep)
{
	return get_user_pages_fast_only(addr, 1, gup_flags, pagep) == 1;
}

> @@ -2797,10 +2803,7 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
>  	 * FOLL_FAST_ONLY is required in order to match the API description of
>  	 * this routine: no fall back to regular ("slow") GUP.
>  	 */
> -	unsigned int gup_flags = FOLL_GET | FOLL_FAST_ONLY;
> -
> -	if (write)
> -		gup_flags |= FOLL_WRITE;
> +	gup_flags = FOLL_GET | FOLL_FAST_ONLY;

Er ... gup_flags |=, surely?