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Message-ID: <YgVntJO3E+8ILRDV@google.com>
Date: Thu, 10 Feb 2022 11:29:56 -0800
From: Minchan Kim <minchan@...nel.org>
To: John Hubbard <jhubbard@...dia.com>
Cc: linux-mm <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: pin_user_pages supports NULL pages arguments?
On Thu, Feb 10, 2022 at 11:20:31AM -0800, John Hubbard wrote:
> On 2/10/22 11:17, Minchan Kim wrote:
> > * pin_user_pages() - pin user pages in memory for use by other devices
> > < snip >
> > * @pages: array that receives pointers to the pages pinned.
> > * Should be at least nr_pages long. Or NULL, if caller
> > * only intends to ensure the pages are faulted in.
> >
> > pin_user_pages(,, pages = NULL, );
> > gup_flags |= FOLL_PIN
> > __get_user_pages_locked
> > __get_user_pages
> > ..
> > VM_BUG_ON(!!pages != !!(gup_flags & (FOLL_GET | FOLL_PIN)));
>
> Only FOLL_GET or FOLL_PIN are supposed to fill in the **pages array. So
> if a caller passes a null **pages arg, then that caller must not also
> set FOLL_GET or FOLL_PIN. That's what the VM_BUG_ON() is expressing.
Yub, but pin_user_pages adds FOLL_PIN unconditinally and the comments
says it supports NUU pages argument. Isn't it conflict?
>
> Perhaps that should be part of the documentation. It sort of is already,
> for get_user_pages*().
I expected it was just copied from get_user_pages.
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