linux-kernel - Re: [PATCH] mm: extend prefault helpers to fault in more than PAGE

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Message-ID: <CAJd=RBDFUZi3_tk2ZRpjF=5a9884zJGv4Ti_kM0pjXnRmw0jKA@mail.gmail.com>
Date:	Fri, 17 Feb 2012 21:06:17 +0800
From:	Hillf Danton <dhillf@...il.com>
To:	Hillf Danton <dhillf@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Intel Graphics Development <intel-gfx@...ts.freedesktop.org>,
	DRI Development <dri-devel@...ts.freedesktop.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux MM <linux-mm@...ck.org>
Cc:	Daniel Vetter <daniel.vetter@...ll.ch>
Subject: Re: [PATCH] mm: extend prefault helpers to fault in more than PAGE_SIZE

On Thu, Feb 16, 2012 at 11:14 PM, Daniel Vetter <daniel@...ll.ch> wrote:
> On Thu, Feb 16, 2012 at 09:32:08PM +0800, Hillf Danton wrote:
>> On Thu, Feb 16, 2012 at 8:01 PM, Daniel Vetter <daniel.vetter@...ll.ch> wrote:
>> > @@ -416,17 +417,20 @@ static inline int fault_in_pages_writeable(char __user *uaddr, int size)
>> >         * Writing zeroes into userspace here is OK, because we know that if
>> >         * the zero gets there, we'll be overwriting it.
>> >         */
>> > -       ret = __put_user(0, uaddr);
>> > +       while (uaddr <= end) {
>> > +               ret = __put_user(0, uaddr);
>> > +               if (ret != 0)
>> > +                       return ret;
>> > +               uaddr += PAGE_SIZE;
>> > +       }
>>
>> What if
>>              uaddr & ~PAGE_MASK == PAGE_SIZE -3 &&
>>                 end & ~PAGE_MASK == 2
>
> I don't quite follow - can you elaborate upon which issue you're seeing?

I concerned that __put_user(0, end) is missed, but it was added below.

And looks good to me.
Hillf

>        if (ret == 0) {
> -               char __user *end = uaddr + size - 1;
> -
>                /*
>                 * If the page was already mapped, this will get a cache miss
>                 * for sure, so try to avoid doing it.
>                 */
> -               if (((unsigned long)uaddr & PAGE_MASK) !=
> +               if (((unsigned long)uaddr & PAGE_MASK) ==
>                                ((unsigned long)end & PAGE_MASK))
> -                       ret = __put_user(0, end);
> +                       ret = __put_user(0, end);
>        }
>        return ret;