linux-kernel - Re: [PATCH] mm: honor FOLL_GET flag in follow_hugetlb

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Message-ID: <CAH3drwZym3+o2cUhB37Zi6ALj65Z7j+N1w9WA-t1+0xi7XjWaw@mail.gmail.com>
Date:	Wed, 8 May 2013 10:33:07 -0400
From:	Jerome Glisse <j.glisse@...il.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
Cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Jerome Glisse <jglisse@...hat.com>
Subject: Re: [PATCH] mm: honor FOLL_GET flag in follow_hugetlb_page v2

On Tue, May 7, 2013 at 10:41 PM, KOSAKI Motohiro
<kosaki.motohiro@...il.com> wrote:
> On Tue, May 7, 2013 at 8:51 PM, Jerome Glisse <j.glisse@...il.com> wrote:
>> On Tue, May 7, 2013 at 8:47 PM, KOSAKI Motohiro
>> <kosaki.motohiro@...il.com> wrote:
>>> On Tue, May 7, 2013 at 6:58 PM,  <j.glisse@...il.com> wrote:
>>>> From: Jerome Glisse <jglisse@...hat.com>
>>>>
>>>> Do not increase page count if FOLL_GET is not set. None of the
>>>> current user can trigger the issue because none of the current
>>>> user call __get_user_pages with both the pages array ptr non
>>>> NULL and the FOLL_GET flags non set in other word all caller
>>>> of __get_user_pages that don't set the FOLL_GET flags also call
>>>> with pages == NULL.
>>>
>>> Because, __get_user_pages() doesn't allow pages==NULL and FOLL_GET is on.
>>
>> Yes but this allow pages != NULL and FOLL_GET not set and as i said
>> there is no such user of that yet and this is exactly what i was
>> trying to use.
>
> Why? The following bug_on inhibit both case.

Yes i get lost on the double negation, but still my patch is correct
and i am not using gup but follow_hugetlb_page directly and i run into
the issue. My patch does not change the behavior for current user,
just fix assumption new user might have when not setting the FOLL_GET
flags.

>>>     VM_BUG_ON(!!pages != !!(gup_flags & FOLL_GET));
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