linux-kernel - Re: [PATCH] mm, gup: introduce concept of "foreign" get_user

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Date:	Wed, 27 Jan 2016 14:59:55 -0800
From:	Dave Hansen <dave@...1.net>
To:	Vlastimil Babka <vbabka@...e.cz>, linux-kernel@...r.kernel.org
Cc:	linux-mm@...ck.org, x86@...nel.org, dave.hansen@...ux.intel.com,
	akpm@...ux-foundation.org, kirill.shutemov@...ux.intel.com,
	aarcange@...hat.com, n-horiguchi@...jp.nec.com,
	srikar@...ux.vnet.ibm.com, jack@...e.cz
Subject: Re: [PATCH] mm, gup: introduce concept of "foreign" get_user_pages()

On 01/27/2016 03:30 AM, Vlastimil Babka wrote:
>> >  long __get_user_pages_unlocked(struct task_struct *tsk, struct mm_struct *mm,
>> >  			       unsigned long start, unsigned long nr_pages,
>> >  			       int write, int force, struct page **pages,
>> >  			       unsigned int gup_flags)
>> >  {
>> >  	long ret;
>> > -	down_read(&mm->mmap_sem);
>> > -	ret = get_user_pages(tsk, mm, start, nr_pages, write, force,
>> > -			     pages, NULL);
>> > -	up_read(&mm->mmap_sem);
>> > +	down_read(&current->mm->mmap_sem);
>> > +	ret = get_user_pages(start, nr_pages, write, force, pages, NULL);
>> > +	up_read(&current->mm->mmap_sem);
> I understand your reply to lkp report also means that this no longer locks
> current's mmap_sem? :)

Ahhh, good point.  Now that we're passing them in again, we've got to
use them.  That also makes it wrong to call get_user_pages() directly.
We should be using the __ variant so we can pass in the tsk/mm that came
into __get_user_pages_unlocked().

Updated version is attached.

View attachment "get_current_user_pages.patch" of type "text/x-patch" (39973 bytes)