linux-kernel - Re: [PATCH 1/5] mm: gup: add get_user_pages_locked and get_user_pages

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Date:	Fri, 16 Jan 2015 14:47:55 +0200
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Michel Lespinasse <walken@...gle.com>,
	Andrew Jones <drjones@...hat.com>,
	Hugh Dickins <hughd@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Andres Lagar-Cavilla <andreslc@...gle.com>,
	Minchan Kim <minchan@...nel.org>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	"\\\"Dr. David Alan Gilbert\\\"" <dgilbert@...hat.com>,
	Peter Feiner <pfeiner@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	David Miller <davem@...emloft.net>,
	Steve Capper <steve.capper@...aro.org>,
	Johannes Weiner <jweiner@...hat.com>
Subject: Re: [PATCH 1/5] mm: gup: add get_user_pages_locked and
 get_user_pages_unlocked

On Tue, Jan 13, 2015 at 05:37:50PM +0100, Andrea Arcangeli wrote:
> We can leverage the VM_FAULT_RETRY functionality in the page fault
> paths better by using either get_user_pages_locked or
> get_user_pages_unlocked.
> 
> The former allow conversion of get_user_pages invocations that will
> have to pass a "&locked" parameter to know if the mmap_sem was dropped
> during the call. Example from:
> 
>     down_read(&mm->mmap_sem);
>     do_something()
>     get_user_pages(tsk, mm, ..., pages, NULL);
>     up_read(&mm->mmap_sem);
> 
> to:
> 
>     int locked = 1;
>     down_read(&mm->mmap_sem);
>     do_something()
>     get_user_pages_locked(tsk, mm, ..., pages, &locked);
>     if (locked)
>         up_read(&mm->mmap_sem);
> 
> The latter is suitable only as a drop in replacement of the form:
> 
>     down_read(&mm->mmap_sem);
>     get_user_pages(tsk, mm, ..., pages, NULL);
>     up_read(&mm->mmap_sem);
> 
> into:
> 
>     get_user_pages_unlocked(tsk, mm, ..., pages);
> 
> Where tsk, mm, the intermediate "..." paramters and "pages" can be any
> value as before. Just the last parameter of get_user_pages (vmas) must
> be NULL for get_user_pages_locked|unlocked to be usable (the latter
> original form wouldn't have been safe anyway if vmas wasn't null, for
> the former we just make it explicit by dropping the parameter).
> 
> If vmas is not NULL these two methods cannot be used.
> 
> Signed-off-by: Andrea Arcangeli <aarcange@...hat.com>
> Reviewed-by: Andres Lagar-Cavilla <andreslc@...gle.com>
> Reviewed-by: Peter Feiner <pfeiner@...gle.com>

Reviewed-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>

-- 
 Kirill A. Shutemov
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