linux-kernel - Re: RFC: get_user_pages_locked|unlocked to leverage VM_FAULT

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Message-ID: <20141001153611.GC2843@worktop.programming.kicks-ass.net>
Date:	Wed, 1 Oct 2014 17:36:11 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	Andres Lagar-Cavilla <andreslc@...gle.com>,
	Gleb Natapov <gleb@...nel.org>,
	Radim Krcmar <rkrcmar@...hat.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Andy Lutomirski <luto@...capital.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Sasha Levin <sasha.levin@...cle.com>,
	Jianyu Zhan <nasa4836@...il.com>,
	Paul Cassella <cassella@...y.com>,
	Hugh Dickins <hughd@...gle.com>,
	Peter Feiner <pfeiner@...gle.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	"Dr. David Alan Gilbert" <dgilbert@...hat.com>
Subject: Re: RFC: get_user_pages_locked|unlocked to leverage VM_FAULT_RETRY

On Fri, Sep 26, 2014 at 07:25:35PM +0200, Andrea Arcangeli wrote:
> diff --git a/drivers/dma/iovlock.c b/drivers/dma/iovlock.c
> index bb48a57..12ea7c3 100644
> --- a/drivers/dma/iovlock.c
> +++ b/drivers/dma/iovlock.c
> @@ -95,17 +95,11 @@ struct dma_pinned_list *dma_pin_iovec_pages(struct iovec *iov, size_t len)
>  		pages += page_list->nr_pages;
>  
>  		/* pin pages down */
> -		down_read(&current->mm->mmap_sem);
> -		ret = get_user_pages(
> -			current,
> -			current->mm,
> +		ret = get_user_pages_fast(
>  			(unsigned long) iov[i].iov_base,
>  			page_list->nr_pages,
>  			1,	/* write */
> -			0,	/* force */
> -			page_list->pages,
> -			NULL);
> -		up_read(&current->mm->mmap_sem);
> +			page_list->pages);
>  
>  		if (ret != page_list->nr_pages)
>  			goto unpin;

> --- a/drivers/misc/sgi-gru/grufault.c
> +++ b/drivers/misc/sgi-gru/grufault.c
> @@ -198,8 +198,7 @@ static int non_atomic_pte_lookup(struct vm_area_struct *vma,
>  #else
>  	*pageshift = PAGE_SHIFT;
>  #endif
> -	if (get_user_pages
> -	    (current, current->mm, vaddr, 1, write, 0, &page, NULL) <= 0)
> +	if (get_user_pages_fast(vaddr, 1, write, &page) <= 0)
>  		return -EFAULT;
>  	*paddr = page_to_phys(page);
>  	put_page(page);

> diff --git a/drivers/scsi/st.c b/drivers/scsi/st.c
> index aff9689..c89dcfa 100644
> --- a/drivers/scsi/st.c
> +++ b/drivers/scsi/st.c
> @@ -4536,18 +4536,12 @@ static int sgl_map_user_pages(struct st_buffer *STbp,
>  		return -ENOMEM;
>  
>          /* Try to fault in all of the necessary pages */
> -	down_read(&current->mm->mmap_sem);
>          /* rw==READ means read from drive, write into memory area */
> -	res = get_user_pages(
> -		current,
> -		current->mm,
> +	res = get_user_pages_fast(
>  		uaddr,
>  		nr_pages,
>  		rw == READ,
> -		0, /* don't force */
> -		pages,
> -		NULL);
> -	up_read(&current->mm->mmap_sem);
> +		pages);
>  
>  	/* Errors and no page mapped should return here */
>  	if (res < nr_pages)


For all these and the other _fast() users, is there an actual limit to
the nr_pages passed in? Because we used to have the 64 pages limit from
DIO, but without that we get rather long IRQ-off latencies.
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