linux-kernel - Re: [patch] agp: don't lock pages

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Date:	Thu, 26 Jul 2007 07:26:53 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	airlied@...il.com, magnade@...il.com
Subject: Re: [patch] agp: don't lock pages

On Wed, 2007-07-25 at 13:19 +0200, Nick Piggin wrote:
> Hi,
> 
> Does this patch solve the X problem? Does anyone see anything wrong
> with it or know why agp was locking the pages?

We need to do a little bit of auditing here, but I suspect it will turn
out all right. I think the reason it locked them in the first place was
to avoid AGP pages mapped into process space from being swapped out.

I think that should be taken care of by appropriate vma flags nowadays,
but we need to double check. It also might have been a way around dodgy
refcounting at one point but I think we got that right nowadays (I
remember fixing issues in that area when we removed PageReserved from
those pages back then).

Ben.

> --
> AGP should not need to lock pages. They are not protecting any race
> because there is no lock_page calls, only SetPageLocked.
> 
> This is causing hangs with d00806b183152af6d24f46f0c33f14162ca1262a.
> 
> Signed-off-by: Nick Piggin <npiggin@...e.de>
> 
> diff --git a/drivers/char/agp/generic.c b/drivers/char/agp/generic.c
> index d535c40..3db4f40 100644
> --- a/drivers/char/agp/generic.c
> +++ b/drivers/char/agp/generic.c
> @@ -1170,7 +1170,6 @@ void *agp_generic_alloc_page(struct agp_
>  	map_page_into_agp(page);
>  
>  	get_page(page);
> -	SetPageLocked(page);
>  	atomic_inc(&agp_bridge->current_memory_agp);
>  	return page_address(page);
>  }
> @@ -1187,7 +1186,6 @@ void agp_generic_destroy_page(void *addr
>  	page = virt_to_page(addr);
>  	unmap_page_from_agp(page);
>  	put_page(page);
> -	unlock_page(page);
>  	free_page((unsigned long)addr);
>  	atomic_dec(&agp_bridge->current_memory_agp);
>  }
> diff --git a/drivers/char/agp/intel-agp.c b/drivers/char/agp/intel-agp.c
> index a124060..2f319f4 100644
> --- a/drivers/char/agp/intel-agp.c
> +++ b/drivers/char/agp/intel-agp.c
> @@ -213,7 +213,6 @@ static void *i8xx_alloc_pages(void)
>  	}
>  	global_flush_tlb();
>  	get_page(page);
> -	SetPageLocked(page);
>  	atomic_inc(&agp_bridge->current_memory_agp);
>  	return page_address(page);
>  }
> @@ -229,7 +228,6 @@ static void i8xx_destroy_pages(void *add
>  	change_page_attr(page, 4, PAGE_KERNEL);
>  	global_flush_tlb();
>  	put_page(page);
> -	unlock_page(page);
>  	__free_pages(page, 2);
>  	atomic_dec(&agp_bridge->current_memory_agp);
>  }
> diff --git a/drivers/char/agp/sgi-agp.c b/drivers/char/agp/sgi-agp.c
> index cda608c..98cf8ab 100644
> --- a/drivers/char/agp/sgi-agp.c
> +++ b/drivers/char/agp/sgi-agp.c
> @@ -51,7 +51,6 @@ static void *sgi_tioca_alloc_page(struct
>  		return NULL;
>  
>  	get_page(page);
> -	SetPageLocked(page);
>  	atomic_inc(&agp_bridge->current_memory_agp);
>  	return page_address(page);
>  }

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