linux-kernel - RE: [linux-next PATCH] mlx5: wrong page mask if CONFIG_ARCH_DMA_ADDR_T

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Message-ID: <DB4PR05MB448D41F0471F690E688F6BCC5E70@DB4PR05MB448.eurprd05.prod.outlook.com>
Date:	Mon, 13 Apr 2015 06:47:11 +0000
From:	Eli Cohen <eli@...lanox.com>
To:	Honggang LI <honli@...hat.com>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [linux-next PATCH] mlx5: wrong page mask if
 CONFIG_ARCH_DMA_ADDR_T_64BIT enabled for 32Bit architectures

 On Mon, Apr 13, 2015 at 08:33:33AM +0800, Honggang LI wrote:
> 
> Yes, replaced all of the PAGE_MASK in the file. Please see attched new
> patch.
> 

I think you need to send the new patch inline in your email.

> > 
> > @@ -241,7 +243,7 @@ static void free_4k(struct mlx5_core_dev *dev, u64 addr)  static int alloc_system_page(struct mlx5_core_dev *dev, u16 func_id)  {
> >     struct page *page;
> > -   u64 addr;
> > +   u64 addr = 0; [Eli] Why is this required?
> 
> For 32bit architectures, if CONFIG_ARCH_DMA_ADDR_T_64BIT disabled and
> physical memory is less than 4GB, dma_map_page will return an u32 integer 
> less than 0xffff_ffff. And if addr had been initialized with random
> rubbish in the stack of alloc_system_page, the high four bytes is
> unpredictable. The new mask, MLX5_NUM_4K_IN_PAGE, will reserve the high
> four bytes. So, free_4k/find_fw_page will randomly failed.
> 

Sorry, I still don't understand the issue here. MLX5_NUM_4K_IN_PAGE is
not a mask and will always get the correct value which is fairly
small.
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