linux-kernel - Re: [PATCH] net: bypass ->sendpage for slab pages

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Date:   Fri, 18 Sep 2020 16:37:24 +0800
From:   Coly Li <colyli@...e.de>
To:     David Miller <davem@...emloft.net>, hch@....de
Cc:     kuba@...nel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net: bypass ->sendpage for slab pages

On 2020/8/22 05:14, David Miller wrote:
> From: Christoph Hellwig <hch@....de>
> Date: Thu, 20 Aug 2020 06:37:44 +0200
> 
>> If you look at who uses sendpage outside the networking layer itself
>> you see that it is basically block driver and file systems.  These
>> have no way to control what memory they get passed and have to deal
>> with everything someone throws at them.
> 
> I see nvme doing virt_to_page() on several things when it calls into
> kernel_sendpage().
> 
> This is the kind of stuff I want cleaned up, and which your patch
> will not trap nor address.
> 
> In nvme it sometimes seems to check for sendpage validity:
> 
> 		/* can't zcopy slab pages */
> 		if (unlikely(PageSlab(page))) {
> 			ret = sock_no_sendpage(queue->sock, page, offset, len,
> 					flags);
> 		} else {
> 			ret = kernel_sendpage(queue->sock, page, offset, len,
> 					flags);
> 		}
> 
> Yet elsewhere does not and just blindly calls:
> 
> 	ret = kernel_sendpage(queue->sock, virt_to_page(pdu),
> 			offset_in_page(pdu) + req->offset, len,  flags);
> 
> This pdu seems to come from a page frag allocation.
> 
> That's the target side.  On the host side:
> 
> 		ret = kernel_sendpage(cmd->queue->sock, page, cmd->offset,
> 					left, flags);
> 
> No page slab check or anything like that.
> 
> I'm hesitent to put in the kernel_sendpage() patch, becuase it provides a
> disincentive to fix up code like this.
> 

Hi David and Christoph,

It has been quiet for a while, what should we go next for the
kernel_sendpage() related issue ?

Will Christoph's or my series be considered as proper fix, or maybe I
should wait for some other better idea to show up? Any is OK for me,
once the problem is fixed.

Thanks in advance.

Coly Li