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

From: Coly Li
Date: Fri Sep 18 2020 - 04:37:34 EST

On 2020/8/22 05:14, David Miller wrote:
> From: Christoph Hellwig <hch@xxxxxx>
> 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