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

[David Miller <davem@davemloft.net>]

From: Christoph Hellwig <hch@lst.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.