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

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.