Re: [PATCH v5 1/3] net: introduce helper sendpage_ok() in include/linux/net.h

[Cong Wang]

On Sun, Aug 16, 2020 at 10:45 PM Christoph Hellwig <hch@xxxxxx> wrote:
>
> On Sun, Aug 16, 2020 at 10:55:09AM -0700, Cong Wang wrote:
> > On Sun, Aug 16, 2020 at 1:36 AM Coly Li <colyli@xxxxxxx> wrote:
> > >
> > > The original problem was from nvme-over-tcp code, who mistakenly uses
> > > kernel_sendpage() to send pages allocated by __get_free_pages() without
> > > __GFP_COMP flag. Such pages don't have refcount (page_count is 0) on
> > > tail pages, sending them by kernel_sendpage() may trigger a kernel panic
> > > from a corrupted kernel heap, because these pages are incorrectly freed
> > > in network stack as page_count 0 pages.
> > >
> > > This patch introduces a helper sendpage_ok(), it returns true if the
> > > checking page,
> > > - is not slab page: PageSlab(page) is false.
> > > - has page refcount: page_count(page) is not zero
> > >
> > > All drivers who want to send page to remote end by kernel_sendpage()
> > > may use this helper to check whether the page is OK. If the helper does
> > > not return true, the driver should try other non sendpage method (e.g.
> > > sock_no_sendpage()) to handle the page.
> >
> > Can we leave this helper to mm subsystem?
> >
> > I know it is for sendpage, but its implementation is all about some
> > mm details and its two callers do not belong to net subsystem either.
> >
> > Think this in another way: who would fix it if it is buggy? I bet mm people
> > should. ;)
>
> No.  This is all about a really unusual imitation in sendpage, which

So netdev people will have to understand and support PageSlab() or
page_count()?

If it is unusual even for mm people, how could netdev people suppose
to understand this unusual mm bug? At least not any better.

> is pretty much unexpected.  In fact the best thing would be to make
> sock_sendpage do the right thing and call sock_no_sendpage based
> on this condition, so that driver writers don't have to worry at all.

Agreed, but kernel_sendpage() still relies on mm to provide a helper
to make the decision and ensure this helper is always up-to-date.

In short, it is all about ownership.

Thanks.