How to determine if a page can be spliced into an skbuff, or if it should be copied/rejected?

[David Howells]

Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:

> On Tue, Apr 11, 2023 at 05:08:50PM +0100, David Howells wrote:
> > Add a function to handle MSG_SPLICE_PAGES being passed internally to
> > sendmsg().  Pages are spliced into the given socket buffer if possible and
> > copied in if not (ie. they're slab pages or have a zero refcount).
> 
> That "ie." would better be "e.g." - that condition is *not* enough for
> tell the unsafe ones from the rest.
> 
> sendpage_ok() would be better off called "might_be_ok_to_sendpage()".
> If it's false, we'd better not grab a reference to the page and expect the
> sucker to stay safe until the reference is dropped.  However, AFAICS
> it might return true on a page that is not safe in that respect.
> 
> What rules do you propose for sendpage users?  "Pass whatever page reference
> you want, it'll do the right thing"?  Anything short of that would better
> be documented as explicitly as possible...

Hmmm...  Fair point.  Is everything passed through splice guaranteed to be
safe, I wonder?  Probably not because vmsplice().  Does that mean the existing
callers of sendpage_ok() are also making unviable assumptions?

So there are the following 'classes' of memory that I can immediately think
of:

 - Zero page				Splice (no ref?)
 - Kernel core data			Splice
 - Module core data (vmalloc'd)		Splice
 - Supervisor stack			Copy
 - Slab objects				Copy
 - Page frags				Splice
 - Other skbuff frags			Splice
 - Arbitrary pages (eg. sunrpc xdr buf)	Splice (probably)
 - Ordinary pipe buffers		Splice
 - Spliced tmpfs			Splice
 - Spliced pagecache (file/block)	Splice
 - Spliced DIO file/block		Splice
 - Vmspliced mmap'd anon		Splice (with pin?)
 - Vmspliced MAP_SHARED pagecache	Splice (with pin?)
 - Vmspliced MAP_SHARED DAX		Splice?
 - Vmspliced MAP_SHARED MTD		Splice?
 - Vmspliced MAP_SHARED other device	Reject? (e.g. graphics card mem)
 - Vmspliced /dev/{mem,kmem}		Reject?
 
Question is how to tell that we're looking at something that must be copied or
rejected?  sendpage_ok() checks the PG_slab bit and the pagecount, for
example.

David