Tariq Toukan [off-list ref] wrote:

Any other debug information that we can provide to progress with the analysis?

Can you see if the problem still happens on this branch of my tree?

https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git/log/?h=sendpage-3-frag

It eases the restriction that the WARN_ON is warning about by (in patch 1[*])
copying slab objects into page fragments.

David

[*] "net: Copy slab data for sendmsg(MSG_SPLICE_PAGES)"