Subject: Re: [PATCH net-next v3 01/18] net: Copy slab data for sendmsg(MSG_SPLICE_PAGES)

From: Jakub Kicinski <[email-masked]>
Date: Thu, 22 Jun 2023 11:12:34 -0700

On Tue, 20 Jun 2023 15:53:20 +0100 David Howells wrote:
> If sendmsg() is passed MSG_SPLICE_PAGES and is given a buffer that contains
> some data that's resident in the slab, copy it rather than returning EIO.

How did that happen? I thought MSG_SPLICE_PAGES comes from former
sendpage users and sendpage can't operate on slab pages.

> This can be made use of by a number of drivers in the kernel, including:
> iwarp, ceph/rds, dlm, nvme, ocfs2, drdb.  It could also be used by iscsi,
> rxrpc, sunrpc, cifs and probably others.
> 
> skb_splice_from_iter() is given it's own fragment allocator as
> page_frag_alloc_align() can't be used because it does no locking to prevent
> parallel callers from racing.

The locking is to local_bh_disable(). Does the milliont^w new frag
allocator have any additional benefits?

>  alloc_skb_frag() uses a separate folio for
> each cpu and locks to the cpu whilst allocating, reenabling cpu migration
> around folio allocation.