Re: [PATCH 03/14] iov_iter: extract a iov_iter_extract_bvecs helper from bio code

[David Howells <dhowells@redhat.com>]

Christoph Hellwig <hch@lst.de> wrote:

> On Fri, Jan 23, 2026 at 11:37:17AM +0000, David Howells wrote:
> > Christoph Hellwig <hch@lst.de> wrote:
> > 
> > > +static unsigned int get_contig_folio_len(struct page **pages,
> > > +		unsigned int *num_pages, size_t left, size_t offset)
> > > +{
> > > +	struct folio *folio = page_folio(pages[0]);
> > 
> > You can't do this.  You cannot assume that pages[0] is of folio type.
> > vmsplice() is unfortunately a thing and the page could be a network read
> > buffer.
> 
> Hmm, this just moves around existing code added in commit ed9832bc08db
> ("block: introduce folio awareness and add a bigger size from folio").
> 
> How do we get these network read buffers into either a user address
> space or a (non-bvec) iter passed to O_DIRECT reads/writes?

Splice from TCP socket to pipe, vmsplice from there into process address
space; DIO write() from there I think should do it.

What you might need to do is write page-sized chunks into one end of the TCP
socket and flush it after each one so that vmsplice() sees page-sized chunks
of data.  I'm not sure how well an external connection would work to get
actual transmission buffers.  The problem is that the received packet is
page-aligned, including the network headers (I think), so if you can, say,
send 8K packets, you'd have to try and guess where the page boundaries are as
vmsplice can only work on whole pages.

Can we make vmsplice() just copy data?

> Can we come up with testcase for xfstests or blktests for this?
> 
> How do we find out if a given page is a folio and that we can do this?

That's a question for Willy.

David