Re: [PATCH v2 6/7] nfs: Optimize direct I/O to use folios for requests

On Thu, Jun 18, 2026 at 07:20:06PM +0100, Matthew Wilcox wrote:

Hi Matthew, Christoph, Trond,

> On Thu, Jun 18, 2026 at 07:10:45AM -0700, Christoph Hellwig wrote:
> > On Tue, Jun 16, 2026 at 05:23:48PM +0000, Pranjal Shrivastava wrote:
> > > AFAIU, the MM subsystem explicitly ensures that every valid struct page
> > > is part of a folio.
> > 
> > It is definitively not what the vision for the folio is, although if
> > I'm not mistaken it actually is still true right now.
> 
> It's not true, eg, for slab.  While there's still a struct page there
> for slab, there's no refcount and flags like PG_locked have different
> meanings.  You'll get into a lot of trouble trying to treat slabs as
> folios (and that will include assertions tripping).
> 
> > This whole
> > area is a minefield unfortunately, and we also ran into it with
> > iov_iter_extract_bvecs and the earlier block code it was extracted
> > from.  Adding the relevant people and lists, but for now your best
> > bet is to stick to what the block code does or even better reuse
> > as much as possible of that code.
> 
> Yes.  Fundamentally, it is no business of the filesystem what the iov_iter
> refers to.  We can do direct io to slab memory, vmalloc memory, memory
> that doesn't have a struct page (eg iomem), or whatever we choose.
> 

Thanks for the clarification. I understand the larger vision of keeping
filesystems agnostic to the underlying memory represented by the iov_iter

The documentation for page_folio() [1] mentions that "Every page is part
of a folio," but it appears there are important nuances regarding slab
and other memory types that I was not aware of.

However, I am a bit confused on one point:
Looking at iov_iter_extract_bvecs() [1] it relies on 
get_contig_folio_len() [2], which calls page_folio() on the pages 
extracted (via iov_iter_extract_pages()) without additional checks for
slab or vmalloc memory. 

I am happy to refactor the NFS Direct I/O path to reuse the same helper
(get_contig_folio_len()) from the bvec extractor, but I'm a little 
confused as the bvec extractor seems to suffer from the same risk?

Is the recommendation to keep these details abstracted by the iov_iter
lib and eventually hide things like iov_iter_extract_pages() and manual
folio conversions from filesystems entirely?

If that's the case, would it help to export get_contig_folio_len() (or 
introduce new helpers) in the iov_iter lib for NFS and other fs to use?

Thanks,
Praan

[1] https://elixir.bootlin.com/linux/v7.1-rc6/source/include/linux/page-flags.h#L291
[2] https://elixir.bootlin.com/linux/v7.1/source/lib/iov_iter.c#L1849