Re: [PATCH v1 0/3] s390/uv: handle folios that cannot be split while dirty

[David Hildenbrand]

On 16.05.25 19:07, Claudio Imbrenda wrote:
> On Fri, 16 May 2025 14:39:43 +0200
> David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> >  From patch #3:
> >
> > "
> > Currently, starting a PV VM on an iomap-based filesystem with large
> > folio support, such as XFS, will not work. We'll be stuck in
> > unpack_one()->gmap_make_secure(), because we can't seem to make progress
> > splitting the large folio.
> >
> > The problem is that we require a writable PTE but a writable PTE under such
> > filesystems will imply a dirty folio.
> >
> > So whenever we have a writable PTE, we'll have a dirty folio, and dirty
> > iomap folios cannot currently get split, because
> > split_folio()->split_huge_page_to_list_to_order()->filemap_release_folio()
> > will fail in iomap_release_folio().
> >
> > So we will not make any progress splitting such large folios.
> > "
> >
> > Let's fix one related problem during unpack first, to then handle such
> > folios by triggering writeback before immediately trying to split them
> > again.
> >
> > This makes it work on XFS with large folios again.
> >
> > Long-term, we should cleanly supporting splitting such folios even
> > without writeback, but that's a bit harder to implement and not a quick
> > fix.
>
> yet another layer of duck tape
>
> I really dislike the current interaction between secure execution and
> I/O, I hope I can get a cleaner solution as soon as possible

I'll be more than happy to review such a series -- hoping we can just 
support large folios naturally :)

> Reviewed-by: Claudio Imbrenda <imbrenda@xxxxxxxxxxxxx>

Thanks!

> David: thanks for fixing this mess!


NP; I had a prototype of patch #3 for a long time. But after rebasing on 
top of your work I saw these weird validation errors and just couldn't 
find the issue. And I only saw them with patch #3 on ordinary pagecache 
folios, not with shmem, which severely confused.

... gave it another try today after ~1month and almost immediately 
spotted the issue.

Some things just need time :)

--
Cheers,

David / dhildenb