s390/uv: Improve splitting of large folios that cannot be split while dirty

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.

Until dirty folios can be split more reliably, let's manually trigger
writeback of the problematic folio using
filemap_write_and_wait_range(), and retry the split immediately
afterwards exactly once, before looking up the folio again.

Should this logic be part of split_folio()? Likely not; most split users
don't have to split so eagerly to make any progress.

For now, this seems to affect xfs, zonefs and erofs, and this patch
makes it work again (tested on xfs only).

While this could be considered a fix for commit 67958013 ("xfs: Support
large folios"), commit df2f9708 ("zonefs: enable support for large
folios") and commit ce529cc2 ("erofs: enable large folios for iomap
mode"), before commit eef88fe4 ("s390/uv: Split large folios in
gmap_make_secure()"), we did not try splitting large folios at all. So it's
all rather part of making SE compatible with file systems that support
large folios. But to have some "Fixes:" tag, let's just use eef88fe4.

Not CCing stable, because there are a lot of dependencies, and it simply
not working is not critical in stable kernels.

Reported-by: Sebastian Mitterle <smitterl@redhat.com>
Closes: https://issues.redhat.com/browse/RHEL-58218


Fixes: eef88fe4 ("s390/uv: Split large folios in gmap_make_secure()")
Signed-off-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20250516123946.1648026-4-david@redhat.com


Message-ID: <20250516123946.1648026-4-david@redhat.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>