Re: [LSF/MM/BPF TOPIC] Migrating the un-migratable

[Matthew Wilcox <willy@infradead.org>]

On Wed, Jan 29, 2025 at 05:10:03PM +0100, David Hildenbrand wrote:
> As one example, in context of FUSE we recently discovered that folios that
> are under writeback cannot be migrated, and user space in control of when
> writeback will end. Something similar can happen ->readahead() where user
> space is in charge of supplying page content. Networking filesystems in
> general seem to be prone to this as well.

You're not wrong.  The question is whether we're willing to put the
asterisk on "In the presence of a misbehaving server (network or fuse),
our usual guarantees do not apply".  I'm not sure it's a soluble
problem, though.  Normally writeback (or, as you observed, readahead)
completes just fine and we don't need to use non-movable pages for them.

But if someone trips over the network cable, anything in flight becomes
unmovable until someone plugs it back in.  We've given the DMA address
of the memory to a network adapter, and that's generally a non-revokable
step (maybe the iommu could save us, but at what cost?)

> As another example, failing to split large folios can prevent migration if
> memory is fragmented. XFS (IOMAP in general) refuses to split folios that
> are dirty [3]. Splitting of folios and page migration have a lot in common.

Welll ... yes and no.  iomap refuses to split a dirty folio because it
has a per-folio data structure which tells us which blocks in the folio
are dirty.  If we split the folio, we have to allocate an extra data
structure for each new folio that we create.  It's not impossible, but
it's a big ask for slab.  It'll be a lot better once Zi Yan's patch is
in to only split folios as needed rather than all the way.

That problem doesn't arise for migration.  filemap_release_folio() is
only called by fallback_migrate_folio(), which is only called if the
filesystem doesn't provide a ->migrate_folio callback.  All iomap
users should use filemap_migrate_folio() which just has to move the
data structure from the old folio to the new.