Re: [PATCH 06/11] migrate: Remove call to ->writepage

[Matthew Wilcox <willy@infradead.org>]

On Thu, Mar 27, 2025 at 11:04:57AM -0400, Zi Yan wrote:
> On Fri Mar 7, 2025 at 8:54 AM EST, Matthew Wilcox (Oracle) wrote:
> > The writepage callback is going away; filesystems must implement
> > migrate_folio or else dirty folios will not be migratable.
> 
> What is the impact of this? Are there any filesystem that has
> a_ops->writepage() without migrate_folio()? I wonder if it could make
> the un-migratable problem worse[1] when such FS exists.

As Christoph and I have been going through filesystems removing their
->writepage operations, we've been careful to add ->migrate_folio
callbacks at the same time.  But we haven't fixed any out-of-tree
filesystems, and we can't fix the filesystems which will be written in
the future.

So maybe what we should do is WARN_ON_ONCE() for filesystems which
have a ->writepages, but do not have a ->migrate_folio()?

> >  static int fallback_migrate_folio(struct address_space *mapping,
> >  		struct folio *dst, struct folio *src, enum migrate_mode mode)
> >  {
> > -	if (folio_test_dirty(src)) {
> > -		/* Only writeback folios in full synchronous migration */
> > -		switch (mode) {
> > -		case MIGRATE_SYNC:
> > -			break;
> > -		default:
> > -			return -EBUSY;
> > -		}
> > -		return writeout(mapping, src);
> > -	}
> 
> Now fallback_migrate_folio() no longer writes out page for FS, so it is
> the responsibilty of migrate_folio()?

->migrate_folio() doesn't need to write out the page.  It can migrate
dirty folios (just not folios currently under writeback, obviously)