Re: [ISSUE] split_folio() and dirty IOMAP folios

[Matthew Wilcox <willy@infradead.org>]

On Thu, Nov 07, 2024 at 05:34:40PM +0100, David Hildenbrand wrote:
> On 07.11.24 17:09, Matthew Wilcox wrote:
> > On Thu, Nov 07, 2024 at 04:07:08PM +0100, David Hildenbrand wrote:
> > > I'm debugging an interesting problem: split_folio() will fail on dirty
> > > folios on XFS, and I am not sure who will trigger the writeback in a timely
> > > manner so code relying on the split to work at some point (in sane setups
> > > where page pinning is not applicable) can make progress.
> > 
> > You could call something like filemap_write_and_wait_range()?
> 
> Thanks, have to look into some details of that.
> 
> Looks like the folio_clear_dirty_for_io() is buried in
> folio_prepare_writeback(), so that part is taken care of.
> 
> Guess I have to fo from folio to "mapping,lstart,lend" such that
> __filemap_fdatawrite_range() would look up the folio again. Sounds doable.
> 
> (I assume I have to drop the folio lock+reference before calling that)

I was thinking you'd do it higher in the callchain than
gmap_make_secure().  Presumably userspace says "I want to make this
256MB range secure" and we can start by writing back that entire
256MB chunk of address space.

That doesn't prevent anybody from dirtying it in-between, of course,
so you can still get -EBUSY and have to loop round again.