On Mon, Jun 28, 2021 at 10:59:55PM +0100, Matthew Wilcox wrote:
> > > so permit pages without an iop to enter writeback and create an iop
> > > *then*. Would that solve your problem?
> >
> > It is the right thing to do, especially when combined with a feature
> > patch to not bother to create the iomap_page structure on small
> > block size file systems when the extent covers the whole page.
>
> We don't know the extent layout at the point where *this* patch creates
> iomap_pages during writeback. I imagine we can delay creating one until
> we find out what our destination layout will be?
Hmm. Actually ->page_mkwrite is always is always called on an uptodate
page and we even assert that. I should have remembered the whole page
fault path better.
So yeah, I think we should take patch 1 from Andreas, then a non-folio
version of your patch as a start. The next steps then would be in
approximate order:
1. remove the iomap_page_create in iomap_page_mkwrite_actor as it
clearly is not needed at that point
2. don't bother to create an iomap_page in iomap_readpage_actor when
the iomap spans the whole page
3. don't create the iomap_page in __iomap_write_begin when the
page is marked uptodate or the write covers the whole page
delaying the creation further in iomap_writepage_map will be harder
as the loop around iomap_add_to_ioend is still fundamentally block
based.
