On Tue, Jun 29, 2021 at 06:29:48AM +0100, Christoph Hellwig wrote:
> 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
Further thoughts for a better series:
1. create iomap_page if needed in iomap_writepage_map
2. do not create the iomap_page at all in iomap_readpage_actor.
->readahead is always called on newly allocated pages, and
->readpage either on a clean !uptodate page or on one that
has seen a write leading to a partial uptodate state. That
is for the case that cares about the iomap_page it is present
already
3. don't create the iomap_page in iomap_page_mkwrite_actor
I think this is the simple initial series that should solve Andreas'
problem. Then we can look into optimizing __iomap_write_begin
and iomap_writepage_map further as needed.
