Re: [PATCH 0/2] iomap: small block problems

[Christoph Hellwig <hch@infradead.org>]

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.