On Mon, 19 Sep 2016 07:11:21 +0100
Al Viro [off-list ref] wrote:

On Mon, Sep 19, 2016 at 01:08:30PM +1000, Nicholas Piggin wrote:

quoted

Without looking through all the patches again, I believe the issue was
just that filesystems were not expecting (or at least, not audited to
expect) pages being added to their pagecache in that particular state
(they'd expect to go through ->readpage or see !uptodate in prepare_write).

If some wanted to attach metadata to uptodate pages for example, this
may have caused a problem. It wasn't some big fundamental problem, just a
mechanical one.  

Umm...  Why not make it non-uptodate/locked, try to replace the original
with it in pagecache and then do full-page ->write_begin immediately
followed by full-page ->write_end?  Looks like that ought to work in
all in-tree cases...

That sounds like it probably should work for that case. IIRC, I was looking
at using a write_begin flag to notify the case of of replacing the page, so
the fs could also handle the case of replacing existing pagecache.