On Tue, Jun 22, 2021 at 06:55:33PM +0100, David Howells wrote:

Linus Torvalds [off-list ref] wrote:

quoted

End result: doing the fault_in_readable "unnecessarily" at the
beginning is likely the better optimization. It's basically free when
it's not necessary, and it avoids an extra fault (and extra
lock/unlock and retry) when it does end up faulting pages in.

It may also cause the read in to happen in the background whilst write_begin
is being done.

Huh?  Last I checked, the fault_in_readable actually read a byte from
the page.  It has to wait for the read to complete before that can
happen.