Re: Do we need to unrevert "fs: do not prefault sys_write() user buffer pages"?

[Matthew Wilcox]

On Tue, Jun 22, 2021 at 06:55:33PM +0100, David Howells wrote:
> Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> 
> > 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.