Re: Unexpected splice "always copy" behavior observed

Nick Piggin
2010-05-19T15:44:39Z

On Wed, May 19, 2010 at 08:30:10AM -0700, Linus Torvalds wrote:
>
>
> On Thu, 20 May 2010, Nick Piggin wrote:
> >
> > Well I mean a full invalidate -- invalidate_mapping_pages -- so there is
> > literally no pagecache there at all.
>
> Umm. That won't work. Think mapped pages. You can't handle them
> atomically, so somebody will page-fault them in.
>
> So you'd have to have a "invalidate_and_replace()" to do it atomically
> while holding the mapping spinlock or something.
>
> And WHAT IS THE POINT? That will be about a million times slower than
> just doing the effing copy in the first place!
>
> Memory copies are _not_ slow. Not compared to taking locks and doing TLB
> invalidates.

No I never thought it would be a good idea to try to avoid all races
or anything. Obviously some cases *cannot* be easily invalidated, if
there is a ref on the page or whatever, so the fallback code has to
be there anyway.

So you would just invalidate and try to insert your page. 99.something%
of the time it will work fine. If the insert fails, fall back to
copying.

And hey you *may* even want a heuristic that avoids trying to invalidate
if the page is mapped, due to cost of TLB flushing and faulting etc.