Re: [patch v3] splice: fix race with page invalidation

[Linus Torvalds <torvalds@linux-foundation.org>]

On Thu, 31 Jul 2008, Jamie Lokier wrote:
> 
> But did you miss the bit where you DON'T COPY ANYTHING EVER*?  COW is
> able provide _correctness_ for the rare corner cases which you're not
> optimising for.  You don't actually copy more than 0.0% (*approx).

The thing is, just even _marking_ things COW is the expensive part. If we 
have to walk page tables - we're screwed.

> The cost of COW is TLB flushes*.  But for splice, there ARE NO TLB
> FLUSHES because such files are not mapped writable!

For splice, there are also no flags to set, no extra tracking costs, etc 
etc.

But yes, we could make splice (from a file) do something like

 - just fall back to copy if the page is already mapped (page->mapcount 
   gives us that)

 - set a bit ("splicemapped") when we splice it in, and increment 
   page->mapcount for each splice copy.

 - if a "splicemapped" page is ever mmap'ed or written to (either through 
   write or truncate), we COW it then (and actually move the page cache 
   page - it would be a "woc": a reverse cow, not a normal one).

 - do all of this with page lock held, to make sure that there are no 
   writers or new mappers happening.

So it's probably doable. 

(We could have a separate "splicecount", and actually allow non-writable 
mappings, but I suspect we cannot afford the space in teh "struct space" 
for a whole new count).

> You're missing the real point of network splice().
> 
> It's not just for speed.
> 
> It's for sharing data.  Your TCP buffers can share data, when the same
> big lump is in flight to lots of clients.  Think static file / web /
> FTP server, the kind with 80% of hits to 0.01% of the files roughly
> the same of your RAM.

Maybe. Does it really show up as a big thing?

			Linus