Re: [rfc][patch] unlock_page speedup

[Nick Piggin <npiggin@suse.de>]

On Fri, Dec 19, 2008 at 09:35:14AM -0800, Linus Torvalds wrote:
> 
> 
> On Thu, 18 Dec 2008, Andrew Morton wrote:
> >
> > On Fri, 19 Dec 2008 08:29:09 +0100 Nick Piggin <npiggin@suse.de> wrote:
> > 
> > > Introduce a new page flag, PG_waiters
> > 
> > Leaving how many?  fs-cache wants to take two more.
> 
> Hmm. Do we ever use lock_page() on anything but page-cache pages and the 
> buffer cache?
> 
> We _could_ decide to try to move the whole locking into the "mapping" 
> field, and use a few more bits in the low bits of the pointer. Right now 
> we just use one bit (PAGE_MAPPING_ANON), but if we just make the rule be 
> that "struct address_space" has to be 8-byte aligned, then we'd have two 
> more bits available there, and we could hide the lock bit and the 
> contention bit there too.
> 
> This actually would have a _really_ nice effect, in that if we do this, 
> then I suspect that we could eventually even make the bits in "flags" be 
> non-atomic. The lock bit really is special. The other bits tend to be 
> either pretty static over allocation, or things that should be set only 
> when the page is locked.
> 
> I dunno. But it sounds like a reasonable thing to do, and it would free 
> one bit from the page flags, rather than use yet another one. And because 
> locking is special and because we already have to access that "mapping" 
> pointer specially, I don't think the impact would be very invasive.

I did a patch for that at one point. It doesn't go very far to allowing
non-atomic page flags, but it allows non-atomic unlock_page. But Hugh
wanted to put PG_swapcache in there, so I put it on the shelf for a while.