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@xxxxxxx> 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. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
