From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail138.messagelabs.com (mail138.messagelabs.com [216.82.249.35]) by kanga.kvack.org (Postfix) with ESMTP id 600E26B0044 for ; Sun, 21 Dec 2008 23:03:34 -0500 (EST) Date: Mon, 22 Dec 2008 04:51:49 +0100 From: Nick Piggin Subject: Re: [rfc][patch] unlock_page speedup Message-ID: <20081222035149.GI26419@wotan.suse.de> References: <20081219072909.GC26419@wotan.suse.de> <20081218233549.cb451bc8.akpm@linux-foundation.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org To: Linus Torvalds Cc: Andrew Morton , Linux Memory Management List , linux-fsdevel@vger.kernel.org List-ID: 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 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, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org