linux-kernel - Re: [rfc] optimise unlock

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Message-ID: <20070517062729.GA14350@wotan.suse.de>
Date:	Thu, 17 May 2007 08:27:30 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Hugh Dickins <hugh@...itas.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	linux-arch@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux Memory Management List <linux-mm@...ck.org>
Subject: Re: [rfc] optimise unlock_page

On Wed, May 16, 2007 at 12:47:54PM -0700, Linus Torvalds wrote:
> 
> On Wed, 16 May 2007, Hugh Dickins wrote:
> 
> > > The other option of moving the bit into ->mapping hopefully avoids all
> > > the issues, and would probably be a little faster again on the P4, at the
> > > expense of being a more intrusive (but it doesn't look too bad, at first
> > > glance)...
> > 
> > Hmm, I'm so happy with PG_swapcache in there, that I'm reluctant to
> > cede it to your PG_locked, though I can't deny your use should take
> > precedence.  Perhaps we could enforce 8-byte alignment of struct
> > address_space and struct anon_vma to make both bits available
> > (along with the anon bit).
> 
> We probably could. It should be easy enough to mark "struct address_space" 
> to be 8-byte aligned.

Yeah, it might be worthwhile, because I agree that PG_swapcache would
work nicely there too.


> > But I think you may not be appreciating how intrusive PG_locked
> > will be.  There are many references to page->mapping (often ->host)
> > throughout fs/ : when we keep anon/swap flags in page->mapping, we
> > know the filesystems will never see those bits set in their pages,
> > so no page_mapping-like conversion is needed; just a few places in
> > common code need to adapt.
> 
> You're right, it could be really painful. We'd have to rename the field, 
> and use some inline function to access it (which masks off the low bits).

Yeah, I realise that the change is intrusive in terms of lines touched,
but AFAIKS, it should not be much more complex than a search/replace...

As far as deprecating things goes... I don't think we have to wait too
long, its more for features, drivers, or more fundamental APIs isn't it?
If we just point out that one must use set_page_mapping/page_mapping
rather than page->mapping, it is trivial to fix any out of tree breakage.

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