linux-kernel - Re: [PATCH 1/8] badpage: simplify page

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Message-ID: <Pine.LNX.4.64.0812020947440.5306@blonde.anvils>
Date:	Tue, 2 Dec 2008 10:39:26 +0000 (GMT)
From:	Hugh Dickins <hugh@...itas.com>
To:	Christoph Lameter <cl@...ux-foundation.org>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Russ Anderson <rja@....com>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Dave Jones <davej@...hat.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 1/8] badpage: simplify page_alloc flag check+clear

On Mon, 1 Dec 2008, Christoph Lameter wrote:
> On Mon, 1 Dec 2008, Hugh Dickins wrote:
> > > > PAGE_FLAGS_CHECK_AT_FREE
> 
> > > Rename this to PAGE_FLAGS_CLEAR_WHEN_FREE?
> >
> > No, that's a list of just the ones it's checking at free;
> > it then (with this patch) goes on to clear all of them.
> 
> But they are always clear on free. The checking is irrelevant.

How about CHECK_PAGE_FLAGS_CLEAR_AT_FREE?

> 
> > One of the problems with PREP is that it's not obvious that it
> > means ALLOC: yes, I'd be happier with PAGE_FLAGS_CLEAR_AT_FREE.
> 
> Ok.

If you like the suggestion above, then for this one
how about CHECK_PAGE_FLAGS_CLEAR_AT_ALLOC?

I just haven't changed those names in the patch, continued to
use the names from before: they're probably not the names I'd
have chosen, but it is hard to write a paragraph in a name.

The one I really disliked was "PAGE_FLAGS" for an obscure
subset of page flags, and have got rid of that.

> > > This is equal to
> > >
> > > page->flags &=~PAGE_FLAGS_CHECK_AT_PREP;
> > >
> > > You can drop the if...
> >
> > I was intentionally following the existing style of
> > 	if (PageDirty(page))
> > 		__ClearPageDirty(page);
> > 	if (PageSwapBacked(page))
> > 		__ClearPageSwapBacked(page);
> > which is going out of its way to avoid dirtying a cacheline.
> >
> > In all the obvious cases, I think the cacheline will already
> > be dirty; but I guess there's an important case (high order
> > but not compound?) which has a lot of clean cachelines.
> 
> Free or alloc dirties the cacheline of the page struct regardless since
> the LRU field is always modified.
> 
> Well, ok. The not compound high order case may be an exception.
> 
> But then lets at least make a single check
> 
> If (page->flags & (all the flags including dirty and SwapBacked))
> 	zap-em.

That's exactly what I did, isn't it?

Hugh
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