linux-kernel - Re: [PATCH 05/10] ksm: no debug in page_dup

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Date:	Tue, 21 Jul 2009 09:16:54 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	Rik van Riel <riel@...hat.com>
Cc:	Izik Eidus <ieidus@...hat.com>, akpm@...ux-foundation.org,
	hugh.dickins@...cali.co.uk, aarcange@...hat.com, chrisw@...hat.com,
	avi@...hat.com, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	nickpiggin@...oo.com.au
Subject: Re: [PATCH 05/10] ksm: no debug in page_dup_rmap()

On Mon, Jul 20, 2009 at 02:11:14PM -0400, Rik van Riel wrote:
> Izik Eidus wrote:
> >From: Hugh Dickins <hugh.dickins@...cali.co.uk>
> >
> >page_dup_rmap(), used on each mapped page when forking,  was originally
> >just an inline atomic_inc of mapcount.  2.6.22 added CONFIG_DEBUG_VM
> >out-of-line checks to it, which would need to be ever-so-slightly
> >complicated to allow for the PageKsm() we're about to define.
> >
> >But I think these checks never caught anything.  And if it's coding
> >errors we're worried about, such checks should be in page_remove_rmap()
> >too, not just when forking; whereas if it's pagetable corruption we're
> >worried about, then they shouldn't be limited to CONFIG_DEBUG_VM.
> 
> Acked-by: Rik van Riel <riel@...hat.com>

I like debug code like this as it helps comment the code a litte
bit too. We've got lots of debug checks in the VM and probably
very few of them catch anything useful... I'd kind of like to see
it be ever-so-slightly complicated with PageKsm, and even a call
to page_check_anon_rmap put into page_remove_rmap (which is a good
idea).

pagetable corruption/struct page corruption I think is good to
check for, but it is fine to have such checks under DEBUG_VM --
we have a couple of orders of magnitude more memory that is not
for struct page, so decent coverage of memory corruption kind of
wants slab and page debugging too, don't you think?

/checks the sky for pigs...

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