linux-kernel - Re: [PATCH] Ensure that walk_page_range()'s start and end are page-aligned

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Message-ID: <alpine.DEB.2.00.1202131350500.17296@chino.kir.corp.google.com>
Date:	Mon, 13 Feb 2012 13:55:31 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Dan Smith <danms@...ibm.com>
cc:	akpm@...ux-foundation.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Ensure that walk_page_range()'s start and end are
 page-aligned

On Mon, 13 Feb 2012, Dan Smith wrote:

> DR> It doesn't "ensure" anything without CONFIG_DEBUG_VM enabled, which
> DR> isn't the default.
> 
> Are you proposing a change in verbiage or a stronger check? A
> VM_BUG_ON() seemed on par with other checks, such as the one in
> get_user_pages_fast().
> 

That's not a precedent, there's a big difference between the performance 
of gup_fast(), where we can't spare an additional compare and branch, and 
walk_page_range().  VM_BUG_ON() is typically used in situations where a 
debug kernel has been built, including CONFIG_DEBUG_VM, and the check 
helps to isolate a problem that would be otherwise difficult to find.  If 
that fits the criteria, fine, but it doesn't "ensure" walk_page_range() 
always has start and end addresses that are page aligned, so the changelog 
needs to be modified to describe why an error in this path wouldn't be 
evident.
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