linux-kernel - Re: [PATCH] mm: introduce for_each

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Message-ID: <1407887208.2695.9.camel@buesod1.americas.hpqcorp.net>
Date:	Tue, 12 Aug 2014 16:46:48 -0700
From:	Davidlohr Bueso <davidlohr@...com>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	"James E.J. Bottomley" <jejb@...isc-linux.org>,
	Helge Deller <deller@....de>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	Robert Richter <rric@...nel.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, aswin@...com
Subject: Re: [PATCH] mm: introduce for_each_vma helpers

On Wed, 2014-08-13 at 00:52 +0300, Kirill A. Shutemov wrote:
> On Tue, Aug 12, 2014 at 10:45:23AM -0700, Davidlohr Bueso wrote:
> > The most common way of iterating through the list of vmas, is via:
> >     for (vma = mm->mmap; vma; vma = vma->vm_next)
> > 
> > This patch replaces this logic with a new for_each_vma(vma) helper,
> > which 1) encapsulates this logic, and 2) make it easier to read.
> 
> Why does it need to be encapsulated?
> Do you have problem with reading plain for()?
> 
> Your for_each_vma(vma) assumes "mm" from the scope. This can be confusing
> for reader: whether it uses "mm" from the scope or "current->mm". This
> will lead to very hard to find bug one day.

I think its fairly obvious to see where the mm is coming from -- the
helpers *do not* necessarily use current, it uses whatever mm was
already there in the first place. I have not changed anything related to
this from the callers. 

The only related change I can think of, is for some callers that do:

for (vma = current->mm->mmap; vma != NULL; vma = vma->vm_next)

So we just add a local mm from current->mm and replace the for() with
for_each_vma(). I don't see anything particularly ambiguous with that.

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