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

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Date:	Fri, 15 Aug 2014 18:28:16 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	Davidlohr Bueso <davidlohr@...com>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	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, 13 Aug 2014, Andrew Morton wrote:
> On Tue, 12 Aug 2014 16:46:48 -0700 Davidlohr Bueso <davidlohr@...com> wrote:
> > 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. 
> 
> It is a bit of a hand-grenade for those (rare) situations where code is
> dealing with other-tasks-mm.  It's simple enough to add an `mm' arg?
> 
> > 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.
> 
> Adding a local to support a macro which secretly uses that local is
> pretty nasty.
> 
> 
> Overall, I'm not really sure that
> 
> -	for (vma = mm->mmap; vma; vma = vma->vm_next) {
> +	for_each_vma(mm, vma) {
> 
> is much of an improvement.  I'll wait to see what others think...

... I'm with Kirill: obscuring a simple for loop is unhelpful -
unless it's a prelude to a grand enhancement under the hood?

As to the hidden mm argument: a momentary lapse of taste, I hope.

Hugh
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