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

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Message-ID: <1407886188.2695.3.camel@buesod1.americas.hpqcorp.net>
Date:	Tue, 12 Aug 2014 16:29:48 -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()?

No problem in particular. But encapsulation is always good to have, and
we have a number of examples similar to what I'm proposing all
throughout the kernel (just like at vma_interval_tree_foreach).

> 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 don't like this.
> 
> > It also updates most of the callers, so its a pretty good start.
> > 
> > Similarly, we also have for_each_vma_start(vma, start) when the user
> > does not want to start at the beginning of the list. And lastly the
> > for_each_vma_start_inc(vma, start, inc) helper in introduced to allow
> > users to create higher level special vma abstractions, such as with
> > the case of ELF binaries.
> 
> for_each_vma_start_inc() is pretty much the plain for() but with
> really_long_and_fancy_name(). Why?

Because we can implement things like for_each_vma_gate() on top.

Thanks,
Davidlohr


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