linux-kernel - Re: [PATCH] mm/mmap: introduce vma_range

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Date: Mon, 22 Jan 2024 19:18:30 -0500
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Yajun Deng <yajun.deng@...ux.dev>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/mmap: introduce vma_range_init()

* Andrew Morton <akpm@...ux-foundation.org> [240122 18:40]:
> On Mon, 22 Jan 2024 17:00:31 -0500 "Liam R. Howlett" <Liam.Howlett@...cle.com> wrote:
> 
> > * Yajun Deng <yajun.deng@...ux.dev> [240110 21:15]:
> > > There is a lot of code needs to set the range of vma, introduce
> > > vma_range_init() to initialize the range of vma.
> > > 
> > > Signed-off-by: Yajun Deng <yajun.deng@...ux.dev>
> > > ---
> > >  include/linux/mm.h |  9 +++++++++
> > >  mm/mmap.c          | 29 +++++++----------------------
> > >  2 files changed, 16 insertions(+), 22 deletions(-)
> > 
> > This isn't a whole lot of code, are there others?  We're losing code
> > clarity in favour of saving 6 lines?
> > 
> 
> Oh.  I thought it was a nice cleanup which made things more clear.

I'm not totally against it; that's why I suggested the changes below. I
think a name change would go a long way for clarity. It's not as much as
I though it would be though.

> 
> > > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > > index f5a97dec5169..abb4534be3cc 100644
> > > --- a/include/linux/mm.h
> > > +++ b/include/linux/mm.h
> > > @@ -3516,6 +3516,15 @@ static inline bool range_in_vma(struct vm_area_struct *vma,
> > >  	return (vma && vma->vm_start <= start && end <= vma->vm_end);
> > >  }
> > >  
> > > +static inline void vma_range_init(struct vm_area_struct *vma,
> > 
> > Any reason this can't be in mm/internal.h ?
> 
> That would be good.

One other thing, do we trust this to be inlined correctly by the
compiler or should this be __always_inline?  I'd expect it to be okay as
it is, but I've been proven wrong in a perf trace before..

> 
> > vma_range_set(), vma_set_range(), or just vma_range() might be a better
> > name?  My thinking is that some of these are actually modifying the vma
> > and not just initializing it, right?
> 
> I'd vote for vma_set_range().
> 

Using vma_set_range() leaves vma_range() or vma_size(), which could be
added for the calculations of vma->vm_end - vma->vm_start.  Davidlohr
suggested such a beast a few years ago, but that one would need to live
in the include/linux/mm.h as it occurs a lot more.

$ git grep "vma->vm_end - vma->vm_start" | wc -l
198

. for just those named vma.