linux-kernel - Re: [PATCH 4/9] mm: use mm_populate() for blocking remap_file

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Message-Id: <20130312134702.a972d9a141bf86f14768ad41@linux-foundation.org>
Date:	Tue, 12 Mar 2013 13:47:02 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Michel Lespinasse <walken@...gle.com>
Cc:	Tommi Rantala <tt.rantala@...il.com>,
	Andy Lutomirski <luto@...capital.net>,
	Ingo Molnar <mingo@...nel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Hugh Dickins <hughd@...gle.com>, Jorn_Engel <joern@...fs.org>,
	Rik van Riel <riel@...hat.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Dave Jones <davej@...hat.com>
Subject: Re: [PATCH 4/9] mm: use mm_populate() for blocking
 remap_file_pages()

On Mon, 11 Mar 2013 17:24:29 -0700 Michel Lespinasse <walken@...gle.com> wrote:
> > --- a/mm/fremap.c~mm-fremapc-fix-oops-on-error-path
> > +++ a/mm/fremap.c
> > @@ -163,7 +163,8 @@ SYSCALL_DEFINE5(remap_file_pages, unsign
> >          * and that the remapped range is valid and fully within
> >          * the single existing vma.
> >          */
> > -       if (!vma || !(vma->vm_flags & VM_SHARED))
> > +       vm_flags = vma->vm_flags;
> > +       if (!vma || !(vm_flags & VM_SHARED))
> >                 goto out;
> 
> Your commit message indicates the vm_flags load here doesn't generate any code, but this seems very brittle and compiler dependent. If the compiler was to generate an actual load here, the issue with vma == NULL would reappear.

I didn't try very hard.  I have a surprisingly strong dislike of adding
"= 0" everywhere just to squish warnings.

There are actually quite a lot of places where this function could use
s/vma->vm_flags/vm_flags/ and might save a bit of code as a result. 
But the function's pretty straggly and I stopped doing it.

> 
> >         if (!vma->vm_ops || !vma->vm_ops->remap_pages)
> > @@ -254,7 +255,8 @@ get_write_lock:
> >          */
> >
> >  out:
> > -       vm_flags = vma->vm_flags;
> > +       if (vma)
> > +               vm_flags = vma->vm_flags;
> >         if (likely(!has_write_lock))
> >                 up_read(&mm->mmap_sem);
> >         else
> 
> 
> 
> Would the following work ? I think it's simpler, and with the compiler
> I'm using here it doesn't emit warnings:
> 
> diff --git a/mm/fremap.c b/mm/fremap.c
> index 0cd4c11488ed..329507e832fb 100644
> --- a/mm/fremap.c
> +++ b/mm/fremap.c
> @@ -254,7 +254,8 @@ get_write_lock:
>  	 */
>  
>  out:
> -	vm_flags = vma->vm_flags;
> +	if (!err)
> +		vm_flags = vma->vm_flags;
>  	if (likely(!has_write_lock))
>  		up_read(&mm->mmap_sem);
>  	else

Yes, this will work.

gcc-4.4.4 does generate the warning with this.

Testing `err' was my v1, but it is not obvious that err==0 always
correlates with vma!= NULL.  This is true (I checked), and it had
better be true in the future, but it just feels safer and simpler to
test `vma' directly.

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