linux-kernel - Re: [PATCH v3] mm/mmap.c: remove incorrect MAP_FIXED flag comparison from mmap

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Message-ID: <20151124161248.GA1414@home.local>
Date:	Tue, 24 Nov 2015 17:12:49 +0100
From:	Piotr Kwapulinski <kwapulinski.piotr@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	mhocko@...e.com, oleg@...hat.com, cmetcalf@...hip.com,
	mszeredi@...e.cz, viro@...iv.linux.org.uk, dave@...olabs.net,
	kirill.shutemov@...ux.intel.com, n-horiguchi@...jp.nec.com,
	aarcange@...hat.com, iamjoonsoo.kim@....com, jack@...e.cz,
	xiexiuqi@...wei.com, vbabka@...e.cz, Vineet.Gupta1@...opsys.com,
	riel@...hat.com, gang.chen.5i5j@...il.com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v3] mm/mmap.c: remove incorrect MAP_FIXED flag comparison
 from mmap_region

On Mon, Nov 23, 2015 at 02:14:01PM -0800, Andrew Morton wrote:
> On Mon, 23 Nov 2015 18:36:42 +0100 Piotr Kwapulinski <kwapulinski.piotr@...il.com> wrote:
> 
> > The following flag comparison in mmap_region makes no sense:
> > 
> > if (!(vm_flags & MAP_FIXED))
> >     return -ENOMEM;
> > 
> > The condition is always false and thus the above "return -ENOMEM" is never
> > executed. The vm_flags must not be compared with MAP_FIXED flag.
> > The vm_flags may only be compared with VM_* flags.
> > MAP_FIXED has the same value as VM_MAYREAD.
> > Hitting the rlimit is a slow path and find_vma_intersection should realize
> > that there is no overlapping VMA for !MAP_FIXED case pretty quickly.
> > 
> > Remove the code that makes no sense.
> > 
> > ...
> >
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -1551,9 +1551,6 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
> >  		 * MAP_FIXED may remove pages of mappings that intersects with
> >  		 * requested mapping. Account for the pages it would unmap.
> >  		 */
> > -		if (!(vm_flags & MAP_FIXED))
> > -			return -ENOMEM;
> > -
> >  		nr_pages = count_vma_pages_range(mm, addr, addr + len);
> >  
> >  		if (!may_expand_vm(mm, (len >> PAGE_SHIFT) - nr_pages))
> 
> Did you intend to retain the stale comment?

It was my intention. This comment is still valid, even after removing the
condition.

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