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

[Piotr Kwapulinski]

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@xxxxxxxxx> 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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