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

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 17 Nov 2015 18:26:38 +0100 Piotr Kwapulinski <kwapulinski.piotr@gmail.com> wrote:

> The following flag comparison in mmap_region is not fully correct:
> 
> if (!(vm_flags & MAP_FIXED))
> 
> The vm_flags should not be compared with MAP_FIXED (0x10). It is a bit
> confusing. This condition is almost always true since VM_MAYREAD (0x10)
> flag is almost always set by default. This patch removes this condition.
> 
> ...
>
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1547,13 +1547,6 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>  	if (!may_expand_vm(mm, len >> PAGE_SHIFT)) {
>  		unsigned long nr_pages;
>  
> -		/*
> -		 * 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))

That looks simpler.

However the changelog doesn't describe the end-user visible effects of
the bug, as changelogs should always do.  Presumably this is causing
incorrect ENOMEM reporting due to RLIMIT_AS being exceeded, but this
isn't very specific.

So can you please fill in the details here?  Such info is needed when
deciding which kernel version(s) need the fix.

Thanks.