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

From: Andrew Morton
Date: Mon Nov 23 2015 - 17:14:11 EST


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?
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