Re: [patch] mmap.2: document the munmap exception for underlying page size

[Mike Kravetz <mike.kravetz-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>]

On 07/21/2015 05:41 PM, David Rientjes wrote:
> munmap(2) will fail with an errno of EINVAL for hugetlb memory if the
> length is not a multiple of the underlying page size.
>
> Documentation/vm/hugetlbpage.txt was updated to specify this behavior
> since Linux 4.1 in commit 80d6b94bd69a ("mm, doc: cleanup and clarify
> munmap behavior for hugetlb memory").
>
> Signed-off-by: David Rientjes <rientjes-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
> ---
>   man2/mmap.2 | 4 ++++
>   1 file changed, 4 insertions(+)
>
> diff --git a/man2/mmap.2 b/man2/mmap.2
> --- a/man2/mmap.2
> +++ b/man2/mmap.2
> @@ -383,6 +383,10 @@ All pages containing a part
>   of the indicated range are unmapped, and subsequent references
>   to these pages will generate
>   .BR SIGSEGV .
> +An exception is when the underlying memory is not of the native page
> +size, such as hugetlb page sizes, whereas
> +.I length
> +must be a multiple of the underlying page size.
>   It is not an error if the
>   indicated range does not contain any mapped pages.
>   .SS Timestamps changes for file-backed mappings
>
> --

Should we also add a similar comment for the mmap offset?  Currently
the man page says:

"offset must be a multiple of the page size as returned by
  sysconf(_SC_PAGE_SIZE)."

For hugetlbfs, I beieve the offset must be a multiple of the
hugetlb page size.  A similar comment/exception about using
the "underlying page size" would apply here as well.

-- 
Mike Kravetz
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