linux-kernel - Re: [patch 1/2] mm, doc: cleanup and clarify munmap behavior for hugetlb memory

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Date:	Fri, 27 Mar 2015 09:58:47 -0400
From:	Eric B Munson <emunson@...mai.com>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Jonathan Corbet <corbet@....net>,
	Davide Libenzi <davidel@...ilserver.org>,
	Luiz Capitulino <lcapitulino@...hat.com>,
	Shuah Khan <shuahkh@....samsung.com>,
	Hugh Dickins <hughd@...gle.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Joern Engel <joern@...fs.org>,
	Jianguo Wu <wujianguo@...wei.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
	linux-doc@...r.kernel.org
Subject: Re: [patch 1/2] mm, doc: cleanup and clarify munmap behavior for
 hugetlb memory

On Thu, 26 Mar 2015, David Rientjes wrote:

> munmap(2) of hugetlb memory requires a length that is hugepage aligned,
> otherwise it may fail.  Add this to the documentation.
> 
> This also cleans up the documentation and separates it into logical
> units: one part refers to MAP_HUGETLB and another part refers to
> requirements for shared memory segments.
> 
> Signed-off-by: David Rientjes <rientjes@...gle.com>
> ---

If this is the route we are going to take, this behavoir needs to be
called out prominently in the mmap/munmap man page.


>  Documentation/vm/hugetlbpage.txt | 21 +++++++++++++--------
>  1 file changed, 13 insertions(+), 8 deletions(-)
> 
> diff --git a/Documentation/vm/hugetlbpage.txt b/Documentation/vm/hugetlbpage.txt
> --- a/Documentation/vm/hugetlbpage.txt
> +++ b/Documentation/vm/hugetlbpage.txt
> @@ -289,15 +289,20 @@ file systems, write system calls are not.
>  Regular chown, chgrp, and chmod commands (with right permissions) could be
>  used to change the file attributes on hugetlbfs.
>  
> -Also, it is important to note that no such mount command is required if the
> +Also, it is important to note that no such mount command is required if
>  applications are going to use only shmat/shmget system calls or mmap with
> -MAP_HUGETLB.  Users who wish to use hugetlb page via shared memory segment
> -should be a member of a supplementary group and system admin needs to
> -configure that gid into /proc/sys/vm/hugetlb_shm_group.  It is possible for
> -same or different applications to use any combination of mmaps and shm*
> -calls, though the mount of filesystem will be required for using mmap calls
> -without MAP_HUGETLB.  For an example of how to use mmap with MAP_HUGETLB see
> -map_hugetlb.c.
> +MAP_HUGETLB.  For an example of how to use mmap with MAP_HUGETLB see map_hugetlb
> +below.
> +
> +Users who wish to use hugetlb memory via shared memory segment should be a
> +member of a supplementary group and system admin needs to configure that gid
> +into /proc/sys/vm/hugetlb_shm_group.  It is possible for same or different
> +applications to use any combination of mmaps and shm* calls, though the mount of
> +filesystem will be required for using mmap calls without MAP_HUGETLB.
> +
> +When using munmap(2) to unmap hugetlb memory, the length specified must be
> +hugepage aligned, otherwise it will fail with errno set to EINVAL.
> +
>  
>  Examples
>  ========
> 
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