linux-kernel - Re: [PATCH v2 23/35] ovl: copy-up on MAP

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Date:   Mon, 7 May 2018 12:28:34 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Miklos Szeredi <mszeredi@...hat.com>, linux-unionfs@...r.kernel.org
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 23/35] ovl: copy-up on MAP_SHARED

On 05/07/2018 01:37 AM, Miklos Szeredi wrote:
> A corner case of a corner case is when
> 
>  - file opened for O_RDONLY
>  - which is then memory mapped SHARED
>  - file opened for O_WRONLY
>  - contents modified
>  - contents read back though the shared mapping
> 
> Unfortunately it looks very difficult to do anything about the established
> shared map after the file is copied up.
> 
> Instead, when a read-only file is mapped shared, copy up the file before
> actually doing the map.  This may result in unnecessary copy-ups (but so
> may copy-up on open(O_RDWR) for exampe).

                              for example).

> 
> We can revisit this later if it turns out to be a performance problem in
> real life.
> 
> Signed-off-by: Miklos Szeredi <mszeredi@...hat.com>
> ---
>  fs/overlayfs/Kconfig     | 21 +++++++++++++++++++++
>  fs/overlayfs/file.c      | 22 ++++++++++++++++++++++
>  fs/overlayfs/overlayfs.h |  7 +++++++
>  fs/overlayfs/ovl_entry.h |  1 +
>  fs/overlayfs/super.c     | 22 ++++++++++++++++++++++
>  5 files changed, 73 insertions(+)
> 
> diff --git a/fs/overlayfs/Kconfig b/fs/overlayfs/Kconfig
> index 17032631c5cf..991c0a5a0e00 100644
> --- a/fs/overlayfs/Kconfig
> +++ b/fs/overlayfs/Kconfig
> @@ -103,3 +103,24 @@ config OVERLAY_FS_XINO_AUTO
>  	  For more information, see Documentation/filesystems/overlayfs.txt
>  
>  	  If unsure, say N.
> +
> +config OVERLAY_FS_COPY_UP_SHARED
> +       bool "Overlayfs: copy up when mapping a file shared"

	                                        a shared file" ??

> +       default n
> +       depends on OVERLAY_FS
> +       help
> +         If this option is enabled then on mapping a file with MAP_SHARED
> +	 overlayfs copies up the file in anticipation of it being modified (just
> +	 like we copy up the file on O_WRONLY and O_RDWR in anticipation of
> +	 modification).  This does not interfere with shared library loading, as
> +	 that uses MAP_PRIVATE.  But there might be use cases out there where
> +	 this impacts performance and disk usage.
> +
> +	 This just selects the default, the feature can also be enabled or
> +	 disabled in the running kernel or individually on each overlay mount.
> +
> +	 To get maximally standard compliant behavior, enable this option.
> +
> +	 To get a maximally backward compatible kernel, disable this option.
> +
> +	 If unsure, say N.

For Kconfig (coding-style.rst):
Lines under a ``config`` definition are indented with one tab, while help text
is indented an additional two spaces.


-- 
~Randy