Re: [PATCH v2 23/35] ovl: copy-up on MAP_SHARED

[Miklos Szeredi]

On Mon, May 7, 2018 at 9:28 PM, Randy Dunlap <rdunlap@xxxxxxxxxxxxx> wrote:
> 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@xxxxxxxxxx>
>> ---
>>  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" ??

This is referring to MAP_SHARED flag of mmap().  So it's the mapping
that is shared, not the 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.

Not sure what went wrong there.  Fixed now.

Thanks,
Miklos