Re: [RFC PATCH 0/7] dax, ext4: Synchronous page faults

[Dan Williams <dan.j.williams@intel.com>]

On Mon, Aug 14, 2017 at 7:04 AM, Boaz Harrosh <boazh@netapp.com> wrote:
>
> Thank you Jan, I'm patiently waiting for this MAP_SYNC flag since I asked for
> it in 2014. I'm so glad its time is finally do.
>
> Thank you for working on this. Please CC me on future patches.
> (note the new Netapp email)
>
> On 13/08/17 12:25, Christoph Hellwig wrote:
>> On Sat, Aug 12, 2017 at 07:44:14PM -0700, Dan Williams wrote:
>>> How about MAP_SYNC == (MAP_SHARED|MAP_PRIVATE)? On older kernels that
>>> should get -EINVAL, and on new kernels it means SYNC+SHARED.
>>
>> Cute trick, but I'd hate to waster it just for our little flag.
>>
>> How about:
>>
>> #define __MAP_VALIDATE                MAP_SHARED|MAP_PRIVATE
>> #define MAP_SYNC              0x??? | __MAP_VALIDATE
>>
>> so that we can reuse that trick for any new flag?
>>
>
> YES! And please create a mask for all new flags and in validation
> code if ((m_flags & __MAP_VALIDATE) == __MAP_VALIDATE) then you
> want that (m_flags & __MAP_NEWFLAGS) does not come empty, this
> way you actually preserve the old check that SHARED and PRIVATE
> do not co exist.
>
> Few Comments on this new MAP_ flag
>
> 0] The name at least needs to be MAP_MSYNC because only meta-data is
>     synced not the data pointed to. That is the responsibility of the app
>
> 1] This flag you have named MAP_SYNC but it is very much related to
>    dax and the ability for user-mode to "flush" the data pointed by this
>    now "synced" meta data.
>    For example in ext4, this flag set on an inode that is *not* IS_DAX
>    should fail the mmap. Because there is no point of synced meta if the
>    data is actually in page-cache and we know for sure it was not yet synced,
>    And there is no way for user-mode to directly "sync" the data as well.
>
> 2] The code should be constructed that the default check for the MAP_SYNC
>    should fail, and only Hopped in FSs are allowed.
>    (So not to modify all Implementations of file_operations->mmap() )
>
> 3] /dev/pmem could start serving DAX pages in mmap, if asked for MAP_MSYNC
>    (which is also an API that says "I know I need to cl_flush". See 1. )
>
> 4] Once we have this flag. And properly implemented at least in one FS
>    and optionally in /dev/pmemX we no longer have any justification for
>    /dev/daxX and it can die a slow and happy death.

I'm all for replacing /dev/dax with filesystem equivalent
functionality, but I don't think MAP_SYNC gets us fully there. That's
what the MAP_DIRECT proposal [1] is meant to address.

[1]: https://lkml.org/lkml/2017/8/13/160