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

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

On Tue, Aug 1, 2017 at 4:26 AM, Jan Kara <jack@suse.cz> wrote:
> On Tue 01-08-17 04:02:41, Christoph Hellwig wrote:
>> On Fri, Jul 28, 2017 at 11:38:21AM +0200, Jan Kara wrote:
>> > Well, you are right I can make the implementation work with struct file
>> > flag as well - let's call it O_DAXDSYNC. However there are filesystem
>> > operations where you may need to answer question: Is there any fd with
>> > O_DAXDSYNC open against this inode (for operations that change file offset
>> > -> block mapping)? And in that case inode flag is straightforward while
>> > file flag is a bit awkward (you need to implement counter of fd's with that
>> > flag in the inode).
>>
>> We can still keep and inode flag as the internal implementation
>> detail.  As mentioned earlier the right flag to control behavior
>> of a mapping is an mmap flag.  And the initial naive implementation
>> would simply mark the inode as sync once the first MAP_SYNC open happens
>> on it.  We could then move to more precise tracking if/when needed.
>
> OK, makes sense and I like the MAP_SYNC proposal. I'll change it in my
> implementation.

Does sys_mmap() reject unknown flag values today? I'm either not
looking in the right place or it's missing and we'll need some
interface/mechanism to check if MAP_SYNC is honored.