On Mon, Aug 14, 2017 at 7:04 AM, Boaz Harrosh <boazh@xxxxxxxxxx> 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
