On Thu, Jul 28, 2016 at 7:21 PM, Dave Chinner <david@xxxxxxxxxxxxx> wrote: > On Thu, Jul 28, 2016 at 10:10:33AM +0200, Jan Kara wrote: >> On Thu 28-07-16 08:19:49, Dave Chinner wrote: [..] >> So DAX doesn't need flushing to maintain consistent view of the data but it >> does need flushing to make sure fsync(2) results in data written via mmap >> to reach persistent storage. > > I thought this all changed with the removal of the pcommit > instruction and wmb_pmem() going away. Isn't it now a platform > requirement now that dirty cache lines over persistent memory ranges > are either guaranteed to be flushed to persistent storage on power > fail or when required by REQ_FLUSH? No, nothing automates cache flushing. The path of a write is: cpu-cache -> cpu-write-buffer -> bus -> imc -> imc-write-buffer -> media The ADR mechanism and the wpq-flush facility flush data thorough the imc (integrated memory controller) to media. dax_do_io() gets writes to the imc, but we still need a posted-write-buffer flush mechanism to guarantee data makes it out to media. > https://lkml.org/lkml/2016/7/9/131 > > And part of that is the wmb_pmem() calls are going away? > > https://lkml.org/lkml/2016/7/9/136 > https://lkml.org/lkml/2016/7/9/140 > > i.e. fsync on pmem only needs to take care of writing filesystem > metadata now, and the pmem driver handles the rest when it gets a > REQ_FLUSH bio from fsync? > > https://lkml.org/lkml/2016/7/9/134 > > Or have we somehow ended up with the fucked up situation where > dax_do_io() writes are (effectively) immediately persistent and > untracked by internal infrastructure, whilst mmap() writes > require internal dirty tracking and fsync() to flush caches via > writeback? dax_do_io() writes are not immediately persistent. They bypass the cpu-cache and cpu-write-bufffer and are ready to be flushed to media by REQ_FLUSH or power-fail on an ADR system. -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
