On Fri 13-11-15 18:32:40, Dan Williams wrote: > On Fri, Nov 13, 2015 at 4:43 PM, Andreas Dilger <adilger@xxxxxxxxx> wrote: > > On Nov 13, 2015, at 5:20 PM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote: > >> > >> On Fri, Nov 13, 2015 at 4:06 PM, Ross Zwisler > >> <ross.zwisler@xxxxxxxxxxxxxxx> wrote: > >>> Currently the PMEM driver doesn't accept REQ_FLUSH or REQ_FUA bios. These > >>> are sent down via blkdev_issue_flush() in response to a fsync() or msync() > >>> and are used by filesystems to order their metadata, among other things. > >>> > >>> When we get an msync() or fsync() it is the responsibility of the DAX code > >>> to flush all dirty pages to media. The PMEM driver then just has issue a > >>> wmb_pmem() in response to the REQ_FLUSH to ensure that before we return all > >>> the flushed data has been durably stored on the media. > >>> > >>> Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx> > >> > >> Hmm, I'm not seeing why we need this patch. If the actual flushing of > >> the cache is done by the core why does the driver need support > >> REQ_FLUSH? Especially since it's just a couple instructions. REQ_FUA > >> only makes sense if individual writes can bypass the "drive" cache, > >> but no I/O submitted to the driver proper is ever cached we always > >> flush it through to media. > > > > If the upper level filesystem gets an error when submitting a flush > > request, then it assumes the underlying hardware is broken and cannot > > be as aggressive in IO submission, but instead has to wait for in-flight > > IO to complete. > > Upper level filesystems won't get errors when the driver does not > support flush. Those requests are ended cleanly in > generic_make_request_checks(). Yes, the fs still needs to wait for > outstanding I/O to complete but in the case of pmem all I/O is > synchronous. There's never anything to await when flushing at the > pmem driver level. > > > Since FUA/FLUSH is basically a no-op for pmem devices, > > it doesn't make sense _not_ to support this functionality. > > Seems to be a nop either way. Given that DAX may lead to dirty data > pending to the device in the cpu cache that a REQ_FLUSH request will > not touch, its better to leave it all to the mm core to handle. I.e. > it doesn't make sense to call the driver just for two instructions > (sfence + pcommit) when the mm core is taking on the cache flushing. > Either handle it all in the mm or the driver, not a mixture. So I think REQ_FLUSH requests *must* end up doing sfence + pcommit because e.g. journal writes going through block layer or writes done through dax_do_io() must be on permanent storage once REQ_FLUSH request finishes and the way driver does IO doesn't guarantee this, does it? Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR -- 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
