On Tue, Jun 20, 2017 at 9:17 AM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote: > On Tue, Jun 20, 2017 at 1:49 AM, Christoph Hellwig <hch@xxxxxx> wrote: >> [stripped giant fullquotes] >> >> On Mon, Jun 19, 2017 at 10:53:12PM -0700, Andy Lutomirski wrote: >>> But that's my whole point. The kernel doesn't really need to prevent >>> all these background maintenance operations -- it just needs to block >>> .page_mkwrite until they are synced. I think that whatever new >>> mechanism we add for this should be sticky, but I see no reason why >>> the filesystem should have to block reflink on a DAX file entirely. >> >> Agreed - IFF we want to support write through semantics this is the >> only somewhat feasible way. It still has massive downsides of forcing >> the full sync machinery to run from the page fauly handler, which >> I'm rather scared off, but that's still better than creating a magic >> special case that isn't managable at all. > > An immutable-extent DAX-file and a reflink-capable DAX-file are not > mutually exclusive, and I have yet to hear a need for reflink support > without fsync/msync. Instead I have heard the need for an immutable > file for RDMA purposes, especially for hardware that can't trigger an > mmu fault. The special management of an immutable file is acceptable > to get these capabilities. I guess this applies to any user of get_user_pages() on a DAX-mapped file. Hmm.
