---- On Fri, 31 Aug 2018 17:42:55 +0800 Jan Kara <jack@xxxxxxx> wrote ---- > On Fri 31-08-18 09:38:09, Dave Chinner wrote: > > On Thu, Aug 30, 2018 at 03:47:32PM -0400, Mikulas Patocka wrote: > > > > > > > > > On Thu, 30 Aug 2018, Jeff Moyer wrote: > > > > > > > Mike Snitzer <snitzer@xxxxxxxxxx> writes: > > > > > > > > > Until we properly add DAX support to dm-snapshot I'm afraid we really do > > > > > need to tolerate this "regression". Since reality is the original > > > > > support for snapshot of a DAX DM device never worked in a robust way. > > > > > > > > Agreed. > > > > > > > > -Jeff > > > > > > You can't support dax on snapshot - if someone maps a block and the block > > > needs to be moved, then what? > > > > This is only a problem for access via mmap and page faults. > > > > At the filesystem level, it's no different to the existing direct IO > > algorithm for read/write IO - we simply allocate new space, copy the > > data we need to copy into the new space (may be no copy needed), and > > then write the new data into the new space. I'm pretty sure that for > > bio-based IO to dm-snapshot devices the algorithm will be exactly > > the same. > > > > However, for direct access via mmap, we have to modify how the > > userspace virtual address is mapped to the physical location. IOWs, > > during the COW operation, we have to invalidate all existing user > > mappings we have for that physical address. This means we have to do > > an invalidation after the allocate/copy part of the COW operation. > > > > If we are doing this during a page fault, it means we'll probably > > have to restart the page fault so it can look up the new physical > > address associated with the faulting user address. After we've done > > the invalidation, any new (or restarted) page fault finds the > > location of new copy we just made, maps it into the user address > > space, updates the ptes and we're all good. > > > > Well, that's the theory. We haven't implemented this for XFS yet, so > > it might end up a little different, and we might yet hit unexpected > > problems (it's DAX, that's what happens :/). > > Yes, that's outline of a plan :) > > > It's a whole different ballgame for a dm-snapshot device - block > > devices are completely unaware of page faults to DAX file mappings. > > Actually, block devices are not completely unaware of DAX page faults - > they will get ->direct_access callback for the fault range. It does not > currently convey enough information - we also need to inform the block > device whether it is read or write. But that's about all that's needed to > add AFAICT. And by comparing returned PFN with the one we have stored in > the radix tree (which we have if that file offset is mapped by anybody), > the filesystem / DAX code can tell whether remapping happened and do the > unmapping. Hi Jan, I am trying to investigate how to make dm-snapshot to support DAX, and I dropped a patchset to upstream for comments. Any suggestion is welcome. # https://lkml.org/lkml/2018/11/21/281 In the beginning, I haven't considered the situation of mmap write faults. >From Dan's reply and this email thread, now I have a more clear understanding. The question is that, even the virtual dm block device has been informed that the mmap may have write operations through PROT_WRITE, if userspace directly operate the virtual address of origin device like memcpy, dm-snapshot doesn't have chance to detect this behavior. Although dm-snapshot can have chance to prepare a COW area to back up origin's blocks within ->direct_access callback for the fault range, how can it to have opportunity to read the data from origin device and save it to COW? --- Cheers, Huaisheng Ye -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
