Re: Snapshot target and DAX-capable devices

[Huaisheng Ye <yehs2007@zoho.com>]

 ---- On Fri, 31 Aug 2018 17:42:55 +0800 Jan Kara <jack@suse.cz> 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@redhat.com> 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

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