[dm-devel] [PATCH] vmalloc: introduce vmap_pfn for persistent memory

[Mikulas Patocka]

On Thu, 9 Nov 2017, Dan Williams wrote:

> On Thu, Nov 9, 2017 at 8:37 AM, Mikulas Patocka <mpatocka at redhat.com> wrote:
> >
> >
> > On Wed, 8 Nov 2017, Dan Williams wrote:
> >
> >> On Wed, Nov 8, 2017 at 12:26 PM, Mikulas Patocka <mpatocka at redhat.com> wrote:
> >> > On Wed, 8 Nov 2017, Christoph Hellwig wrote:
> >> >
> >> >> Can you start by explaining what you actually need the vmap for?
> >> >
> >> > It is possible to use lvm on persistent memory. You can create linear or
> >> > striped logical volumes on persistent memory and these volumes still have
> >> > the direct_access method, so they can be mapped with the function
> >> > dax_direct_access().
> >> >
> >> > If we create logical volumes on persistent memory, the method
> >> > dax_direct_access() won't return the whole device, it will return only a
> >> > part. When dax_direct_access() returns the whole device, my driver just
> >> > uses it without vmap. When dax_direct_access() return only a part of the
> >> > device, my driver calls it repeatedly to get all the parts and then
> >> > assembles the parts into a linear address space with vmap.
> >>
> >> I know I proposed "call dax_direct_access() once" as a strawman for an
> >> in-kernel driver user, but it's better to call it per access so you
> >> can better stay in sync with base driver events like new media errors
> >> and unplug / driver-unload. Either that, or at least have a plan how
> >> to handle those events.
> >
> > Calling it on every access would be inacceptable performance overkill. How
> > is it supposed to work anyway? - if something intends to move data on
> > persistent memory while some driver accesse it, then we need two functions
> > - dax_direct_access() and dax_relinquish_direct_access(). The current
> > kernel lacks a function dax_relinquish_direct_access() that would mark a
> > region of data as moveable, so we can't move the data anyway.
> 
> We take a global reference on the hosting device while pages are
> registered, see the percpu_ref usage in kernel/memremap.c, and we hold
> the dax_read_lock() over calls to dax_direct_access() to temporarily
> hold the device alive for the duration of the call.

If would be good if you provided some function that locks down persistent 
memory in the long-term. Locking it on every access just kills performance 
unacceptably.

For changing mapping, you could provide a callback. When the callback is 
called, the driver that uses persistent memory could quiesce itself, 
release the long-term lock and let the system change the mapping.

> While pages are pinned for DMA the devm_memremap_pages() mapping is
> pinned. Otherwise, an error reading persistent memory is identical to
> an error reading DRAM.

The question is if storage controllers and their drivers can react to this 
in a sensible way. Did someone test it?

Mikulas