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

[Dan Williams <dan.j.williams@intel.com>]

On Thu, Nov 9, 2017 at 10:13 AM, Mikulas Patocka <mpatocka@redhat.com> wrote:
>
>
> On Thu, 9 Nov 2017, Dan Williams wrote:
>
>> On Thu, Nov 9, 2017 at 8:37 AM, Mikulas Patocka <mpatocka@redhat.com> wrote:
>> >
>> >
>> > On Wed, 8 Nov 2017, Dan Williams wrote:
>> >
>> >> On Wed, Nov 8, 2017 at 12:26 PM, Mikulas Patocka <mpatocka@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.

I'll take a look at this. It dovetails with some of the discussions we
are having about how to support RDMA to persistent memory and
notification/callback to tear down memory registrations.

>> 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?

The drivers don't need to react, once the pages are pinned for dma the
hot-unplug will not progress until all those page references are
dropped.

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