Re: [PATCH V1 1/2] libnvdimm/nsio: differentiate between probe mapping and runtime mapping

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

On Wed, Oct 16, 2019 at 8:19 PM Aneesh Kumar K.V
<aneesh.kumar@linux.ibm.com> wrote:
>
> On 10/17/19 8:34 AM, Dan Williams wrote:
> > On Wed, Oct 16, 2019 at 7:43 PM Aneesh Kumar K.V
>
>
> ....
>
> >>>
> >> The error is printed by generic code and the failures are due to fixed
> >> size. We can't workaround that using vmalloc=<size> option.
> >
> > Darn.
> >
> > Ok, explain to me again how this patch helps. This just seems to delay
> > the inevitable failure a bit, but the end result is that the user
> > needs to pick and choose which namespaces to enable after the kernel
> > has tried to auto-probe namespaces.
> >
>
> Right now we map the entire namespace using I/O remap range while
> probing and if we find the namespace mode to be fsdax or devdax we unmap
> the namespace and map them back using direct-map range. This temporary
> mapping cause transient failures if two large namespaces were probed at
> the same time. We try to map the second names space in the I/O remap
> range while the first one is already mapped there. ie,
>
> If we have two 6TB namespaces with an I/O remap range limit of 8TB. We
> can individually create these namespaces and enable/disable them because
> they are within the 8TB limit. But if we try to probe them together,
> and if namespace1 is already mapped in the I/O remap range we have only
> 2TB space left in the I/O remap range and trying to probe the second
> namespace at the same time fails due to lack of space to map the
> namespace during probing.
>
> This is not an issue with raw/btt, because they keep the namespace
> always mapped in the I/O remap range and if we are able to create a
> namespace we can enable them together. (we do have the issue of creating
> large raw/btt namespace while other raw/btt namespaces are disabled and
> trying to enable them all together).
>
> The fix proposed in this patch is to not map the full namespace range
> while probing. We just need to map the reserve block to find the
> namespace mode. Based on the detected namespace mode, we either use
> direct-map range or I/O range to map the full namespace.
>

Ah, ok, you're not trying to address the case where raw and btt
namespaces don't fit you're trying to fix the case where raw and btt
namespaces *do* fit in the vmap, but pfn / fsdax namespaces exceed the
vmap.

I would highlight that in the changelog, namely that an otherwise
working configuration with two namespaces (one btt and one pfn), where
the btt namespace consumes almost all the vmap space, will fail to
enable due to needing too much temporary vmap space to initialize the
pfn mode namespace.

Let's also make this a bit more self contained to devm_nsio_enable()
and add a length parameter. Then when btt is ready to start accessing
more than the initial 8K it can be just be called again with the full
namespace size. Then devm_nsio_enable can notice when the mapping
needs to be expanded vs established.

Although btt should not be making assumptions about the underlying
namepsace type so it would need to call a new devm_ndns_enable()
wrapper that calls devm_nsio_enable() for nsio namespaces and is a nop
for ndblk namespaces.
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