Re: [PATCH v4 10/45] libnvdimm/pfn_dev: increase MAX_STRUCT_PAGE_SIZE

Alexander Potapenko
2023-01-30T08:35:35Z

On Thu, Jan 5, 2023 at 11:09 PM Dan Williams <dan.j.williams@intel.com> wrote:
>
> Alexander Potapenko wrote:
> > (+ Dan Williams)
> > (resending with patch context included)
> >
> > On Mon, Jul 11, 2022 at 6:27 PM Marco Elver <elver@google.com> wrote:
> > >
> > > On Fri, 1 Jul 2022 at 16:23, Alexander Potapenko <glider@google.com> wrote:
> > > >
> > > > KMSAN adds extra metadata fields to struct page, so it does not fit into
> > > > 64 bytes anymore.
> > >
> > > Does this somehow cause extra space being used in all kernel configs?
> > > If not, it would be good to note this in the commit message.
> > >
> > I actually couldn't verify this on QEMU, because the driver never got loaded.
> > Looks like this increases the amount of memory used by the nvdimm
> > driver in all kernel configs that enable it (including those that
> > don't use KMSAN), but I am not sure how much is that.
> >
> > Dan, do you know how bad increasing MAX_STRUCT_PAGE_SIZE can be?
>
> Apologies I missed this several months ago. The answer is that this
> causes everyone creating PMEM namespaces on v6.1+ to lose double the
> capacity of their namespace even when not using KMSAN which is too
> wasteful to tolerate. So, I think "6e9f05dc66f9 libnvdimm/pfn_dev:
> increase MAX_STRUCT_PAGE_SIZE" needs to be reverted and replaced with
> something like:
>
> diff --git a/drivers/nvdimm/Kconfig b/drivers/nvdimm/Kconfig
> index 79d93126453d..5693869b720b 100644
> --- a/drivers/nvdimm/Kconfig
> +++ b/drivers/nvdimm/Kconfig
> @@ -63,6 +63,7 @@ config NVDIMM_PFN
> bool "PFN: Map persistent (device) memory"
> default LIBNVDIMM
> depends on ZONE_DEVICE
> + depends on !KMSAN
> select ND_CLAIM
> help
> Map persistent memory, i.e. advertise it to the memory
>

Looks like we still don't have a resolution for this problem.
I have the following options in mind:

1. Set MAX_STRUCT_PAGE_SIZE to 80 (i.e. increase it by 2*sizeof(struct
page *) added by KMSAN) instead of 128.
2. Disable storing of struct pages on device for KMSAN builds.

, but if those are infeasible, we can always go for:

3. Disable KMSAN for NVDIMM and reflect it in Documentation. I am
happy to send the patch if we decide this is the best option.