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

[Alexander Potapenko <glider@google.com>]

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
>
>
> ...otherwise, what was the rationale for increasing this value? Were you
> actually trying to use KMSAN for DAX pages?

I was just building the kernel with nvdimm driver and KMSAN enabled.
Because KMSAN adds extra data to every struct page, it immediately hit
the following assert:

drivers/nvdimm/pfn_devs.c:796:3: error: call to
__compiletime_assert_330 declared with 'error' attribute: BUILD_BUG_ON
fE
                BUILD_BUG_ON(sizeof(struct page) > MAX_STRUCT_PAGE_SIZE);

The comment before MAX_STRUCT_PAGE_SIZE declaration says "max struct
page size independent of kernel config", but maybe we can afford
making it dependent on CONFIG_KMSAN (and possibly other config options
that increase struct page size)?

I don't mind disabling the driver under KMSAN, but having an extra
ifdef to keep KMSAN support sounds reasonable, WDYT?



-- 
Alexander Potapenko
Software Engineer

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