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

[Dan Williams]

Alexander Potapenko wrote:
> On Thu, Jan 5, 2023 at 11:09 PM Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
> >
> > Alexander Potapenko wrote:
> > > (+ Dan Williams)
> > > (resending with patch context included)
> > >
> > > On Mon, Jul 11, 2022 at 6:27 PM Marco Elver <elver@xxxxxxxxxx> wrote:
> > > >
> > > > On Fri, 1 Jul 2022 at 16:23, Alexander Potapenko <glider@xxxxxxxxxx> 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?

How about a module parameter to opt-in to the increased permanent
capacity loss?

-- >8 --