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

From: Alexander Potapenko <glider@google.com>
To: Dan Williams <dan.j.williams@intel.com>
Cc: Marco Elver <elver@google.com>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	 Alexei Starovoitov <ast@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	 Andrey Konovalov <andreyknvl@google.com>,
	Andy Lutomirski <luto@kernel.org>, Arnd Bergmann <arnd@arndb.de>,
	 Borislav Petkov <bp@alien8.de>, Christoph Hellwig <hch@lst.de>,
	Christoph Lameter <cl@linux.com>,
	 David Rientjes <rientjes@google.com>,
	Dmitry Vyukov <dvyukov@google.com>,
	 Eric Dumazet <edumazet@google.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	 Herbert Xu <herbert@gondor.apana.org.au>,
	Ilya Leoshkevich <iii@linux.ibm.com>,
	 Ingo Molnar <mingo@redhat.com>, Jens Axboe <axboe@kernel.dk>,
	Joonsoo Kim <iamjoonsoo.kim@lge.com>,
	 Kees Cook <keescook@chromium.org>,
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Subject: Re: [PATCH v4 10/45] libnvdimm/pfn_dev: increase MAX_STRUCT_PAGE_SIZE
Date: Mon, 30 Jan 2023 09:34:54 +0100	[thread overview]
Message-ID: <CAG_fn=XNfrpTxWYYLnG5L-ogKmxvWvLGTzgqbT7sWxnFgnu7_w@mail.gmail.com> (raw)
In-Reply-To: <63b74a6e6a909_c81f0294a5@dwillia2-xfh.jf.intel.com.notmuch>

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.