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From:   Alexander Potapenko <glider@google.com>
Date:   Tue, 10 Jan 2023 10:14:44 +0100
Message-ID: <CAG_fn=W36J9FNtkdFN6Ygua+hZKd3cUg9akJvx6rJknr9T--rA@mail.gmail.com>
Subject: Re: [PATCH v4 10/45] libnvdimm/pfn_dev: increase MAX_STRUCT_PAGE_SIZE
To:     Eric Dumazet <edumazet@google.com>,
        Dan Williams <dan.j.williams@intel.com>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        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>,
        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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        Matthew Wilcox <willy@infradead.org>,
        "Michael S. Tsirkin" <mst@redhat.com>,
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> > > Unfortunately, it can not be dynamically enabled because the size of
> > > 'struct page' is unfortunately recorded in the metadata of the device.
> > > Recall this is for supporting platform configurations where the capacity
> > > of the persistent memory exceeds or consumes too much of System RAM.
> > > Consider 4TB of PMEM consumes 64GB of space just for 'struct page'. So,
> > > NVDIMM subsystem has a mode to store that page array in a reservation on
> > > the PMEM device itself.
> >
> > Sorry, I might be missing something, but why cannot we have
> >
> > #ifdef CONFIG_KMSAN
> > #define MAX_STRUCT_PAGE_SIZE 128
> > #else
> > #define MAX_STRUCT_PAGE_SIZE 64
> > #endif
> >
>
> Possibly because this needs to be a fixed size on permanent storage
> (like an inode on a disk file system)
>

Ah, thank you, that makes sense.
Then I'm assuming some contents of struct pages are also stored in the
persistent memory. What happens if sizeof(struct page) stays below
MAX_STRUCT_PAGE_SIZE, but the layout changes?
E.g. doesn't it cause problems if the user enables/disables CONFIG_MEMCG?