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From: Dan Williams <dan.j.williams@intel.com>
Date: Thu, 7 Nov 2019 07:54:21 -0800
Message-ID: <CAPcyv4j-rSy-T5Qv6GbcDcmUerhQQYsMKbUY7EaGHz-GecKDtQ@mail.gmail.com>
Subject: Re: [QUESTION] Error on initializing dax by using different struct
 page size
To: Won-Kyo Choe <wkyo.choe@gmail.com>
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On Thu, Nov 7, 2019 at 7:30 AM Won-Kyo Choe <wkyo.choe@gmail.com> wrote:
>
> Hi, there. I'm using Opatne DC memory to use it a volatile memory. Recently,
> I found that if sizeof(struct page) is above 64 bytes (e.g. 128 byes),
> `device_dax` cannot be initialized when system boots. I am aware that
> for some reason there is a function, `__mm_zero_struct_page`, which limits
> the size of struct page when it exceeds 80 bytes. However, due to the
> research purpose, I do not use that constraint and I'm quite certain
> that using different page size is usable in main memory. So, I'm
> wondering why this is not possible in persistent memory and which
> patches are related to this problem.
>
> I will attach the system log for clarification. The test is run in
> linux-5.3.9 and linuxt-5.3-rc5

How did you manage to build the kernel with a 128byte struct page
size? This build assert in drivers/nvdimm/pfn_devs.c

                BUILD_BUG_ON(sizeof(struct page) > MAX_STRUCT_PAGE_SIZE);

...will start to trigger in v5.4 to explicitly prevent this going
forward. See commit e96f0bf2ec92 "libnvdimm/pfn_dev: Add a build check
to make sure we notice when struct page size change" for more details.

In general 64-bytes per page is already expensive 128 bytes is a
gigantic struct page.
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