Re: [QUESTION] Error on initializing dax by using different struct page size

[Won-Kyo Choe <wkyo.choe@gmail.com>]

On Thu, Nov 07, 2019 at 07:54:21AM -0800, Dan Williams wrote:
> 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.
> 
Thanks for the related commit. The kernel that I am using (5.3.9 / 5.3-rc5) does
not have the assert so that I was able to build it by little bit modifying lines
in include/linux/mm.h

        BUILD_BUG_ON(sizeof(struct page) > 80);
        ...

, which is quite similar with the assert you referred.

> In general 64-bytes per page is already expensive 128 bytes is a
> gigantic struct page.
Yes. I am aware that issue. I just wanted to add hot-page tracking
feature by inserting some data structure collecting it inside struct page
but the size is matter. I should find another way to get that stat :)

(Sorry, I should've put cc on this mail)

Thanks,

Won-Kyo
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