linux-kernel - Re: [PATCH v2] fs/dax: include <asm/page.h> to fix build error on ARC

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Date:   Tue, 5 Jan 2021 04:17:38 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Randy Dunlap <rdunlap@...radead.org>
Cc:     linux-kernel@...r.kernel.org, kernel test robot <lkp@...el.com>,
        Vineet Gupta <vgupta@...opsys.com>,
        linux-snps-arc@...ts.infradead.org,
        Dan Williams <dan.j.williams@...el.com>,
        Vineet Gupta <vgupts@...opsys.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        linux-fsdevel@...r.kernel.org, linux-nvdimm@...ts.01.org
Subject: Re: [PATCH v2] fs/dax: include <asm/page.h> to fix build error on ARC

On Thu, Dec 31, 2020 at 08:29:14PM -0800, Randy Dunlap wrote:
> fs/dax.c uses copy_user_page() but ARC does not provide that interface,
> resulting in a build error.
> 
> Provide copy_user_page() in <asm/page.h> (beside copy_page()) and
> add <asm/page.h> to fs/dax.c to fix the build error.
> 
> ../fs/dax.c: In function 'copy_cow_page_dax':
> ../fs/dax.c:702:2: error: implicit declaration of function 'copy_user_page'; did you mean 'copy_to_user_page'? [-Werror=implicit-function-declaration]

Could somebody explain what the force-cast is doing in there?
I mean, the call is
        copy_user_page(vto, (void __force *)kaddr, vaddr, to);
kaddr is a local variable there, declared as void *; AFAICS, that
had been pure cargo-cult since
commit 7a9eb20666317794d0279843fbd091af93907780
Author: Dan Williams <dan.j.williams@...el.com>
Date:   Fri Jun 3 18:06:47 2016 -0700

    pmem: kill __pmem address space

I mean, it's been more than 4 years, time to bury that body...