linux-kernel - Re: [PATCH] mm-move-mremap-from-file_operations-to-vm_operations

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Date:	Thu, 16 Jul 2015 16:24:44 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Joonsoo Kim <js1304@...il.com>, Benjamin LaHaise <bcrl@...ck.org>,
	Fengguang Wu <fengguang.wu@...el.com>,
	Jeff Moyer <jmoyer@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	linux-next@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH]
 mm-move-mremap-from-file_operations-to-vm_operations_struct-fix

On Fri, 17 Jul 2015 01:14:05 +0200 Oleg Nesterov <oleg@...hat.com> wrote:

> fs/aio.c can't be compiled if CONFIG_MMU=n, filemap_page_mkwrite()
> is not defined in this case. Add yet another "must not be called"
> helper into nommu.c to make the linker happy.
> 
> I still think this is pointless, afaics sys_io_setup() simply can't
> succeed if CONFIG_MMU=n. Instead we should make CONFIG_AIO depend
> on CONFIG_MMU.
> 
> ..
>
> --- a/mm/nommu.c
> +++ b/mm/nommu.c
> @@ -2008,6 +2008,12 @@ void filemap_map_pages(struct vm_area_struct *vma, struct vm_fault *vmf)
>  }
>  EXPORT_SYMBOL(filemap_map_pages);
>  
> +int filemap_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
> +{
> +	BUG();
> +	return 0;
> +}
> +
>  static int __access_remote_vm(struct task_struct *tsk, struct mm_struct *mm,
>  		unsigned long addr, void *buf, int len, int write)
>  {

So if anyone starts testing aio on NOMMU, this patch will make the
whole thing immediately go BUG.  This isn't helpful :(

Yes, making AIO depend on MMU sounds better.  Because if it wasn't
busted before, it sure is now!
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