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Subject: Re: [PATCH] m68k: mm: Remove check for VM_IO to fix deferred I/O
To:     Geert Uytterhoeven <geert@linux-m68k.org>,
        linux-m68k@lists.linux-m68k.org
References: <20220128173006.1713210-1-geert@linux-m68k.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-fbdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        Finn Thain <fthain@linux-m68k.org>
From:   Michael Schmitz <schmitzmic@gmail.com>
Message-ID: <b1cd50eb-e476-b4bd-0b2f-b4a8699660f6@gmail.com>
Date:   Sat, 29 Jan 2022 10:26:01 +1300
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Hi Geert,

for hwregs_present(), the exception fixup will handle any access error 
(through send_fault_sig()), so this should continue to work.

Why the special handling of VM_IO pages? Maybe hp300 had marked all IO 
register pages VM_IO to distinguish IO faults from VM faults...

The only other area I can imagine this might have an impact is the Mac's 
pseudo-DMA - FInn might want to give this some testing.

Cheers,

	Michael


Am 29.01.2022 um 06:30 schrieb Geert Uytterhoeven:
> When an application accesses a mapped frame buffer backed by deferred
> I/O, it receives a segmentation fault.  Fix this by removing the check
> for VM_IO in do_page_fault().
>
> Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
> ---
> This check was never present in a fault handler on any other
> architecture than m68k.
> Some digging revealed that it was added in v2.1.106, but I couldn't find
> an email with a patch adding it.  That same kernel version extended the
> use of the hwreg_present() helper to HP9000/300, so the check might have
> been needed there, perhaps only during development?
> The Atari kernel relies heavily on hwreg_present() (both the success and
> failure cases), and these still work, at least on ARAnyM.
> ---
>  arch/m68k/mm/fault.c | 2 --
>  1 file changed, 2 deletions(-)
>
> diff --git a/arch/m68k/mm/fault.c b/arch/m68k/mm/fault.c
> index 1493cf5eac1e7a39..71aa9f6315dc8028 100644
> --- a/arch/m68k/mm/fault.c
> +++ b/arch/m68k/mm/fault.c
> @@ -93,8 +93,6 @@ int do_page_fault(struct pt_regs *regs, unsigned long address,
>  	vma = find_vma(mm, address);
>  	if (!vma)
>  		goto map_err;
> -	if (vma->vm_flags & VM_IO)
> -		goto acc_err;
>  	if (vma->vm_start <= address)
>  		goto good_area;
>  	if (!(vma->vm_flags & VM_GROWSDOWN))
>