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Date: Tue, 23 Apr 2024 13:39:33 +0200
From: Nam Cao <namcao@linutronix.de>
To: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Jaya Kumar <jayalk@intworks.biz>, Daniel Vetter <daniel@ffwll.ch>,
	Helge Deller <deller@gmx.de>,
	Javier Martinez Canillas <javierm@redhat.com>,
	linux-fbdev@vger.kernel.org, dri-devel@lists.freedesktop.org,
	linux-kernel@vger.kernel.org, tiwai@suse.de, bigeasy@linutronix.de,
	patrik.r.jakobsson@gmail.com,
	Vegard Nossum <vegard.nossum@oracle.com>,
	George Kennedy <george.kennedy@oracle.com>,
	Darren Kenny <darren.kenny@oracle.com>, chuansheng.liu@intel.com,
	Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>,
	stable@vger.kernel.org
Subject: Re: [PATCH] fbdev: fix incorrect address computation in deferred IO
Message-ID: <20240423113933.IpMjHGw7@linutronix.de>
References: <20240419190032.40490-1-namcao@linutronix.de>
 <666d986e-5227-4b6d-829c-95ff16115488@suse.de>
 <20240423095538.m79ML6a0@linutronix.de>
 <64c1585d-70e0-47d4-9d78-405b483b433c@suse.de>
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On Tue, Apr 23, 2024 at 01:27:09PM +0200, Thomas Zimmermann wrote:
> Am 23.04.24 um 11:55 schrieb Nam Cao:
..
> > > The page-fault handler at [1] use vm_fault.pgoff to retrieve the page
> > > structure. Can we do the same here and avoid that computation?
> > Yes, thanks for the suggestion.
> > 
> > It will change things a bit: offset will not be the exact value anymore,
> > but will be rounded down to multiple of PAGE_SIZE. But that doesn't matter,
> > because it will only be used to calculate the page offset later on.
> > 
> > We can clean this up and rename this "offset" to "pg_offset". But that's
> > for another day.
> 
> But can't we use struct vm_fault.pgoff directly? The page-fault handler has
> used it since forever. The look-up code for the pageref should probably do
> the same, because there's a 1:1 connection between the page and the pageref.
> The pageref structure only exists because we cannot store its data in struct
> page directly.
> 
> AFAICT pgoff is exactly the value want to compute. See [1] and? the
> calculation at [2].

Completely agree. I just wanted to point out that the value inside the
variable "offset" won't be the same as before, but it's still correct.

I just tested the patch on real hardware, will send it shortly.

Best regards,
Nam