linux-kernel - Re: [PATCH v10 01/10] drm/shmem-helper: Simplify page offset calculation in fault handler

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Message-ID: <20251201090507.1ee10c65@fedora>
Date: Mon, 1 Dec 2025 09:05:07 +0100
From: Boris Brezillon <boris.brezillon@...labora.com>
To: Loïc Molinari <loic.molinari@...labora.com>
Cc: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Maxime Ripard
 <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>, David Airlie
 <airlied@...il.com>, Simona Vetter <simona@...ll.ch>, Jani Nikula
 <jani.nikula@...ux.intel.com>, Joonas Lahtinen
 <joonas.lahtinen@...ux.intel.com>, Rodrigo Vivi <rodrigo.vivi@...el.com>,
 Tvrtko Ursulin <tursulin@...ulin.net>, Rob Herring <robh@...nel.org>,
 Steven Price <steven.price@....com>, Liviu Dudau <liviu.dudau@....com>,
 Melissa Wen <mwen@...lia.com>, Maíra Canal
 <mcanal@...lia.com>, Hugh Dickins <hughd@...gle.com>, Baolin Wang
 <baolin.wang@...ux.alibaba.com>, Andrew Morton <akpm@...ux-foundation.org>,
 Al Viro <viro@...iv.linux.org.uk>, Mikołaj Wasiak
 <mikolaj.wasiak@...el.com>, Christian Brauner <brauner@...nel.org>, Nitin
 Gote <nitin.r.gote@...el.com>, Andi Shyti <andi.shyti@...ux.intel.com>,
 Jonathan Corbet <corbet@....net>, Christopher Healy <healych@...zon.com>,
 Matthew Wilcox <willy@...radead.org>, Bagas Sanjaya <bagasdotme@...il.com>,
 linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
 intel-gfx@...ts.freedesktop.org, linux-mm@...ck.org,
 linux-doc@...r.kernel.org, kernel@...labora.com
Subject: Re: [PATCH v10 01/10] drm/shmem-helper: Simplify page offset
 calculation in fault handler

On Fri, 28 Nov 2025 19:52:43 +0100
Loïc Molinari <loic.molinari@...labora.com> wrote:

> For a fault at address addr, the page offset is
>   page_offset = (vmf->address - vma->vm_start) >> PAGE_SHIFT
>               = ((addr & PAGE_MASK) - vma->vm_start) >> PAGE_SHIFT
> 	      = (addr - vma->vm_start) >> PAGE_SHIFT
> 
> Since the faulty logical page offset based on VMA is
>   vmf->pgoff = vma->vm_pgoff + ((addr - vma->vm_start) >> PAGE_SHIFT)
> 
> We can slightly simplify the calculation using
>   page_offset = vmf->pgoff - vma->vm_pgoff
> 
> Signed-off-by: Loïc Molinari <loic.molinari@...labora.com>

Reviewed-by: Boris Brezillon <boris.brezillon@...labora.com>

One nit below

> ---
>  drivers/gpu/drm/drm_gem_shmem_helper.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> index dc94a27710e5..be89be1c804c 100644
> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> @@ -577,8 +577,8 @@ static vm_fault_t drm_gem_shmem_fault(struct vm_fault *vmf)
>  	struct page *page;
>  	pgoff_t page_offset;
>  
> -	/* We don't use vmf->pgoff since that has the fake offset */
> -	page_offset = (vmf->address - vma->vm_start) >> PAGE_SHIFT;
> +	/* Offset to faulty address in the VMA (without the fake offset). */

It's weird to say "without the fake offset" here, because IIUC, both
vmf->pgoff and vma->vm_pgoff contain the fake offset. And that's fine,
the problem really is when one of the subtraction operand is not
relative to the fake offset.

> +	page_offset = vmf->pgoff - vma->vm_pgoff;
>  
>  	dma_resv_lock(shmem->base.resv, NULL);
>