linux-kernel - Re: [PATCH v3 02/13] device/dax: update devdax to use mmap

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Message-ID: <2jvm2x7krh7bkt43goiufksuxntncu2hxx67jos3i7zwj63jhh@rw47665pa25y>
Date: Wed, 17 Sep 2025 11:37:07 +0100
From: Pedro Falcato <pfalcato@...e.de>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 
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Subject: Re: [PATCH v3 02/13] device/dax: update devdax to use mmap_prepare

On Tue, Sep 16, 2025 at 03:11:48PM +0100, Lorenzo Stoakes wrote:
> The devdax driver does nothing special in its f_op->mmap hook, so
> straightforwardly update it to use the mmap_prepare hook instead.
> 
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> Acked-by: David Hildenbrand <david@...hat.com>
> Reviewed-by: Jan Kara <jack@...e.cz>

Acked-by: Pedro Falcato <pfalcato@...e.de>

> ---
>  drivers/dax/device.c | 32 +++++++++++++++++++++-----------
>  1 file changed, 21 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/dax/device.c b/drivers/dax/device.c
> index 2bb40a6060af..c2181439f925 100644
> --- a/drivers/dax/device.c
> +++ b/drivers/dax/device.c
> @@ -13,8 +13,9 @@
>  #include "dax-private.h"
>  #include "bus.h"
>  
> -static int check_vma(struct dev_dax *dev_dax, struct vm_area_struct *vma,
> -		const char *func)
> +static int __check_vma(struct dev_dax *dev_dax, vm_flags_t vm_flags,
> +		       unsigned long start, unsigned long end, struct file *file,
> +		       const char *func)
>  {
>  	struct device *dev = &dev_dax->dev;
>  	unsigned long mask;
> @@ -23,7 +24,7 @@ static int check_vma(struct dev_dax *dev_dax, struct vm_area_struct *vma,
>  		return -ENXIO;
>  
>  	/* prevent private mappings from being established */
> -	if ((vma->vm_flags & VM_MAYSHARE) != VM_MAYSHARE) {
> +	if ((vm_flags & VM_MAYSHARE) != VM_MAYSHARE) {
>  		dev_info_ratelimited(dev,
>  				"%s: %s: fail, attempted private mapping\n",
>  				current->comm, func);
> @@ -31,15 +32,15 @@ static int check_vma(struct dev_dax *dev_dax, struct vm_area_struct *vma,
>  	}
>  
>  	mask = dev_dax->align - 1;
> -	if (vma->vm_start & mask || vma->vm_end & mask) {
> +	if (start & mask || end & mask) {
>  		dev_info_ratelimited(dev,
>  				"%s: %s: fail, unaligned vma (%#lx - %#lx, %#lx)\n",
> -				current->comm, func, vma->vm_start, vma->vm_end,
> +				current->comm, func, start, end,
>  				mask);
>  		return -EINVAL;
>  	}
>  
> -	if (!vma_is_dax(vma)) {
> +	if (!file_is_dax(file)) {
>  		dev_info_ratelimited(dev,
>  				"%s: %s: fail, vma is not DAX capable\n",
>  				current->comm, func);
> @@ -49,6 +50,13 @@ static int check_vma(struct dev_dax *dev_dax, struct vm_area_struct *vma,
>  	return 0;
>  }
>  
> +static int check_vma(struct dev_dax *dev_dax, struct vm_area_struct *vma,
> +		     const char *func)
> +{
> +	return __check_vma(dev_dax, vma->vm_flags, vma->vm_start, vma->vm_end,
> +			   vma->vm_file, func);
> +}
> +

Side comment: I'm no DAX expert at all, but this check_vma() thing looks... smelly?
Besides the !dax_alive() check, I don't see the need to recheck vma limits at
every ->huge_fault() call. Even taking mremap() into account,
->get_unmapped_area() should Do The Right Thing, no?

-- 
Pedro