Re: [PATCH 2/2] dax/bus: fix mapping attribute error reporting

[Dave Jiang]

On 9/15/26 2:56 AM, Muchun Song wrote:
> After the DAX configuration locking was converted to rwsems, successful
> lock acquisition leaves rc set to zero in mapping_store(). If the requested
> range size is misaligned, the allocation is skipped and the zero rc is
> converted to len. The sysfs write therefore reports success without
> allocating the requested range.
> 
> Call alloc_dev_dax_range() unconditionally and let its full range
> validation return -EINVAL for a misaligned start or size.
> 
> Fixes: c05ae9d85b47 ("dax/bus.c: replace driver-core lock usage by a local rwsem")
> Assisted-by: LLM
> Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> ---
>  drivers/dax/bus.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/drivers/dax/bus.c b/drivers/dax/bus.c
> index 54e4bbc98218..f232001ff5b7 100644
> --- a/drivers/dax/bus.c
> +++ b/drivers/dax/bus.c
> @@ -1276,8 +1276,7 @@ static ssize_t mapping_store(struct device *dev, struct device_attribute *attr,
>  	}
>  
>  	to_alloc = range_len(&r);
> -	if (size_is_aligned(dev_dax, to_alloc))
> -		rc = alloc_dev_dax_range(dev_dax, r.start, to_alloc);
> +	rc = alloc_dev_dax_range(dev_dax, r.start, to_alloc);
>  	up_write(&dax_dev_rwsem);
>  	up_write(&dax_region_rwsem);
>  

So this patch becomes somewhat of a backport issue as it depends on patch 1. Please consider swap the ordering of your patch series and fix this issue first. Maybe something like so the fix can be independently backported:

diff --git a/drivers/dax/bus.c b/drivers/dax/bus.c
index b809e1a264af..e40c25401cf0 100644
--- a/drivers/dax/bus.c
+++ b/drivers/dax/bus.c
@@ -1192,7 +1192,7 @@ static ssize_t mapping_store(struct device *dev, struct de
vice_attribute *attr,
                return rc;
        if (!dax_region->dev->driver) {
                up_write(&dax_region_rwsem);
-               return rc;
+               return -ENXIO;
        }
        rc = down_write_killable(&dax_dev_rwsem);
        if (rc) {
@@ -1201,8 +1201,12 @@ static ssize_t mapping_store(struct device *dev, struct d
evice_attribute *attr,
        }

        to_alloc = range_len(&r);
-       if (alloc_is_aligned(dev_dax, to_alloc))
+       if (!alloc_is_aligned(dev_dax, to_alloc)) {
+               dev_dbg(dev, "%s: size: %zu misaligned\n", __func__, to_alloc);
+               rc = -EINVAL;
+       } else {
                rc = alloc_dev_dax_range(dev_dax, r.start, to_alloc);
+       }
        up_write(&dax_dev_rwsem);
        up_write(&dax_region_rwsem);