Re: [PATCH RFC v2 13/18] dax/bus: Factor out dev dax resize logic

From: Jonathan Cameron <Jonathan.Cameron@Huawei.com>
To: Ira Weiny <ira.weiny@intel.com>
Cc: Dan Williams <dan.j.williams@intel.com>,
	Navneet Singh <navneet.singh@intel.com>,
	Fan Ni <fan.ni@samsung.com>, Davidlohr Bueso <dave@stgolabs.net>,
	Dave Jiang <dave.jiang@intel.com>,
	Alison Schofield <alison.schofield@intel.com>,
	Vishal Verma <vishal.l.verma@intel.com>,
	<linux-cxl@vger.kernel.org>, <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH RFC v2 13/18] dax/bus: Factor out dev dax resize logic
Date: Wed, 30 Aug 2023 12:27:41 +0100	[thread overview]
Message-ID: <20230830122741.00007628@Huawei.com> (raw)
In-Reply-To: <20230604-dcd-type2-upstream-v2-13-f740c47e7916@intel.com>

On Mon, 28 Aug 2023 22:21:04 -0700
Ira Weiny <ira.weiny@intel.com> wrote:

> Dynamic Capacity regions must limit dev dax resources to those areas
> which have extents backing real memory.  Four alternatives were
> considered to manage the intersection of region space and extents:
> 
> 1) Create a single region resource child on region creation which
>    reserves the entire region.  Then as extents are added punch holes in
>    this reservation.  This requires new resource manipulation to punch
>    the holes and still requires an additional iteration over the extent
>    areas which may already have existing dev dax resources used.
> 
> 2) Maintain an ordered xarray of extents which can be queried while
>    processing the resize logic.  The issue is that existing region->res
>    children may artificially limit the allocation size sent to
>    alloc_dev_dax_range().  IE the resource children can't be directly
>    used in the resize logic to find where space in the region is.
> 
> 3) Maintain a separate resource tree with extents.  This option is the
>    same as 2) but with a different data structure.  Most ideally we have
>    some unified representation of the resource tree.
> 
> 4) Create region resource children for each extent.  Manage the dax dev
>    resize logic in the same way as before but use a region child
>    (extent) resource as the parents to find space within each extent.
> 
> Option 4 can leverage the existing resize algorithm to find space within
> the extents.
> 
> In preparation for this change, factor out the dev_dax_resize logic.
> For static regions use dax_region->res as the parent to find space for
> the dax ranges.  Future patches will use the same algorithm with
> individual extent resources as the parent.
> 
> Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Hi Ira,

Some trivial comments on comments, but in general this indeed seems to be doing what you
say and factoring out the static allocation part.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

> ---
>  drivers/dax/bus.c | 128 +++++++++++++++++++++++++++++++++---------------------
>  1 file changed, 79 insertions(+), 49 deletions(-)
> 
> diff --git a/drivers/dax/bus.c b/drivers/dax/bus.c
> index b76e49813a39..ea7ae82b4687 100644
> --- a/drivers/dax/bus.c
> +++ b/drivers/dax/bus.c
> @@ -817,11 +817,10 @@ static int devm_register_dax_mapping(struct dev_dax *dev_dax, int range_id)
>  	return 0;
>  }
>  

> -static ssize_t dev_dax_resize(struct dax_region *dax_region,
> -		struct dev_dax *dev_dax, resource_size_t size)
> +/*

/**

Suitable builds will then check this doc matches the function etc
even if this is never included into any of the docs build.

> + * dev_dax_resize_static - Expand the device into the unused portion of the
> + * region. This may involve adjusting the end of an existing resource, or
> + * allocating a new resource.
> + *
> + * @parent: parent resource to allocate this range in.
> + * @dev_dax: DAX device we are creating this range for

Trivial: Doesn't seem to be consistent on . or not

> + * @to_alloc: amount of space to alloc; must be <= space available in @parent
> + *
> + * Return the amount of space allocated or -ERRNO on failure
> + */
> +static ssize_t dev_dax_resize_static(struct resource *parent,
> +				     struct dev_dax *dev_dax,
> +				     resource_size_t to_alloc)