linux-kernel - Re: [PATCH] DAX: warn when kmem regions are truncated for memory block alignment.

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Message-ID: <Z-wv9NQr2pE1U8w5@gourry-fedora-PF4VCD3F>
Date: Tue, 1 Apr 2025 14:27:00 -0400
From: Gregory Price <gourry@...rry.net>
To: Dan Williams <dan.j.williams@...el.com>
Cc: David Hildenbrand <david@...hat.com>, linux-cxl@...r.kernel.org,
	nvdimm@...ts.linux.dev, linux-kernel@...r.kernel.org,
	kernel-team@...a.com, vishal.l.verma@...el.com,
	dave.jiang@...el.com
Subject: Re: [PATCH] DAX: warn when kmem regions are truncated for memory
 block alignment.

On Tue, Apr 01, 2025 at 10:47:09AM -0700, Dan Williams wrote:
> David Hildenbrand wrote:
> > diff --git a/drivers/dax/kmem.c b/drivers/dax/kmem.c
> > index e97d47f42ee2e..23a68ff809cdf 100644
> > --- a/drivers/dax/kmem.c
> > +++ b/drivers/dax/kmem.c
> > @@ -67,8 +67,8 @@ static void kmem_put_memory_types(void)
> >   
> >   static int dev_dax_kmem_probe(struct dev_dax *dev_dax)
> >   {
> > +       unsigned long total_len = 0, orig_len = 0;
> >          struct device *dev = &dev_dax->dev;
> > -       unsigned long total_len = 0;
> >          struct dax_kmem_data *data;
> >          struct memory_dev_type *mtype;
> >          int i, rc, mapped = 0;
> > @@ -97,6 +97,7 @@ static int dev_dax_kmem_probe(struct dev_dax *dev_dax)
> >          for (i = 0; i < dev_dax->nr_range; i++) {
> >                  struct range range;
> >   
> > +               orig_len += range_len(&dev_dax->ranges[i].range);
> >                  rc = dax_kmem_range(dev_dax, i, &range);
> >                  if (rc) {
> >                          dev_info(dev, "mapping%d: %#llx-%#llx too small after alignment\n",
> > @@ -109,6 +110,9 @@ static int dev_dax_kmem_probe(struct dev_dax *dev_dax)
> >          if (!total_len) {
> >                  dev_warn(dev, "rejecting DAX region without any memory after alignment\n");
> >                  return -EINVAL;
> > +       } else if (total_len != orig_len) {
> > +               dev_warn(dev, "DAX region truncated by %lu bytes due to alignment\n",
> > +                        orig_len - total_len);
> 
> This looks good, I agree with it being a warn because the user has lost
> usable capacity and maybe this eventually pressures platform BIOS to
> avoid causing Linux warnings.
> 
> In terms of making that loss easier for people to report / understand
> how about use string_get_size() to convert raw bytes to power of 10 and
> power of 2 values? I.e.
> 
> "DAX region truncated by X.XX GiB (Y.YY GB) due to alignment."

Will pick this up in v2.