linux-kernel - Re: [PATCH 11/12] device-dax: Add dis-contiguous resource support

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Date:   Fri, 10 Jul 2020 17:44:48 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Joao Martins <joao.m.martins@...cle.com>
Cc:     Linux MM <linux-mm@...ck.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Christoph Hellwig <hch@....de>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 11/12] device-dax: Add dis-contiguous resource support

On Tue, Mar 24, 2020 at 9:12 AM Joao Martins <joao.m.martins@...cle.com> wrote:
>
> On 3/23/20 11:55 PM, Dan Williams wrote:
> >  static ssize_t dev_dax_resize(struct dax_region *dax_region,
> >               struct dev_dax *dev_dax, resource_size_t size)
> >  {
> >       resource_size_t avail = dax_region_avail_size(dax_region), to_alloc;
> > -     resource_size_t dev_size = range_len(&dev_dax->range);
> > +     resource_size_t dev_size = dev_dax_size(dev_dax);
> >       struct resource *region_res = &dax_region->res;
> >       struct device *dev = &dev_dax->dev;
> > -     const char *name = dev_name(dev);
> >       struct resource *res, *first;
> > +     resource_size_t alloc = 0;
> > +     int rc;
> >
> >       if (dev->driver)
> >               return -EBUSY;
> > @@ -684,38 +766,47 @@ static ssize_t dev_dax_resize(struct dax_region *dax_region,
> >        * allocating a new resource.
> >        */
> >       first = region_res->child;
> > -     if (!first)
> > -             return __alloc_dev_dax_range(dev_dax, dax_region->res.start,
> > -                             to_alloc);
>
> You probably want to retain the condition above?
>
> Otherwise it removes the ability to create new devices or resizing it , once we
> have zero-ed the last one.
>
> > -     for (res = first; to_alloc && res; res = res->sibling) {
> > +retry:
> > +     rc = -ENOSPC;
> > +     for (res = first; res; res = res->sibling) {
> >               struct resource *next = res->sibling;
> > -             resource_size_t free;
> >
> >               /* space at the beginning of the region */
> > -             free = 0;
> > -             if (res == first && res->start > dax_region->res.start)
> > -                     free = res->start - dax_region->res.start;
> > -             if (free >= to_alloc && dev_size == 0)
> > -                     return __alloc_dev_dax_range(dev_dax,
> > -                                     dax_region->res.start, to_alloc);
> > -
> > -             free = 0;
> > +             if (res == first && res->start > dax_region->res.start) {
> > +                     alloc = min(res->start - dax_region->res.start,
> > +                                     to_alloc);
> > +                     rc = __alloc_dev_dax_range(dev_dax,
> > +                                     dax_region->res.start, alloc);
> > +                     break;
> > +             }
> > +
> > +             alloc = 0;
> >               /* space between allocations */
> >               if (next && next->start > res->end + 1)
> > -                     free = next->start - res->end + 1;
> > +                     alloc = min(next->start - (res->end + 1), to_alloc);
> >
> >               /* space at the end of the region */
> > -             if (free < to_alloc && !next && res->end < region_res->end)
> > -                     free = region_res->end - res->end;
> > -
> > -             if (free >= to_alloc && strcmp(name, res->name) == 0)
> > -                     return __adjust_dev_dax_range(dev_dax, res,
> > -                                     resource_size(res) + to_alloc);
> > -             else if (free >= to_alloc && dev_size == 0)
> > -                     return __alloc_dev_dax_range(dev_dax, res->end + 1,
> > -                                     to_alloc);
> > +             if (!alloc && !next && res->end < region_res->end)
> > +                     alloc = min(region_res->end - res->end, to_alloc);
> > +
> > +             if (!alloc)
> > +                     continue;
> > +
> > +             if (adjust_ok(dev_dax, res)) {
> > +                     rc = __adjust_dev_dax_range(dev_dax, res,
> > +                                     resource_size(res) + alloc);
> > +                     break;
> > +             }
> > +             rc = __alloc_dev_dax_range(dev_dax, res->end + 1,
> > +                             alloc);
>
> I am wondering if we should switch to:
>
>         if (adjust_ok(...))
>                 rc = __adjust_dev_dax_range(...);
>         else
>                 rc = __alloc_dev_dax_range(...);
>
> And then a debug print at the end depicting whether and how did we grabbed
> space? Something like:
>
>         dev_dbg(&dev_dax->dev, "%s(%d) %d", action, location, rc);
>
> Assuming we set @location to its values when we allocate space at the end,
> beginning or middle; and @action to whether we adjusted up/down or allocated new
> range.
>
> Essentially, something similar to namespaces scan_allocate() just to help
> troubleshoot?

I went ahead and just added "alloc", "extend", "shrink", and "delete
debug prints in the right places.