linux-kernel - Re: [PATCH v5 1/2] mm/memory_hotplug: split memmap_on

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Date:   Fri, 6 Oct 2023 22:01:19 +0000
From:   "Verma, Vishal L" <vishal.l.verma@...el.com>
To:     "Williams, Dan J" <dan.j.williams@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "osalvador@...e.de" <osalvador@...e.de>,
        "david@...hat.com" <david@...hat.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
CC:     "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "Huang, Ying" <ying.huang@...el.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "aneesh.kumar@...ux.ibm.com" <aneesh.kumar@...ux.ibm.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-cxl@...r.kernel.org" <linux-cxl@...r.kernel.org>,
        "Hocko, Michal" <mhocko@...e.com>,
        "nvdimm@...ts.linux.dev" <nvdimm@...ts.linux.dev>,
        "jmoyer@...hat.com" <jmoyer@...hat.com>,
        "Jonathan.Cameron@...wei.com" <Jonathan.Cameron@...wei.com>
Subject: Re: [PATCH v5 1/2] mm/memory_hotplug: split memmap_on_memory requests
 across memblocks

On Fri, 2023-10-06 at 14:52 +0200, David Hildenbrand wrote:
> On 05.10.23 20:31, Vishal Verma wrote:
> > 
<..>
> > @@ -2167,47 +2221,28 @@ static int __ref try_remove_memory(u64 start, u64 size)
> >         if (rc)
> >                 return rc;
> >   
> > +       mem_hotplug_begin();
> > +
> >         /*
> > -        * We only support removing memory added with MHP_MEMMAP_ON_MEMORY in
> > -        * the same granularity it was added - a single memory block.
> > +        * For memmap_on_memory, the altmaps could have been added on
> > +        * a per-memblock basis. Loop through the entire range if so,
> > +        * and remove each memblock and its altmap.
> >          */
> >         if (mhp_memmap_on_memory()) {
> > -               rc = walk_memory_blocks(start, size, &mem, test_has_altmap_cb);
> > -               if (rc) {
> > -                       if (size != memory_block_size_bytes()) {
> > -                               pr_warn("Refuse to remove %#llx - %#llx,"
> > -                                       "wrong granularity\n",
> > -                                       start, start + size);
> > -                               return -EINVAL;
> > -                       }
> > -                       altmap = mem->altmap;
> > -                       /*
> > -                        * Mark altmap NULL so that we can add a debug
> > -                        * check on memblock free.
> > -                        */
> > -                       mem->altmap = NULL;
> > -               }
> > +               unsigned long memblock_size = memory_block_size_bytes();
> > +               u64 cur_start;
> > +
> > +               for (cur_start = start; cur_start < start + size;
> > +                    cur_start += memblock_size)
> > +                       remove_memory_block_and_altmap(nid, cur_start,
> > +                                                      memblock_size);
> > +       } else {
> > +               remove_memory_block_and_altmap(nid, start, size);
> 
> Better call remove_memory_block_devices() and arch_remove_memory(start, 
> size, altmap) here explicitly instead of using 
> remove_memory_block_and_altmap() that really can only handle a single
> memory block with any inputs.
> 
I'm not sure I follow. Even in the non memmap_on_memory case, we'd have
to walk_memory_blocks() to get to the memory_block->altmap, right?

Or is there a more direct way? If we have to walk_memory_blocks, what's
the advantage of calling those directly instead of calling the helper
created above?

Agreed with and fixed up all the other comments.