linux-kernel - Re: [PATCH 2/3] mm/sparsemem: get physical address to page struct instead of virtual address to pfn

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Date:   Fri, 7 Feb 2020 08:44:57 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Wei Yang <richard.weiyang@...il.com>
Cc:     Baoquan He <bhe@...hat.com>,
        Wei Yang <richardw.yang@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oscar Salvador <osalvador@...e.de>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH 2/3] mm/sparsemem: get physical address to page struct
 instead of virtual address to pfn

On Fri, Feb 7, 2020 at 4:15 AM Wei Yang <richard.weiyang@...il.com> wrote:
>
> On Thu, Feb 06, 2020 at 07:21:49PM -0800, Dan Williams wrote:
> >On Thu, Feb 6, 2020 at 7:10 PM Baoquan He <bhe@...hat.com> wrote:
> >>
> >> Hi Dan,
> >>
> >> On 02/06/20 at 06:19pm, Dan Williams wrote:
> >> > On Thu, Feb 6, 2020 at 3:17 PM Wei Yang <richardw.yang@...ux.intel.com> wrote:
> >> > > diff --git a/mm/sparse.c b/mm/sparse.c
> >> > > index b5da121bdd6e..56816f653588 100644
> >> > > --- a/mm/sparse.c
> >> > > +++ b/mm/sparse.c
> >> > > @@ -888,7 +888,7 @@ int __meminit sparse_add_section(int nid, unsigned long start_pfn,
> >> > >         /* Align memmap to section boundary in the subsection case */
> >> > >         if (IS_ENABLED(CONFIG_SPARSEMEM_VMEMMAP) &&
> >> > >                 section_nr_to_pfn(section_nr) != start_pfn)
> >> > > -               memmap = pfn_to_kaddr(section_nr_to_pfn(section_nr));
> >> > > +               memmap = pfn_to_page(section_nr_to_pfn(section_nr));
> >> >
> >> > Yes, this looks obviously correct. This might be tripping up
> >> > makedumpfile. Do you see any practical effects of this bug? The kernel
> >> > mostly avoids ->section_mem_map in the vmemmap case and in the
> >> > !vmemmap case section_nr_to_pfn(section_nr) should always equal
> >> > start_pfn.
> >>
> >> The practical effects is that the memmap for the first unaligned section will be lost
> >> when destroy namespace to hot remove it. Because we encode the ->section_mem_map
> >> into mem_section, and get memmap from the related mem_section to free it in
> >> section_deactivate(). In fact in vmemmap, we don't need to encode the ->section_mem_map
> >> with memmap.
> >
> >Right, but can you actually trigger that in the SPARSEMEM_VMEMMAP=n case?
> >
> >> By the way, sub-section support is only valid in vmemmap case, right?
> >
> >Yes.
>
> Just one question from curiosity. Why we don't want sub-section for !vmemmap
> case? Because it will wast memory for memmap?

The effort and maintenance burden outweighs the benefit.