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:   Thu, 6 Feb 2020 19:21:49 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Baoquan He <bhe@...hat.com>
Cc:     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 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.

> Seems yes from code, but I don't find any document to prove it.

check_pfn_span() enforces this requirement.