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From: Dan Williams <dan.j.williams@intel.com>
To: Wei Yang <richard.weiyang@gmail.com>
Cc: Baoquan He <bhe@redhat.com>,
	Wei Yang <richardw.yang@linux.intel.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Oscar Salvador <osalvador@suse.de>, Linux MM <linux-mm@kvack.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	David Hildenbrand <david@redhat.com>
Subject: Re: [PATCH 2/3] mm/sparsemem: get physical address to page struct instead of virtual address to pfn
Date: Fri, 7 Feb 2020 08:44:57 -0800	[thread overview]
Message-ID: <CAPcyv4h=CRYjFK7AxmLX21B-AwmnAGL0=Vtp+a5PkLi63=KUKw@mail.gmail.com> (raw)
In-Reply-To: <20200207121453.pgi4axyvx6peqgeo@master>

On Fri, Feb 7, 2020 at 4:15 AM Wei Yang <richard.weiyang@gmail.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@redhat.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@linux.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.

  reply	other threads:[~2020-02-07 16:45 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-06 23:16 [PATCH 0/3] Fixes "mm/sparsemem: support sub-section hotplug" Wei Yang
2020-02-06 23:16 ` [PATCH 1/3] mm/sparsemem: adjust memmap only for SPARSEMEM_VMEMMAP Wei Yang
2020-02-07  2:00   ` Dan Williams
2020-02-07 11:06     ` Wei Yang
2020-02-06 23:16 ` [PATCH 2/3] mm/sparsemem: get physical address to page struct instead of virtual address to pfn Wei Yang
2020-02-07  2:19   ` Dan Williams
2020-02-07  3:10     ` Baoquan He
2020-02-07  3:21       ` Dan Williams
2020-02-07  3:36         ` Baoquan He
2020-02-07  4:05           ` Dan Williams
2020-02-07 11:13           ` Wei Yang
2020-02-07 12:14         ` Wei Yang
2020-02-07 16:44           ` Dan Williams [this message]
2020-02-07 10:51     ` Wei Yang
2020-02-07 11:26     ` Wei Yang
2020-02-09 13:50       ` Baoquan He
2020-02-09 14:14         ` David Hildenbrand
2020-02-10  0:36           ` Wei Yang
2020-02-07  4:11   ` Baoquan He
2020-02-07 10:53     ` Wei Yang
2020-02-06 23:16 ` [PATCH 3/3] mm/sparsemem: avoid memmap overwrite for non-SPARSEMEM_VMEMMAP Wei Yang
2020-02-07  2:06   ` Dan Williams
2020-02-07  3:50     ` Baoquan He
2020-02-07 11:02     ` Wei Yang

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