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Date:   Fri, 7 Feb 2020 15:23:13 +0800
From:   Baoquan He <bhe@redhat.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     Wei Yang <richardw.yang@linux.intel.com>,
        akpm@linux-foundation.org, osalvador@suse.de,
        dan.j.williams@intel.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm/sparsemem: pfn_to_page is not valid yet on SPARSEMEM
Message-ID: <20200207072313.GH26758@MiWiFi-R3L-srv>
References: <20200206125343.9070-1-richardw.yang@linux.intel.com>
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On 02/06/20 at 03:37pm, David Hildenbrand wrote:
> On 06.02.20 15:07, Baoquan He wrote:
> > On 02/06/20 at 02:55pm, David Hildenbrand wrote:
> >> On 06.02.20 14:50, Baoquan He wrote:
> >>> On 02/06/20 at 02:28pm, David Hildenbrand wrote:
> >>>> On 06.02.20 13:53, Wei Yang wrote:
> >>>>> When we use SPARSEMEM instead of SPARSEMEM_VMEMMAP, pfn_to_page()
> >>>>> doesn't work before sparse_init_one_section() is called. This leads to a
> >>>>> crash when hotplug memory.
> >>>>>
> >>>>> We should use memmap as it did.
> >>>>>
> >>>>> Fixes: ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
> >>>>> Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
> >>>>> CC: Dan Williams <dan.j.williams@intel.com>
> >>>>> ---
> >>>>>  mm/sparse.c | 2 +-
> >>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>>>>
> >>>>> diff --git a/mm/sparse.c b/mm/sparse.c
> >>>>> index 5a8599041a2a..2efb24ff8f96 100644
> >>>>> --- a/mm/sparse.c
> >>>>> +++ b/mm/sparse.c
> >>>>> @@ -882,7 +882,7 @@ int __meminit sparse_add_section(int nid, unsigned long start_pfn,
> >>>>>  	 * Poison uninitialized struct pages in order to catch invalid flags
> >>>>>  	 * combinations.
> >>>>>  	 */
> >>>>> -	page_init_poison(pfn_to_page(start_pfn), sizeof(struct page) * nr_pages);
> >>>>> +	page_init_poison(memmap, sizeof(struct page) * nr_pages);
> >>>>
> >>>> If you add sub-sections that don't fall onto the start of the section,
> >>>>
> >>>> pfn_to_page(start_pfn) != memmap
> >>>>
> >>>> and your patch would break that under SPARSEMEM_VMEMMAP if I am not wrong.
> >>>
> >>> It returns the pfn_to_page(pfn) from __populate_section_memmap() and
> >>> assign to memmap in vmemmap case, how come it breaks anything. Correct
> >>> me if I was wrong.
> >>
> >> I'm sorry, I can't follow :) Can you elaborate?
> >>
> >> Was your comment targeted at why the old code cannot be broken or why
> >> this patch cannot be broken?
> > 
> > Sorry for the confusion :-) the latter. I mean the returned memmap has been
> > at the pfn_to_page(start_pfn) in SPARSEMEM_VMEMMAP case.
> 
> Yeah, at least for SPARSEMEM_VMEMMAP it is indeed right. Thanks :)
> 
> 
> Now, about SPARSEMEM:
> 
> populate_section_memmap() does not care about nr_pages and will allocate
> a memmap for the whole section. So, whenever we add sub-sections to a
> section, we allocate a new memmap for the whole section. And we do
> overwrite the memmap pointer in our section. ( sparse_add_section() )
> 
> That makes me assume that sub-section hot-add under SPARSEMEM is either
> 
> a) never enabled and only works with SPARSEMEM_VMEMMAP
> b) horribly broken
> 
> And I think a) applies (looking at pfn_section_valid()). Therefore, we
> don't have to care about sub-section hot-add specifics (and I would be
> broken already)

Yeah, I have the same thought as you. And later Dan's words confirms it
in another threaad.