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@xxxxxxxxxx> 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@xxxxxxxxxxxxxxx> 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? > >> Seems yes from code, but I don't find any document to prove it. > >check_pfn_span() enforces this requirement. -- Wei Yang Help you, Help me
