On Fri, Feb 7, 2020 at 4:15 AM Wei Yang <richard.weiyang@xxxxxxxxx> 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@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? The effort and maintenance burden outweighs the benefit.
