On Fri, Feb 07, 2020 at 11:36:36AM +0800, Baoquan He wrote: >On 02/06/20 at 07:21pm, 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? > >I think no, the lost memmap should only happen in vmemmap 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. I saw this function, but those combination of vmemmap and !vmemmap make my brain not work properly. > >Thanks for your confirmation. Do you mind if I add some document >sentences somewhere make clear this? Thanks, hope this would help the future audience. -- Wei Yang Help you, Help me
