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. Thanks for your confirmation. Do you mind if I add some document sentences somewhere make clear this?
