On 02/25/20 at 10:57am, Michal Hocko wrote: > On Thu 20-02-20 12:33:13, Baoquan He wrote: > > Currently, subsection map is used when SPARSEMEM is enabled, including > > VMEMMAP case and !VMEMMAP case. However, subsection hotplug is not > > supported at all in SPARSEMEM|!VMEMMAP case, subsection map is unnecessary > > and misleading. Let's adjust code to only allow subsection map being > > used in VMEMMAP case. > > This really needs more explanation I believe. What exactly happens if > somebody tries to hotremove a part of the section with !VMEMMAP? I can > see that clear_subsection_map returns 0 but that is not an error code. > Besides that section_deactivate doesn't propagate the error upwards. > /me stares into the code > > OK, I can see it now. It is relying on check_pfn_span to use the proper > subsection granularity. This really begs for a comment in the code > somewhere. Yes, check_pfn_span() guards it. People have no way to hot add/remove on non-section aligned block with !VMEMMAP. I have added extra comment to above section_activate() to note this, please check patch 5/7. Let me see how to add words to reflect the check_pfn_span() guard thing.
