On Wed 26-02-20 11:53:36, Baoquan He wrote: > 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. An explicit note about check_pfn_span gating the proper alignement and sizing sounds sufficient to me. -- Michal Hocko SUSE Labs
