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charset="UTF-8" X-Rspamd-Queue-Id: 62AE1180458DF X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam03 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Tue, Jun 23, 2020 at 11:14 PM Wei Yang wrote: > > On Tue, Jun 23, 2020 at 05:18:28PM +0200, Michal Hocko wrote: > >On Tue 23-06-20 17:42:58, Wei Yang wrote: > >> For early sections, we assumes its memmap will never be partially > >> removed. But current behavior breaks this. > >> > >> Let's correct it. > >> > >> Fixes: ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug") > >> Signed-off-by: Wei Yang > > > >Can a user trigger this or is this a theoretical bug? > > Let me rewrite the changelog a little. Look forward any comments. > > For early sections, its memmap is handled specially even sub-section is > enabled. The memmap could only be populated as a whole. > > Quoted from the comment of section_activate(): > > * The early init code does not consider partially populated > * initial sections, it simply assumes that memory will never be > * referenced. If we hot-add memory into such a section then we > * do not need to populate the memmap and can simply reuse what > * is already there. > > While current section_deactivate() breaks this rule. When hot-remove a > sub-section, section_deactivate() would depopulate its memmap. The > consequence is if we hot-add this subsection again, its memmap never get > proper populated. Ok, forgive the latency as re-fetched this logic into my mental cache. So what I was remembering was the initial state of the code that special cased early sections, and that still seems to be the case in pfn_valid(). IIRC early_sections / bootmem are blocked from being removed entirely. Partial / subsection removals are ok.