Re: [PATCH v10 03/13] mm/sparsemem: Add helpers track active portions of a section at boot

[Oscar Salvador]

On Tue, 2019-06-18 at 22:51 -0700, Dan Williams wrote:
> Prepare for hot{plug,remove} of sub-ranges of a section by tracking a
> sub-section active bitmask, each bit representing a PMD_SIZE span of
> the
> architecture's memory hotplug section size.
> 
> The implications of a partially populated section is that pfn_valid()
> needs to go beyond a valid_section() check and either determine that
> the
> section is an "early section", or read the sub-section active ranges
> from the bitmask. The expectation is that the bitmask
> (subsection_map)
> fits in the same cacheline as the valid_section() / early_section()
> data, so the incremental performance overhead to pfn_valid() should
> be
> negligible.
> 
> The rationale for using early_section() to short-ciruit the
> subsection_map check is that there are legacy code paths that use
> pfn_valid() at section granularity before validating the pfn against
> pgdat data. So, the early_section() check allows those traditional
> assumptions to persist while also permitting subsection_map to tell
> the
> truth for purposes of populating the unused portions of early
> sections
> with PMEM and other ZONE_DEVICE mappings.
> 
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Logan Gunthorpe <logang@xxxxxxxxxxxx>
> Cc: Oscar Salvador <osalvador@xxxxxxx>
> Cc: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
> Reported-by: Qian Cai <cai@xxxxxx>
> Tested-by: Jane Chu <jane.chu@xxxxxxxxxx>
> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>

Reviewed-by: Oscar Salvador <osalvador@xxxxxxx>

-- 
Oscar Salvador
SUSE L3