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

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Date:   Mon, 24 Jun 2019 19:57:55 +0200
From:   Oscar Salvador <osalvador@...e.de>
To:     Dan Williams <dan.j.williams@...el.com>, akpm@...ux-foundation.org
Cc:     Michal Hocko <mhocko@...e.com>, Vlastimil Babka <vbabka@...e.cz>,
        Logan Gunthorpe <logang@...tatee.com>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Qian Cai <cai@....pw>, Jane Chu <jane.chu@...cle.com>,
        linux-mm@...ck.org, linux-nvdimm@...ts.01.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v10 03/13] mm/sparsemem: Add helpers track active
 portions of a section at boot

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@...e.com>
> Cc: Vlastimil Babka <vbabka@...e.cz>
> Cc: Logan Gunthorpe <logang@...tatee.com>
> Cc: Oscar Salvador <osalvador@...e.de>
> Cc: Pavel Tatashin <pasha.tatashin@...een.com>
> Reported-by: Qian Cai <cai@....pw>
> Tested-by: Jane Chu <jane.chu@...cle.com>
> Signed-off-by: Dan Williams <dan.j.williams@...el.com>

Reviewed-by: Oscar Salvador <osalvador@...e.de>

-- 
Oscar Salvador
SUSE L3