linux-kernel - Re: [PATCH v6 01/12] mm/sparsemem: Introduce struct mem_section

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Message-ID: <CA+CK2bCVAuYFFee+P09H_5fN4w2BHXUS1ZeSVN7hxcCTwgobqA@mail.gmail.com>
Date:   Sat, 4 May 2019 11:55:50 -0400
From:   Pavel Tatashin <pasha.tatashin@...een.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Logan Gunthorpe <logang@...tatee.com>,
        Linux MM <linux-mm@...ck.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH v6 01/12] mm/sparsemem: Introduce struct mem_section_usage

> > I'm ok with it being 16M for now unless it causes a problem in
> > practice, i.e. something like the minimum hardware mapping alignment
> > for physical memory being less than 16M.
>
> On second thought, arbitrary differences across architectures is a bit
> sad. The most common nvdimm namespace alignment granularity is
> PMD_SIZE, so perhaps the default sub-section size should try to match
> that default.

I think that even if you keep it 16M for now, at very least you should
make the map_active bitmap scalable so it will be possible to change
as required later without revisiting all functions that use it. Making
it a static array won't slowdown x86, as it will be still a single
64-bit word on x86.

Pasha