linux-kernel - Re: [PATCH] sparse: Track the boundaries of memory sections for accurate checks

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Message-ID: <CAPcyv4gQZ-=6SdsGc-YafcAUz0WWxtGuh56CPan1xqSkWbd9=A@mail.gmail.com>
Date:   Wed, 14 Sep 2016 15:05:51 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     "Raslan, KarimAllah" <karahmed@...zon.de>
Cc:     Michal Hocko <mhocko@...nel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Vlastimil Babka <vbabka@...e.cz>,
        Johannes Weiner <hannes@...xchg.org>,
        Yaowei Bai <baiyaowei@...s.chinamobile.com>,
        Joe Perches <joe@...ches.com>, Tejun Heo <tj@...nel.org>,
        "Liguori, Anthony" <aliguori@...zon.com>,
        "Schoenherr, Jan H." <jschoenh@...zon.de>
Subject: Re: [PATCH] sparse: Track the boundaries of memory sections for
 accurate checks

On Wed, Sep 14, 2016 at 2:40 PM, Raslan, KarimAllah <karahmed@...zon.de> wrote:
>
>
> On 6/20/16, 10:23 AM, "Michal Hocko" <mhocko@...nel.org> wrote:
>
>     On Sat 18-06-16 12:11:19, KarimAllah Ahmed wrote:
>     > When sparse memory model is used an array of memory sections is created to
>     > track each block of contiguous physical pages. Each element of this array
>     > contains PAGES_PER_SECTION pages. During the creation of this array the actual
>     > boundaries of the memory block is lost, so the whole block is either considered
>     > as present or not.
>     >
>     > pfn_valid() in the sparse memory configuration checks which memory sections the
>     > pfn belongs to then checks whether it's present or not. This yields sub-optimal
>     > results when the available memory doesn't cover the whole memory section,
>     > because pfn_valid will return 'true' even for the unavailable pfns at the
>     > boundaries of the memory section.
>
>     Please be more verbose of _why_ the patch is needed. Why those
>     "sub-optimal results" matter?
>
> Does this make sense to you ?

[ channeling my inner akpm ]

What's the user visible effect of this change?  What code is getting
tripped up by pfn_valid() being imprecise, and why is changing
pfn_valid() the preferred fix?