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

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Date:   Wed, 14 Sep 2016 22:11:00 +0000
From:   "Raslan, KarimAllah" <karahmed@...zon.de>
To:     Dan Williams <dan.j.williams@...el.com>
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


Ahmed, Karim Allah
karahmed@...zon.de



> On Sep 15, 2016, at 12:05 AM, Dan Williams <dan.j.williams@...el.com> wrote:
> 
> 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?

I did expand the commit message in v2 of this patch to answer these questions:

https://patchwork.kernel.org/patch/9190737/

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