linux-kernel - Re: uninitialized pmem struct pages

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Message-ID: <dd49a67a-109e-b5c1-2010-572587fe4ed6@redhat.com>
Date:   Tue, 5 Jan 2021 10:37:38 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Michal Hocko <mhocko@...e.com>, Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: uninitialized pmem struct pages

>> Yeah, obviously the first one. Being able to add+use PMEM is more
>> important than using each and every last MB of main memory.
>>
>> I wonder if we can just stop adding any system RAM like
>>
>> [     Memory Section    ]
>> [ RAM ] [      Hole     ]
>>
>> When there could be the possibility that the hole might actually be
>> PMEM. (e.g., with CONFIG_ZONE_DEVICE and it being the last section in a
>> sequence of sections, not just a tiny hole)
> 
> I like the simplicity of it... I worry that the capacity loss
> regression is easy to notice by looking at the output of free(1) from
> one kernel to the next and someone screams.

Well, you can always make it configurable and then simply fail to add
PMEM later if impossible (trying to sub-section hot-add into early
section). It's in the hands of the sysadmin then ("max out system ram"
vs. "support any PMEM device that could eventually be there at
runtime"). Distros would go for the second.

I agree that it's not optimal, but sometimes simplicity has to win.

-- 
Thanks,

David / dhildenb