linux-kernel - Re: uninitialized pmem struct pages

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Date:   Tue, 5 Jan 2021 10:58:46 +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

On 05.01.21 10:56, Dan Williams wrote:
> On Tue, Jan 5, 2021 at 1:37 AM David Hildenbrand <david@...hat.com> wrote:
>>
>>>> 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.
> 
> Here's where we left it last time, open to pfn_to_online_page hacks...
> 
> https://lore.kernel.org/linux-mm/CAPcyv4ivq=EPUePXiX2ErcVyF7+dV9Yv215Oue7X_Y2X_Jfw8Q@mail.gmail.com
> 

Yeah, I recall. That's why I favor simple approaches right now - less
brain power to waste ;)

> I don't think a slow-path flag in the mem-section is too onerous, but
> I'll withhold judgement until I have the patch I'm thinking of
> in-hand. Let me give it a shot, you can always nack the final result.

Sure!


-- 
Thanks,

David / dhildenb