linux-kernel - Re: [PATCH 0/8] implement "memmap on memory" feature on s390

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Date:   Fri, 17 Nov 2023 14:56:50 +0100
From:   Sumanth Korikkar <sumanthk@...ux.ibm.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-mm <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oscar Salvador <osalvador@...e.de>,
        Michal Hocko <mhocko@...e.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        linux-s390 <linux-s390@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/8] implement "memmap on memory" feature on s390

On Fri, Nov 17, 2023 at 12:08:31AM +0100, David Hildenbrand wrote:
> On 14.11.23 19:02, Sumanth Korikkar wrote:
> > Hi All,
> > 
> > The patch series implements "memmap on memory" feature on s390 and
> > provides the necessary fixes for it.
> 
> Thinking about this, one thing that makes s390x different from all the other
> architectures in this series is the altmap handling.
> 
> I'm curious, why is that even required?
> 
> A memmep that is not marked as online in the section should not be touched
> by anybody (except memory onlining code :) ). And if we do, it's usually a
> BUG because that memmap might contain garbage/be poisoned or completely
> stale, so we might want to track that down and fix it in any case.
> 
> So what speaks against just leaving add_memory() populate the memmap from
> the altmap? Then, also the page tables for the memmap are already in place
> when onlining memory.
>

we do have page_init_poison() in sparse_add_section() which should be
handled later then. not in add_pages()
> 
> Then, adding two new notifier calls on start of memory_block_online() called
> something like MEM_PREPARE_ONLINE and end the end of memory_block_offline()
> called something like MEM_FINISH_OFFLINE is still suboptimal, but that's
> where standby memory could be activated/deactivated, without messing with
> the altmap.
> 
> That way, the only s390x specific thing is that the memmap that should not
> be touched by anybody is actually inaccessible, and you'd
> activate/deactivate simply from the new notifier calls just the way we used
> to do.
ok. 

Thanks