linux-kernel - Re: [RFC PATCH V2 2/10] mm: expose is_mem_section

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Message-ID: <99a6db0c-6d73-d982-58b3-7a0172748ae4@redhat.com>
Date:   Fri, 10 Jan 2020 14:41:36 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>, lantianyu1986@...il.com
Cc:     kys@...rosoft.com, haiyangz@...rosoft.com, sthemmin@...rosoft.com,
        sashal@...nel.org, akpm@...ux-foundation.org,
        michael.h.kelley@...rosoft.com,
        Tianyu Lan <Tianyu.Lan@...rosoft.com>,
        linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, vkuznets@...hat.com, eric.devolder@...cle.com,
        vbabka@...e.cz, osalvador@...e.de, pavel.tatashin@...rosoft.com,
        rppt@...ux.ibm.com
Subject: Re: [RFC PATCH V2 2/10] mm: expose is_mem_section_removable() symbol

On 07.01.20 14:36, Michal Hocko wrote:
> On Tue 07-01-20 21:09:42, lantianyu1986@...il.com wrote:
>> From: Tianyu Lan <Tianyu.Lan@...rosoft.com>
>>
>> Hyper-V balloon driver will use is_mem_section_removable() to
>> check whether memory block is removable or not when receive
>> memory hot remove msg. Expose it.
> 
> I do not think this is a good idea. The check is inherently racy. Why
> cannot the balloon driver simply hotremove the region when asked?
> 

It's not only racy, it also gives no guarantees. False postives and
false negatives are possible.

If you want to avoid having to loop forever trying to offline when
calling offline_and_remove_memory(), you could try to
alloc_contig_range() the memory first and then play the
PG_offline+notifier game like virtio-mem.

I don't remember clearly, but I think pinned pages can make offlining
loop for a long time. And I remember there were other scenarios as well
(including out of memory conditions and similar).

I sent an RFC [1] for powerpc/memtrace that does the same (just error
handling is more complicated as it wants to offline and remove multiple
consecutive memory blocks) - if you want to try to go down that path.

[1] https://lkml.kernel.org/r/20191217123851.8854-1-david@redhat.com

-- 
Thanks,

David / dhildenb