linux-kernel - Re: [PATCH RFC 0/4] mm: place pages to the freelist tail when onling and undoing isolation

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Message-ID: <5c0910c2cd0d9d351e509392a45552fb@suse.de>
Date:   Wed, 16 Sep 2020 20:50:41 +0200
From:   osalvador@...e.de
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-hyperv@...r.kernel.org, xen-devel@...ts.xenproject.org,
        linux-acpi@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Michael Ellerman <mpe@...erman.id.au>,
        Michal Hocko <mhocko@...nel.org>,
        Mike Rapoport <rppt@...nel.org>,
        Scott Cheloha <cheloha@...ux.ibm.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Vlastimil Babka <vbabka@...e.cz>, Wei Liu <wei.liu@...nel.org>,
        Wei Yang <richard.weiyang@...ux.alibaba.com>
Subject: Re: [PATCH RFC 0/4] mm: place pages to the freelist tail when onling
 and undoing isolation

On 2020-09-16 20:34, David Hildenbrand wrote:
> When adding separate memory blocks via add_memory*() and onlining them
> immediately, the metadata (especially the memmap) of the next block 
> will be
> placed onto one of the just added+onlined block. This creates a chain
> of unmovable allocations: If the last memory block cannot get
> offlined+removed() so will all dependant ones. We directly have 
> unmovable
> allocations all over the place.
> 
> This can be observed quite easily using virtio-mem, however, it can 
> also
> be observed when using DIMMs. The freshly onlined pages will usually be
> placed to the head of the freelists, meaning they will be allocated 
> next,
> turning the just-added memory usually immediately un-removable. The
> fresh pages are cold, prefering to allocate others (that might be hot)
> also feels to be the natural thing to do.
> 
> It also applies to the hyper-v balloon xen-balloon, and ppc64 dlpar: 
> when
> adding separate, successive memory blocks, each memory block will have
> unmovable allocations on them - for example gigantic pages will fail to
> allocate.
> 
> While the ZONE_NORMAL doesn't provide any guarantees that memory can 
> get
> offlined+removed again (any kind of fragmentation with unmovable
> allocations is possible), there are many scenarios (hotplugging a lot 
> of
> memory, running workload, hotunplug some memory/as much as possible) 
> where
> we can offline+remove quite a lot with this patchset.

Hi David,

I did not read through the patchset yet, so sorry if the question is 
nonsense, but is this not trying to fix the same issue the vmemmap 
patches did? [1]

I was about to give it a new respin now that thw hwpoison stuff has been 
settled.

[1] https://patchwork.kernel.org/cover/11059175/
>