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Date: Thu, 09 Nov 2017 14:30:18 +0100 From: Vitaly Kuznetsov <vkuznets@...hat.com> To: Michal Hocko <mhocko@...nel.org> Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>, Vlastimil Babka <vbabka@...e.cz>, Mel Gorman <mgorman@...hsingularity.net>, YASUAKI ISHIMATSU <yasu.isimatu@...il.com>, Hillf Danton <hillf.zj@...baba-inc.com>, Johannes Weiner <hannes@...xchg.org>, "K. Y. Srinivasan" <kys@...rosoft.com>, Stephen Hemminger <sthemmin@...rosoft.com>, Alex Ng <alexng@...rosoft.com> Subject: Re: [PATCH RFC] mm/memory_hotplug: make it possible to offline blocks with reserved pages Michal Hocko <mhocko@...nel.org> writes: > On Wed 08-11-17 17:16:19, Vitaly Kuznetsov wrote: >> Michal Hocko <mhocko@...nel.org> writes: >> >> > On Wed 08-11-17 16:39:49, Vitaly Kuznetsov wrote: >> >> Michal Hocko <mhocko@...nel.org> writes: >> >> >> >> > On Wed 08-11-17 14:01:55, Vitaly Kuznetsov wrote: >> >> >> Hyper-V balloon driver needs to hotplug memory in smaller chunks and to >> >> >> workaround Linux's 128Mb allignment requirement so it does a trick: partly >> >> >> populated 128Mb blocks are added and then a custom online_page_callback >> >> >> hook checks if the particular page is 'backed' during onlining, in case it >> >> >> is not backed it is left in Reserved state. When the host adds more pages >> >> >> to the block we bring them online from the driver (see >> >> >> hv_bring_pgs_online()/hv_page_online_one() in drivers/hv/hv_balloon.c). >> >> >> Eventually the whole block becomes fully populated and we hotplug the next >> >> >> 128Mb. This all works for quite some time already. >> >> > >> >> > Why does HyperV needs to workaround the section size limit in the first >> >> > place? We are allocation memmap for the whole section anyway so it won't >> >> > save any memory. So the whole thing sounds rather dubious to me. >> >> > >> >> >> >> Memory hotplug requirements in Windows are different, they have 2Mb >> >> granularity, not 128Mb like we have in Linux x86. >> >> >> >> Imagine there's a request to add 32Mb of memory comming from the >> >> Hyper-V host. What can we do? Don't add anything at all and wait till >> >> we're suggested to add > 128Mb and then add a section or the current >> >> approach. >> > >> > Use a different approach than memory hotplug. E.g. memory balloning. >> > >> >> But how? When we boot we may have very little memory and later on we >> hotplug a lot so we may not even be able to ballon all possible memory >> without running out of memory. > > Just add more memory and make part of it unusable and return it back to > the host via standard ballooning means. We don't have control over how much memory host gives us and we have no way to return anything to the host. > > How realistic is that the host gives only such a small amount of memory > btw? It happens all the time, Hyper-V host will gradually increase guest's memory when Dynamic Memory is enabled. Moreover, there's a manual interface when host's admin can hotplug memory to guests (starting WS2016) with 2M granularity. -- Vitaly
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