On 01.04.19 16:09, David Hildenbrand wrote: >>> Thinking about your approach, there is one elementary thing to notice: >>> >>> Giving the guest pages from the buffer while hinting requests are being >>> processed means that the guest can and will temporarily make use of more >>> memory than desired. Essentially up to the point where MADV_FREE is >>> finally called for the hinted pages. >> >> Right - but that seems like exactly the reverse of the issue with the current >> approach which is guest can temporarily use less memory than desired. >> >>> Even then the guest will logicall >>> make use of more memory than desired until core MM takes pages away. >> >> That sounds more like a host issue though. If it wants to >> it can use e.g. MAD_DONTNEED. > > Indeed. But MADV_DONTNEED is somewhat undesired for performance reasons. > You want to do the work when swapping not when hinting. > > But what I wanted to say here: Looking at the pure size of your guest > will at least not help you to identify if more memory than desired will > be used. > >> >>> So: >>> 1) Unmodified guests will make use of more memory than desired. >> >> One interesting possibility for this is to add the buffer memory >> by hotplug after the feature has been negotiated. >> I agree this sounds complex. > > Yes it is, and it goes into the direction of virtio-mem that essentially > does that. But bad news: memory hotplug is complicated stuff, both on > the hypervisor and guest side. And things like NUMA make it more involved. > > But even then, malicious guest can simply fake feature negotiation and > make use of all hotplugged memory. Won't work, at least not for > malicious guests. > >> >> But I have an idea: how about we include the hint size in the >> num_pages counter? Then unmodified guests put >> it in the balloon and don't use it. Modified ones >> will know to use it just for hinting. > > These are the nightmares I was talking about. I would like to decouple > this feature as far as possible from balloon inflation/deflation. > Ballooning is 4k based and might have other undesirable side effect. > Just because somebody wants to use page hinting does not mean he wants > to use ballooning. Effectively, many people will want to avoid > ballooning completely by using page hinting for their use case. > >> >> >>> 2) Malicious guests will make use of more memory than desired. >> >> Well this limitation is fundamental to balloon right? > > Yep, it is the fundamental issue of ballooning. If memory is available > right from the boot, the system is free to do with it whatever it wants. > (one of the main things virtio-mem will do differently/better) > >> If host wants to add tracking of balloon memory, it >> can enforce the limits. So far no one bothered, >> but maybe with this feature we should start to do that. > > I think I already had endless rants about why this is not possible. > Ballooning as it is currently implemented by virtio-balloon is broken by > design. Period. You can and never will be able to distinguish unmodified > guests from malicious guests. Please don't design new approaches based > on broken design. > >> >>> 3) Sane, modified guests will make use of more memory than desired. >>> >>> Instead, we could make our life much easier by doing the following: >>> >>> 1) Introduce a parameter to cap the amount of memory concurrently hinted >>> similar like you suggested, just don't consider it a buffer value. >>> "-device virtio-balloon,hinting_size=1G". This gives us control over the >>> hinting proceess. >>> >>> hinting_size=0 (default) disables hinting >>> >>> The admin can tweak the number along with memory requirements of the >>> guest. We can make suggestions (e.g. calculate depending on #cores,#size >>> of memory, or simply "1GB") >> >> So if it's all up to the guest and for the benefit of the guest, and >> with no cost/benefit to the host, then why are we supplying this value >> from the host? > > See 3), the admin has to be aware of hinting behavior. > >> >>> 2) In the guest, track the size of hints in progress, cap at the >>> hinting_size. >>> >>> 3) Document hinting behavior >>> >>> "When hinting is enabled, memory up to hinting_size might temporarily be >>> removed from your guest in order to be hinted to the hypervisor. This is >>> only for a very short time, but might affect applications. Consider the >>> hinting_size when sizing your guest. If your application was tested with >>> XGB and a hinting size of 1G is used, please configure X+1GB for the >>> guest. Otherwise, performance degradation might be possible." >> >> OK, so let's start with this. Now let us assume that guest follows >> the advice. We thus know that 1GB is not needed for guest applications. >> So why do we want to allow applications to still use this extra memory? > > If the application does not need the 1GB, the 1GB will be hinted to the > hypervisor and are effectively only a buffer for the OOM scenario. > (ignoring page cache discussions for now). > > "So why do we want to allow applications to still use this extra memory" > is the EXACT same issue you have with your buffer approach. Any guest > can make use of the buffer and you won't be able to detect it. Very same > problem. Only in your approach, the guest might agree to play nicely by > not making use of the 1G you provided. Just as if the application does > not need/use the additional 1GB. > > The interesting thing is most probably: Will the hinting size usually be > reasonable small? At least I guess a guest with 4TB of RAM will not > suddenly get a hinting size of hundreds of GB. Most probably also only > something in the range of 1GB. But this is an interesting question to > look into. > > Also, if the admin does not care about performance implications when > already close to hinting, no need to add the additional 1Gb to the ram size.
"close to OOM" is what I meant. -- Thanks, David / dhildenb
