On 18.07.2018 15:43, Michal Hocko wrote: > On Wed 18-07-18 15:39:29, David Hildenbrand wrote: >> On 18.07.2018 15:19, Michal Hocko wrote: >>> [got back to this really late. Sorry about that] >>> >>> On Thu 24-05-18 23:07:23, David Hildenbrand wrote: >>>> On 24.05.2018 16:22, Michal Hocko wrote: >>>>> I will go over the rest of the email later I just wanted to make this >>>>> point clear because I suspect we are talking past each other. >>>> >>>> It sounds like we are now talking about how to solve the problem. I like >>>> that :) >>>> >>>>> >>>>> On Thu 24-05-18 16:04:38, David Hildenbrand wrote: >>>>> [...] >>>>>> The point I was making is: I cannot allocate 8MB/128MB using the buddy >>>>>> allocator. All I want to do is manage the memory a virtio-mem device >>>>>> provides as flexible as possible. >>>>> >>>>> I didn't mean to use the page allocator to isolate pages from it. We do >>>>> have other means. Have a look at the page isolation framework and have a >>>>> look how the current memory hotplug (ab)uses it. In short you mark the >>>>> desired physical memory range as isolated (nobody can allocate from it) >>>>> and then simply remove it from the page allocator. And you are done with >>>>> it. Your particular range is gone, nobody will ever use it. If you mark >>>>> those struct pages reserved then pfn walkers should already ignore them. >>>>> If you keep those pages with ref count 0 then even hotplug should work >>>>> seemlessly (I would have to double check). >>>>> >>>>> So all I am arguing is that whatever your driver wants to do can be >>>>> handled without touching the hotplug code much. You would still need >>>>> to add new ranges in the mem section units and manage on top of that. >>>>> You need to do that anyway to keep track of what parts are in use or >>>>> offlined anyway right? Now the mem sections. You have to do that anyway >>>>> for memmaps. Our sparse memory model simply works in those units. Even >>>>> if you make a part of that range unavailable then the section will still >>>>> be there. >>>>> >>>>> Do I make at least some sense or I am completely missing your point? >>>>> >>>> >>>> I think we're heading somewhere. I understand that you want to separate >>>> this "semi" offline part from the general offlining code. If so, we >>>> should definitely enforce segment alignment for online_pages/offline_pages. >>>> >>>> Importantly, what I need is: >>>> >>>> 1. Indicate and prepare memory sections to be used for adding memory >>>> chunks (right now add_memory()) >>> >>> Yes, this is section based. So you will always get memmap (struct page) >>> for the whole section. >>> >>>> 2. Make memory chunks of a section available to the system (right now >>>> online_pages()) >>> >>> Yes, this doesn't have to be section based. All you need is to mark >>> remaining pages as offline. They are reserved at this moment so nobody >>> should touch tehem. >>> >>>> 3. Remove memory chunks of a section from the system (right now >>>> offline_pages()) >>> >>> Yes. All we need is to note that those reserved pages are actually good >>> to offline. I have mentioned that reserved pages are yours at this stage >>> so you can note the special state without an additional page flag. >>> >>> The generic hotplug code just have to learn about this new state. >>> has_unmovable_pages sounds like a proper place to do that. You simply >>> clear the offline state and the PageReserved and you are done with the >>> page. >>> >> >> I agree. This would be minimal invassive - notifiers are still called on >> whole segment. > > That shouldn't matter because notifiers should never step on pages they > do not manage or own. > >>>> 4. Remove memory sections from the system (right now remove_memory()) >>> >>> no change needed >>> >>>> 5. Hinder dumping tools from reading memory chunks that are logically >>>> offline (right now PageOffline()) >>> >>> I still fail to see why do we even care about some dumping tools. Pages >>> are reserved so they simply shouldn't touch that memory at all. >>> >> >> Thanks for having a look! >> >> I wonder why reserved pages never got excluded by dump tools. So I >> assume there is some kind of magic hidden in it. >> >> `git grep SetPageReserved` returns a number of buffers that are not to >> be swapped. So "reserved" there is used for: >> "PG_reserved is set for special pages, which can never be swapped out" > > That was an ancient menaing of the flag. The flag in general means that > you shouldn't touch it unless you own it. > >> And my point would be that these pages are still to be dumped (just as >> it is being done now). They are valid memory. > > Then fix kdump or what ever is touching them. If the rule is really reserved -> dontouch, then I agree. > >> It seems like this bit is used for two different purposes. My take would >> be then to have another way of indicating "don't swap" vs. "page not >> accessible / offline". And that's why I propose PageOffline. >> >> I would even go one step further and rename "reserved" to "dontswap". > > No, it really doesn't have that meaning for years. > So would you agree to change the comment in page-flags.h to something like "PG_reserved is set for special pages, that should never be touched (read/written). Some of them might not even exist." Thanks! -- Thanks, David / dhildenb
