On Thu 28-01-21 11:22:59, Mike Rapoport wrote: > On Tue, Jan 26, 2021 at 01:08:23PM +0100, Michal Hocko wrote: > > On Tue 26-01-21 12:56:48, David Hildenbrand wrote: > > > On 26.01.21 12:46, Michal Hocko wrote: > > > > On Thu 21-01-21 14:27:19, Mike Rapoport wrote: > > > > > From: Mike Rapoport <rppt@xxxxxxxxxxxxx> > > > > > > > > > > Removing a PAGE_SIZE page from the direct map every time such page is > > > > > allocated for a secret memory mapping will cause severe fragmentation of > > > > > the direct map. This fragmentation can be reduced by using PMD-size pages > > > > > as a pool for small pages for secret memory mappings. > > > > > > > > > > Add a gen_pool per secretmem inode and lazily populate this pool with > > > > > PMD-size pages. > > > > > > > > > > As pages allocated by secretmem become unmovable, use CMA to back large > > > > > page caches so that page allocator won't be surprised by failing attempt to > > > > > migrate these pages. > > > > > > > > > > The CMA area used by secretmem is controlled by the "secretmem=" kernel > > > > > parameter. This allows explicit control over the memory available for > > > > > secretmem and provides upper hard limit for secretmem consumption. > > > > > > > > OK, so I have finally had a look at this closer and this is really not > > > > acceptable. I have already mentioned that in a response to other patch > > > > but any task is able to deprive access to secret memory to other tasks > > > > and cause OOM killer which wouldn't really recover ever and potentially > > > > panic the system. Now you could be less drastic and only make SIGBUS on > > > > fault but that would be still quite terrible. There is a very good > > > > reason why hugetlb implements is non-trivial reservation system to avoid > > > > exactly these problems. > > So, if I understand your concerns correct this implementation has two > issues: > 1) allocation failure at page fault that causes unrecoverable OOM and > 2) a possibility for an unprivileged user to deplete secretmem pool and > cause (1) to others > > I'm not really familiar with OOM internals, but when I simulated an > allocation failure in my testing only the allocating process and it's > parent were OOM-killed and then the system continued normally. If you kill the allocating process then yes, it would work, but your process might be the very last to be selected. > You are right, it would be better if we SIGBUS instead of OOM but I don't > agree SIGBUS is terrible. As we started to draw parallels with hugetlbfs > even despite it's complex reservation system, hugetlb_fault() may fail to > allocate pages from CMA and this still will cause SIGBUS. This is an unexpected runtime error. Unless you make it an integral part of the API design. > And hugetlb pools may be also depleted by anybody by calling > mmap(MAP_HUGETLB) and there is no any limiting knob for this, while > secretmem has RLIMIT_MEMLOCK. Yes it can fail. But it would fail at the mmap time when the reservation fails. Not during the #PF time which can be at any time. > That said, simply replacing VM_FAULT_OOM with VM_FAULT_SIGBUS makes > secretmem at least as controllable and robust than hugeltbfs even without > complex reservation at mmap() time. Still sucks huge! > > > > So unless I am really misreading the code > > > > Nacked-by: Michal Hocko <mhocko@xxxxxxxx> > > > > > > > > That doesn't mean I reject the whole idea. There are some details to > > > > sort out as mentioned elsewhere but you cannot really depend on > > > > pre-allocated pool which can fail at a fault time like that. > > > > > > So, to do it similar to hugetlbfs (e.g., with CMA), there would have to be a > > > mechanism to actually try pre-reserving (e.g., from the CMA area), at which > > > point in time the pages would get moved to the secretmem pool, and a > > > mechanism for mmap() etc. to "reserve" from these secretmem pool, such that > > > there are guarantees at fault time? > > > > yes, reserve at mmap time and use during the fault. But this all sounds > > like a self inflicted problem to me. Sure you can have a pre-allocated > > or more dynamic pool to reduce the direct mapping fragmentation but you > > can always fall back to regular allocatios. In other ways have the pool > > as an optimization rather than a hard requirement. With a careful access > > control this sounds like a manageable solution to me. > > I'd really wish we had this discussion for earlier spins of this series, > but since this didn't happen let's refresh the history a bit. I am sorry but I am really fighting to find time to watch for all the moving targets... > One of the major pushbacks on the first RFC [1] of the concept was about > the direct map fragmentation. I tried really hard to find data that shows > what is the performance difference with different page sizes in the direct > map and I didn't find anything. > > So presuming that large pages do provide advantage the first implementation > of secretmem used PMD_ORDER allocations to amortise the effect of the > direct map fragmentation and then handed out 4k pages at each fault. In > addition there was an option to reserve a finite pool at boot time and > limit secretmem allocations only to that pool. > > At some point David suggested to use CMA to improve overall flexibility > [3], so I switched secretmem to use CMA. > > Now, with the data we have at hand (my benchmarks and Intel's report David > mentioned) I'm even not sure this whole pooling even required. I would still like to understand whether that data is actually representative. With some underlying reasoning rather than I have run these XYZ benchmarks and numbers do not look terrible. > I like the idea to have a pool as an optimization rather than a hard > requirement but I don't see why would it need a careful access control. As > the direct map fragmentation is not necessarily degrades the performance > (and even sometimes it actually improves it) and even then the degradation > is small, trying a PMD_ORDER allocation for a pool and then falling back to > 4K page may be just fine. Well, as soon as this is a scarce resource then an access control seems like a first thing to think of. Maybe it is not really necessary but then this should be really justified. I am also still not sure why this whole thing is not just a ramdisk/ramfs which happens to unmap its pages from the direct map. Wouldn't that be a much more easier model to work with? You would get an access control for free as well. -- Michal Hocko SUSE Labs
