On Tue 26-01-21 11:20:11, Mike Rapoport wrote: > On Tue, Jan 26, 2021 at 10:00:13AM +0100, Michal Hocko wrote: > > On Tue 26-01-21 10:33:11, Mike Rapoport wrote: > > > On Tue, Jan 26, 2021 at 08:16:14AM +0100, Michal Hocko wrote: > > > > On Mon 25-01-21 23:36:18, Mike Rapoport wrote: > > > > > On Mon, Jan 25, 2021 at 06:01:22PM +0100, Michal Hocko wrote: > > > > > > On Thu 21-01-21 14:27:18, Mike Rapoport wrote: > > > > > > > From: Mike Rapoport <rppt@xxxxxxxxxxxxx> > > > > > > > > > > > > > > Introduce "memfd_secret" system call with the ability to create memory > > > > > > > areas visible only in the context of the owning process and not mapped not > > > > > > > only to other processes but in the kernel page tables as well. > > > > > > > > > > > > > > The user will create a file descriptor using the memfd_secret() system > > > > > > > call. The memory areas created by mmap() calls from this file descriptor > > > > > > > will be unmapped from the kernel direct map and they will be only mapped in > > > > > > > the page table of the owning mm. > > > > > > > > > > > > > > The secret memory remains accessible in the process context using uaccess > > > > > > > primitives, but it is not accessible using direct/linear map addresses. > > > > > > > > > > > > > > Functions in the follow_page()/get_user_page() family will refuse to return > > > > > > > a page that belongs to the secret memory area. > > > > > > > > > > > > > > A page that was a part of the secret memory area is cleared when it is > > > > > > > freed. > > > > > > > > > > > > > > The following example demonstrates creation of a secret mapping (error > > > > > > > handling is omitted): > > > > > > > > > > > > > > fd = memfd_secret(0); > > > > > > > ftruncate(fd, MAP_SIZE); > > > > > > > ptr = mmap(NULL, MAP_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0); > > > > > > > > > > > > I do not see any access control or permission model for this feature. > > > > > > Is this feature generally safe to anybody? > > > > > > > > > > The mappings obey memlock limit. Besides, this feature should be enabled > > > > > explicitly at boot with the kernel parameter that says what is the maximal > > > > > memory size secretmem can consume. > > > > > > > > Why is such a model sufficient and future proof? I mean even when it has > > > > to be enabled by an admin it is still all or nothing approach. Mlock > > > > limit is not really useful because it is per mm rather than per user. > > > > > > > > Is there any reason why this is allowed for non-privileged processes? > > > > Maybe this has been discussed in the past but is there any reason why > > > > this cannot be done by a special device which will allow to provide at > > > > least some permission policy? > > > > > > Why this should not be allowed for non-privileged processes? This behaves > > > similarly to mlocked memory, so I don't see a reason why secretmem should > > > have different permissions model. > > > > Because appart from the reclaim aspect it fragments the direct mapping > > IIUC. That might have an impact on all others, right? > > It does fragment the direct map, but first it only splits 1G pages to 2M > pages and as was discussed several times already it's not that clear which > page size in the direct map is the best and this is very much workload > dependent. I do appreciate this has been discussed but this changelog is not specific on any of that reasoning and I am pretty sure nobody will remember details in few years in the future. Also some numbers would be appropriate. > These are the results of the benchmarks I've run with the default direct > mapping covered with 1G pages, with disabled 1G pages using "nogbpages" in > the kernel command line and with the entire direct map forced to use 4K > pages using a simple patch to arch/x86/mm/init.c. > > https://docs.google.com/spreadsheets/d/1tdD-cu8e93vnfGsTFxZ5YdaEfs2E1GELlvWNOGkJV2U/edit?usp=sharing A good start for the data I am asking above. -- Michal Hocko SUSE Labs
