Messages in this thread |  | | Subject | Re: [PATCH v17 00/10] mm: introduce memfd_secret system call to create "secret" memory areas | From | David Hildenbrand <> | Date | Tue, 9 Feb 2021 11:30:53 +0100 |
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On 09.02.21 11:23, David Hildenbrand wrote: >>>> A lot of unevictable memory is a concern regardless of CMA/ZONE_MOVABLE. >>>> As I've said it is quite easy to land at the similar situation even with >>>> tmpfs/MAP_ANON|MAP_SHARED on swapless system. Neither of the two is >>>> really uncommon. It would be even worse that those would be allowed to >>>> consume both CMA/ZONE_MOVABLE. >>> >>> IIRC, tmpfs/MAP_ANON|MAP_SHARED memory >>> a) Is movable, can land in ZONE_MOVABLE/CMA >>> b) Can be limited by sizing tmpfs appropriately >>> >>> AFAIK, what you describe is a problem with memory overcommit, not with zone >>> imbalances (below). Or what am I missing? >> >> It can be problem for both. If you have just too much of shm (do not >> forget about MAP_SHARED|MAP_ANON which is much harder to size from an >> admin POV) then migrateability doesn't really help because you need a >> free memory to migrate. Without reclaimability this can easily become a >> problem. That is why I am saying this is not really a new problem. >> Swapless systems are not all that uncommon. > > I get your point, it's similar but still different. "no memory in the > system" vs. "plenty of unusable free memory available in the system". > > In many setups, memory for user space applications can go to > ZONE_MOVABLE just fine. ZONE_NORMAL etc. can be used for supporting user > space memory (e.g., page tables) and other kernel stuff. > > Like, have 4GB of ZONE_MOVABLE with 2GB of ZONE_NORMAL. Have an > application (database) that allocates 4GB of memory. Works just fine. > The zone ratio ends up being a problem for example with many processes > (-> many page tables). > > Not being able to put user space memory into the movable zone is a > special case. And we are introducing yet another special case here > (besides vfio, rdma, unmigratable huge pages like gigantic pages). > > With plenty of secretmem, looking at /proc/meminfo Total vs. Free can be > a big lie of how your system behaves. > >> >>>> One has to be very careful when relying on CMA or movable zones. This is >>>> definitely worth a comment in the kernel command line parameter >>>> documentation. But this is not a new problem. >>> >>> I see the following thing worth documenting: >>> >>> Assume you have a system with 2GB of ZONE_NORMAL/ZONE_DMA and 4GB of >>> ZONE_MOVABLE/CMA. >>> >>> Assume you make use of 1.5GB of secretmem. Your system might run into OOM >>> any time although you still have plenty of memory on ZONE_MOVAVLE (and even >>> swap!), simply because you are making excessive use of unmovable allocations >>> (for user space!) in an environment where you should not make excessive use >>> of unmovable allocations (e.g., where should page tables go?). >> >> yes, you are right of course and I am not really disputing this. But I >> would argue that 2:1 Movable/Normal is something to expect problems >> already. "Lowmem" allocations can easily trigger OOM even without secret >> mem in the picture. It all just takes to allocate a lot of GFP_KERNEL or >> even GFP_{HIGH}USER. Really, it is CMA/MOVABLE that are elephant in the >> room and one has to be really careful when relying on them. > > Right, it's all about what the setup actually needs. Sure, there are > cases where you need significantly more GFP_KERNEL/GFP_{HIGH}USER such > that a 2:1 ratio is not feasible. But I claim that these are corner cases. > > Secretmem gives user space the option to allocate a lot of > GFP_{HIGH}USER memory. If I am not wrong, "ulimit -a" tells me that each > application on F33 can allocate 16 GiB (!) of secretmem.
Got to learn to do my math. It's 16 MiB - so as a default it's less dangerous than I thought!
-- Thanks,
David / dhildenb
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